GTA

All Springer/NP/PCP Air Gun Discussion General => "Bob and Lloyds Workshop" => Topic started by: Greg_E on November 01, 2021, 10:47:07 PM

Title: How much jump before pellet hits the grooves?
Post by: Greg_E on November 01, 2021, 10:47:07 PM
Down in my velocity after valve thread, I discovered something that I'm not sure about.

When I chamber a round in my Bulldog, it embossed the rifling pretty far back. Should the projectile be able to travel a short distance before encountering the rifling?

Or do we want the projectile engaged in the rifling to help seal the barrel for optimum performance?

With firearms you normally want at least a small jump distance, or else chamber pressures can get high. Not sure if the same is true for airguns, just without the pressure spike since the pressure is the pressure with air.
Title: Re: How much jump before pellet hits the grooves?
Post by: rsterne on November 01, 2021, 11:11:26 PM
I cut my chamber so that the bullet is slightly engaged into the tapered leade portion.... I position the taper so that the longest bullet is nearly fully engaged in the taper, while the shortest one just touches it.... I don't want any "jump", nor do I want to have to force the bullet into the full depth of the rifling.... Here is a selection of .257 bullets....

(https://hosting.photobucket.com/albums/oo221/rsterne/6mm Sporter/.highres/257 Bullets_zpshef0w8ul.jpg?width=1920&height=1080&fit=bounds) (https://app.photobucket.com/u/rsterne/a/2f8e7dd4-f82d-4776-a4ea-72f8a8d34c0a/p/449a64b5-430b-43dc-a7e5-6f8ab0776dbe)

I have a very shallow taper on the leade, just 1 degree per side.... If you look carefully, you can see marks on all of those bullets....

Bob
Title: Re: How much jump before pellet hits the grooves?
Post by: oldpro on November 01, 2021, 11:18:07 PM
 If you add 1/8 of and inch of jump you will see pretty big velocity gains BUT it can cause some degree of accuracy concerns but not always. I use jump in most of my bullet shooters but none in my pellet shooters. JMHO
Title: Re: How much jump before pellet hits the grooves?
Post by: Greg_E on November 02, 2021, 01:01:32 AM
I'm pretty sure the taper is much steeper than Bob's.

Since I'm probably never going to shoot pellets in this rifle, I'll have to think about about cutting the leade deeper for a little jump, or at least get them to be on the taper. If I can get a shallower angle reamer, I might be able to get what I want by simply changing the angle.

Anyone know a place in NY that can do this work for me? After that any place I can send it in the US?

I can buy a teamed for around $60 plus shipping from the Ukraine, but you know how some of those things go. I don't have a lathe to make my own or I'd try it.
Title: Re: How much jump before pellet hits the grooves?
Post by: rsterne on November 02, 2021, 12:59:03 PM
You can modify tapered pin reamers into leade reamers.... They are just about ideal for the job, the taper is very shallow, just 1/4" per foot (1.2 deg.)... You just grind away the big end of the reamer behind the chamber diameter you want....

Bob
Title: Re: How much jump before pellet hits the grooves?
Post by: Greg_E on November 02, 2021, 02:38:07 PM
That's what I needed, thanks! Finding throat reamers has been a challenge, but chamber reamers in the shape of XYZ cartridge are everywhere. I'll order up a taper pin reamer of the appropriate size and see if I can make a brass bushing for the small end, should be able to do that with the tools I have. Not a lot of grinding needed for big end on the reamers I was seeing.
Title: Re: How much jump before pellet hits the grooves?
Post by: lennyk on November 05, 2021, 08:30:04 AM
whats the best way to grind them down ?
spin it up in reverse or forward ? and hold a stone to it ?

You can modify tapered pin reamers into leade reamers.... They are just about ideal for the job, the taper is very shallow, just 1/4" per foot (1.2 deg.)... You just grind away the big end of the reamer behind the chamber diameter you want....

Bob
Title: Re: How much jump before pellet hits the grooves?
Post by: mr007s on November 05, 2021, 10:34:34 AM
This may be of some help. https://conicalendmills.com/
Title: Re: How much jump before pellet hits the grooves?
Post by: lennyk on November 05, 2021, 11:16:45 AM
.....................
Title: Re: How much jump before pellet hits the grooves?
Post by: nervoustrigger on November 05, 2021, 11:38:14 AM
The direction of rotation when grinding off the flutes doesn’t seem to matter in my experience if you do a cleanup pass with a sufficiently fine abrasive like a Dremel emery wheel, sneaking up on the final OD.  Done carefully, it should neither leave a burr nor round over the sharp edge.  In other words you can remove the bulk of it with something aggressive like a 4-1/2” angle grinder before moving to the emery wheel.  I like to finish up by dressing the edge with a fine set of diamond needle files.
Title: Re: How much jump before pellet hits the grooves?
Post by: Greg_E on November 05, 2021, 12:04:48 PM
I should have ordered 2 of them, but gorilla comes to mind for grinding the first one.

Probably just going to use a bench grinder and drill to try and get it to stay round and centered for the big end. For the little end I may just machine a small bushing and glue it in place, but I have a lot of short end I can work with since the maximum diameter reamer I'm needing is very close to the finished size in the barrel.
Title: Re: How much jump before pellet hits the grooves?
Post by: JuryRigger on November 05, 2021, 12:19:15 PM
It might be easier to keep it round if you grind it in a drill press, holding the grinder to the reamer, as you can pivot the grinder into the rotating reamer more 'evenly' to keep things concentric than you can by trying to hold the rotating reamer to the also rotating grinding wheel... If your drill press isn't too sloppy, you could even clamp your grinder of choice to the table, and stroke the reamer with the quill, rotating the table over ever so slowly to increase the depth of cut.
Similar to doing this, but preferably with the grinding wheel turned 90-degrees from what I have it here, so that the lay of the finish isn't perpendicular to the direction of rotation-I would suggest trying the process on a test piece first, as there are some quirks to the process regarding angles and such that will tend to create chatter...
(https://www.gatewaytoairguns.org/GTA/MGalleryItem.php?id=8169)
Jesse
Title: Re: How much jump before pellet hits the grooves?
Post by: nervoustrigger on November 05, 2021, 01:12:49 PM
Agreed, I think the drill press will produce a better result.  Or do a hybrid…use the bench grinder/drill approach to remove the bulk of the material.  Then chuck the reamer into a drill press (or hand drill clamped to a surface) and spin it up, then slowly bring in a spinning emery wheel and just kiss the shoulder.  Be patient and continue to just barely let the wheel abrade the shoulder so it takes down only the high spots.  Eventually it will get to the point where it is touching all flutes equally for the full rotation.  Usually can tell by the way it sounds.  If uncertain, color the shoulder with a Sharpie (or blue layout dye) and spin it up again and briefly touch it with the emery wheel and stop to see if it removed the marking from all flutes.
Title: Re: How much jump before pellet hits the grooves?
Post by: Greg_E on November 05, 2021, 04:30:32 PM
I was also thinking grab it in my rotary table and tail stock, then use a cup wheel on my milling machine to grind away. Having trouble finding a decent small cup wheel with like a 1/4 inch shaft. Haven't checked the machine tools suppliers or McMaster yet. Wanted to wait until the reamer was in hand so I could figure out some options. But a bench grinder is still a choice for this. As long as the large end is smaller than the cut, and the edges of the cut are decent, I think it will be OK as long as I have a good bushing on the front to guide the cut.

A lathe would make all of this so much easier!
Title: Re: How much jump before pellet hits the grooves?
Post by: Paulemar on November 05, 2021, 04:53:04 PM

With firearms you normally want at least a small jump distance, or else chamber pressures can get high. Not sure if the same is true for airguns, just without the pressure spike since the pressure is the pressure with air.

Absolutely true, and we mustn't forget, with powder burners the bullet is most likely still within the neck of the cartridge case and needs that small jump distance before engaging. Air guns don't have that to deal with.
Title: Re: How much jump before pellet hits the grooves?
Post by: Greg_E on November 06, 2021, 01:17:27 PM
Seems I can't even buy a break... Received a straight reamer that is around 1/4 in diameter. Trying to work on a .357.
Title: Re: How much jump before pellet hits the grooves?
Post by: rsterne on November 06, 2021, 01:58:42 PM
 ::)

Bob
Title: Re: How much jump before pellet hits the grooves?
Post by: subscriber on November 07, 2021, 04:40:39 PM
The subject of projectile to land jump reveals an apparent conflict - all else being equal:

Conflicting goals
1. A projectile that is at least partially pre-engraved in the lands on loading is pre-aligned, to start down the bore coaxially for tighter groups.
2. A projectile that is loaded such that it has a running start when it engraves the lands produces a higher muzzle velocity - for a given volume of air used.   Possibly, at the expense of grouping ability, due to ambiguous alignment with the barrel bore.

Rather than have a "chamber" so large that a slug is sometimes free to drop through the freebore when the rifle is held muzzle down (while in the field), why not follow Scott's newly discovered approach (quote below) that slugs sized just under groove diameter provide the best combination of power and accuracy.   This is an approach that Dubber et al have been following with .22 slugs for some time. 

I can quote many large bore airgun shooters on this forum, stating that bullet diameter should be sized equal to, or just over barrel groove diameter.   This statement seems right, and yet may in fact not be a truism for airgun shooting.  Yet, I think it makes sense to use such sizing practice because of our PB experience:   Shooting undersized lead bullets is anathema to PB shooters, because hot gas leaking past the projectile tends to melt off lead from the "bearing surface", and solder it to the bore.  Not a risk with ambient temperature air, that cools on expansion in PCP barrels.  So, a good principle that does not apply to airguns...

Pushing a few .300 slugs, NSA & others, there is no groove contact, only land.
....
As to the seal or a slight lacking of is doubtful to creating much loss or air being the total area on .30 is so great.

As long as the power loss due to air leakage around the undersize slug is less than the friction reduction it produces (all the way down the barrel - not just on loading), this "airgun slug" approach is a good and possibly better one.  One might experiment with "labyrinth seal" drive bands on airgun slugs (images below), or keep it simple with just a narrow front and rear band to minimize friction.    Such a front & rear band approach could be combined with a series of smaller non-contact labyrinth seal bands in-between.  I suspect that any gains achieved may be useful only, if you "need" to just break 100 FPE, or some other actually arbitrary benchmark.

When it comes to shooting pellets, they need to have their heads supported by the lands on loading before firing (although this is not the case with the Huben and Leshiy mark 2, that both fire directly from the mag "chambers".  So, massive "jump" to the rifling).  This appears to be impossible to achieve with a deep shallow leade, cut for slug shooting.  Rather than lament the loss of pellet shooting ability, why not create a longer probe for pellets?   Or a version of retractable probe to "deep seat" pellets consistently, just the right amount? 

Or, a simpler approach; find, cast or bump up pellet head diameters so that they center earlier into the "land diameter ramp" on loading, with no possibility of starting anything but straight and coaxial.  Providing the skirt diameter is larger than groove diameter by the typical amount, the rear of the pellet should also start coaxial.

Of course, if the barrel's rifling twist is fast enough for long heavy slugs, it is probably way too fast for short, waisted pellets to shoot well at extended distances. Meaning, over 50 yards.


The attached image below shows a conventional "full friction" airgun lead slug at the top.   Below that is a reduced friction "relieved shank" slug design, common in "large bore" airguns.  It has only two robust driving bands, with the front band tangent to the nose radius.

Third from the top is essentially the same projectile as the top one, but sporting many small driving bands to reduce the total shank length rubbing against the barrel.  The point of that is to not only reduce friction down the barrel, but to minimize air leakage by means of what amounts to a "labyrinth seal".

The bottom slug has a narrow front and rear drive band at full diameter, with labyrinth seal bands that do not contact the groove diameter.

The top slug can be cast or swaged.  The others would need to be cast; or use split swaging dies as are used to make pellets.  Perhaps this amounts to a whole lot of trouble for theoretical gains, over either sizing the top slug perhaps 0.0005  below groove diameter; or the second slug from the top to exactly groove diameter at the breech....


(https://www.gatewaytoairguns.org/GTA/index.php?action=dlattach;topic=193067.0;attach=379774;image)
Title: Re: How much jump before pellet hits the grooves?
Post by: Nvreloader on November 07, 2021, 08:23:25 PM
IMHO

There is several factors that come into play, until there is some standardization of everything,
you are left out in the cold and have to experiment on your own for EACH CALIBER and SIZE,
to find what fits in your case.

1. Are we talking about pellets or bullets, makes a difference?
2. There is NO STANDARD DIMENSIONS for each of the calibers for:
A. Size of pellet, head and skirt dimensions etc.
B. Spec's of the Dimensions of the barrel/bore, lands and groove spec's etc.

177 Caliber example
                  
Smallest Head size--------.169"            
Largest Head size---------.193"            
Smallest Skirt Dia---------.173"         
Largest Skirt Dia-----------.189"         
Shortest Skirt Depth-------.040"      
Longest Skirt Depth--------.179"      
Shortest Bearing Length---.130"   
Longest Bearing Length----.240"   
Shortest OA Length---------.178"
Longest OA Length----------.350"


YMMV,

Tia,
Don
Title: Re: How much jump before pellet hits the grooves?
Post by: Greg_E on November 07, 2021, 08:27:52 PM
Since I'm waiting for a replacement reamer, I can try sizing to .356 and see what happens. But I've shot other .356 in the past and they were not as accurate as .357 and .358 sizes.

When you look at the Benjamin Nosler ammo, you'll see that they went for 3 thin driving bands, and the first two are fully engaged in the rifling.

I'll be cutting the "bad" barrel first and I'll lob a few shots through it to see if I get any speed increase.
Title: Re: How much jump before pellet hits the grooves?
Post by: subscriber on November 09, 2021, 01:34:50 AM
When you look at the Benjamin Nosler ammo, you'll see that they went for 3 thin driving bands, and the first two are fully engaged in the rifling.

Benjamin's narrow band strategy for reducing loading force and friction down the barrel is a good one.  The snag is the cost of the ammo, at more than a US$ per shot:  https://www.pyramydair.com/product/benjamin-nosler-ballistic-tip-extreme-air-rifle-bullet-357-cal-145?p=889 (https://www.pyramydair.com/product/benjamin-nosler-ballistic-tip-extreme-air-rifle-bullet-357-cal-145?p=889) 

(https://www.pyramydair.com/images/zoomed/Benjamin-Nosler-Ballistic-357_Benj-BPN357_zm2.jpg)


I don't know what you guys are willing to pay, but even 25 cents a shot for an airgun projectile makes a PB seem like an attractive alterative.  Of course, I am thinking about what I paid a few years ago, rather than for a desperate purchase now.

So, I assume that anyone shooting airgun calibers over .25 are, or will be casting their own.  As such, the question becomes what molds are available.  A few on this forum even make their own molds to cast very good looking projectiles.



Title: Re: How much jump before pellet hits the grooves?
Post by: Greg_E on November 09, 2021, 09:28:18 AM
Yup, if you want to shoot a reasonable amount, you must cast your own. Griffin ammo is better priced at around $0.50 a shot, but it still adds up fast.

But more airgun specific molds would be nice, Bob's designs are hard to buy due to shortages.

I may buy the Nosler style mold, only place I've seen it is from Africa, and it comes in over 150 grains from the solid nose. I think that mold is flat base where the Nosler is slight cup/dish base.
Title: Re: How much jump before pellet hits the grooves?
Post by: subscriber on November 09, 2021, 02:22:57 PM
Here is a GTA thread on making one's own mold:  https://www.gatewaytoairguns.org/GTA/index.php?topic=186722.msg156143915#msg156143915 (https://www.gatewaytoairguns.org/GTA/index.php?topic=186722.msg156143915#msg156143915)

And Lloyd's friction reduced slug thread:  https://www.gatewaytoairguns.org/GTA/index.php?topic=9843.msg81912#msg81912 (https://www.gatewaytoairguns.org/GTA/index.php?topic=9843.msg81912#msg81912)
Title: Re: How much jump before pellet hits the grooves?
Post by: subscriber on November 09, 2021, 04:50:44 PM
The CAD models below show how deep the breech cone is for a 2 degree included angle, and for a standard taper pin reamer included angle of 1.194 degrees.   The other factor that directly affects the length of the cone is the rifling depth.  In this case, the land height (or groove depth) is 0.004". This is typical for calibers in the .357 to .45 range.  For PB and airguns.

As such, the fact that the model depicts a groove diameter of .452" does not detract from a discussion about a .357 caliber airgun barrel. The cylindrical section behind the cone is depicted as 0.001" larger than groove diameter.  The TP diameter is show as 70% of the groove diameter, with the TP placement somewhat arbitrary, based on the assumed length of the projectile and loading depth.

I happened to have a barrel design in .45 that I then modified to show the breech cone depth.  As a reference point, the breech cone spec for the .45 ACP pistol caliber has a 4.7 degree included angle at the start of the lands.  Very abrupt, compared to the typical 2 degrees, popular for PCP air rifles on this forum.  But then the pistol bullet has some jump, from the case to the lands.  Too shallow and angle would result in the bullet skidding excessively in the rifling.  (Slight skidding can normally be observed in fired bullets, just at the leading edge of rifling impressions of gently captured round nose bullets). 

While common with PCPs, loading bullets partially or fully engraved into the rifling would eliminate skidding, it would not be practical with PBs for a number of reasons (with few exceptions, such as schutzen rifles:  https://www.pyramydair.com/blog/2015/06/ruger-model-3-32-40-schuetzen-rifle-part-1/ (https://www.pyramydair.com/blog/2015/06/ruger-model-3-32-40-schuetzen-rifle-part-1/) ).

Title: Re: How much jump before pellet hits the grooves?
Post by: Greg_E on November 09, 2021, 11:29:09 PM
Wish I could measure the stock length, it looked shorter than the .228 I had drilled for the TP in the old barrel, that makes me think it was closer to 3-4 degrees based on your drawings. But groove depth might be slightly less too, which will shorten the length. Wish I could get a ruler in there, even one marked in 0.010 inch would be good. Maybe I should buy a cheap one and grind it to a thin strip.

The .357 ammo is just barely scuffing the major diameter, and I'm getting around .350 to .352 for a minor diameter, its harder to measure with calipers.

I'll have to try get some dychem in there and get before and after images to see if we can determine the original angle.
Title: Re: How much jump before pellet hits the grooves?
Post by: subscriber on November 10, 2021, 12:13:36 AM
I wonder if you could lay a strip of paper with some reference marks in the breech end of the barrel. (ink dots or caliper "inside measurement beak point holes").  Then, with your bore scope, compare the rifling cone length to the marks on the paper (or whatever material is stable and convenient).

You could wrap tinfoil a few times around some plasticine (with closed ends) to make a slightly smaller than .357 "slug".  Insert that slug into the barrel breech so it can take an imprint of the rifling, including just ahead of the breech cone, after you apply light pressure from the muzzle with a cleaning rod (with a tissue blob to spread the load wider), while resisting the force at the breech end.

You will have to experiment with how hard to push to form an impression, while still being able to push the "slug" out without distorting it.  You might find that just a tinfoil slug randomly balled up provides the most stable impression.  Obviously that would take more foil than if the "core" were plasticine.
Title: Re: How much jump before pellet hits the grooves?
Post by: Nvreloader on November 10, 2021, 11:09:28 AM
Guys

How about making a chamber cast with Cerrosafe metal,
that way you can have a good solid cast to take measurements from etc.

https://www.ebay.com/itm/301891304226 (https://www.ebay.com/itm/301891304226)

Very easy to take a cast of a chamber.

HTH's
Don
Title: Re: How much jump before pellet hits the grooves?
Post by: Greg_E on November 10, 2021, 11:46:59 AM
Guys

How about making a chamber cast with Cerrosafe metal,
that way you can have a good solid cast to take measurements from etc.

https://www.ebay.com/itm/301891304226 (https://www.ebay.com/itm/301891304226)

Very easy to take a cast of a chamber.

HTH's
Don

That's the proper way to do it, we can't do things the proper way.  ;D I should probably order some so that all of us can have clear info for the next person.
Title: Re: How much jump before pellet hits the grooves?
Post by: subscriber on November 10, 2021, 02:21:39 PM
I got hung up on suggesting a proper chamber cast because the cast would get  stuck in the TP and breech O-ring groove; so did not suggest it.  The TP hole can be "masked" with a tinfoil wad or clay plug, but how to fill the O-ring groove stumped me.  Perhaps filling it with modelling clay flush with the surrounding metal can be done, and then cleaned up later.

The point being that you must not have stray clay anywhere in the chamber or leade where you want to take a measurable imprint.

Perhaps you can push or tap a bullet into the chamber until its base is just deeper than the front edge of the O-ring groove.  After filling the O-ring groove with clay, push or tap the bullet out with a cleaning rod from the muzzle.  The bullet should wipe off the excess clay in a direction where stray bits won't corrupt a chamber cast. 

While the bullet is "loaded" in this (shallow) position, fill the TP with clay too.  Wrap the outside of the barrel with some tape to prevent the clay in the TP from being pushed out.  Then push or tap the bullet back out slowly.  You don't want to get clay on the cleaning rod, that could be deposited in the leade where you want to take the cast...

The other reason for suggesting tinfoil is that there is a 99% chance you already have that.  While Cerrosafe is more likely to be ordered.  If Greg had Cerrosafe, we would not need to suggest it :) .

Another possibility it to bridge both the TP and O-ring groove with a plastic tube having an OD that is a slip fit into the breech.  Then pour your chamber cast trough the plastic tube so no Cerrosafe ends up near the TP or O-ring groove.  A paper or thin carboard tube might work too.

It is my nature to suggest approach that address the obvious problems, rather than, "this would be ideal, but here is why it won't work".  Of course, you could figure out how to overcome the above complications too, all by yourself.  My over-explanation is a reaction to an idea being dismissed because someone else had a problem.  Even though the failed attempt had constraints that do not apply to the person asking the question.  For instance; if the Bulldog has no breech O-ring groove, then the chamber cast sticking in that is not a consideration or limitation.


This thread, for instance suggests that shooting lead ball from an air rifle is going to produce disappointing results.  Yet, this is one of those, "it depends" situations that can be made to work:  https://www.gatewaytoairguns.org/GTA/index.php?topic=192997.msg156238014#msg156238014 (https://www.gatewaytoairguns.org/GTA/index.php?topic=192997.msg156238014#msg156238014)
Title: Re: How much jump before pellet hits the grooves?
Post by: Greg_E on November 10, 2021, 02:36:34 PM
I was thinking that I'd just plug the barrel an inch past the beginning of the rifling, and then pour in the "stuff" from the breach end and not fill past the TP. As long as the o ring has been removed, it should slide right out. Maybe I'll need to increase interest and buy some, never done anything like this so it might be a good bit of knowledge to have.

Is this alloy something I can make at home? I have some tin, I have some unknown lead alloy, and I have a ladle that I could heat with a torch. I also have a barrel sitting in a box that I can use for testing since it is not what I call serviceable. I'm also going to ream this test barrel first to make sure I know what I'm doing before moving on to the good barrel.
Title: Re: How much jump before pellet hits the grooves?
Post by: Greg_E on November 10, 2021, 02:39:44 PM
Looked it up, nope, can't make it at home.

Looks like you have about an hour to measure, but need to wait half an hour for it to expand. Strange stuff, think I need to buy some now to play with.

[edit] bought some from the ebay link, should be neat to mess with
Title: Re: How much jump before pellet hits the grooves?
Post by: subscriber on November 10, 2021, 02:50:32 PM
Cerrosafe is a very specific bismuth alloy that first shrinks after it cools (so you can get it out).  Then a specified amount of time after, it expands back to the real dimension for accurate measurement.

You can get all manner of low temperature bismuth casting alloys from amazon.  All made by rotometals (as is Cerrosafe).  So, if knowing the exact land and groove diameter is not important, then I think other alloys can be used.

Pouring in just enough casting metal to stop ahead of the TP is likely to be more challenging than you imagine.  You need a certain amount of heat in the cast, and that is somewhat connected to the cast volume.  Anyway, there is nothing to be lost by trying; providing you use a low melting point metal that can easily be melted out of the barrel.

If you go too deep into the rifling, then removing the cast will be difficult due to the friction.  If you can manage to stop the cast just in front of the TP (at the shallow end), then perhaps that problem goes away.

You want a thin oil film in the chamber area to be cast to ease extraction.  Perhaps you can heat the barrel with a torch to keep the casting metal molten and pour out the excess from the TP, so there is no way too get it jammed in there later.  Once the depth of material is where you want it, hold the barrel vertical and remove the heat source.  Wait until the barrel has cooled to below the temperature where the cast shrinks, and push it out from the front.

https://www.brownells.com/gunsmith-tools-supplies/barrel-tools/barrel-chamfering-accessories/cerrosafe-chamber-casting-alloy-prod384.aspx (https://www.brownells.com/gunsmith-tools-supplies/barrel-tools/barrel-chamfering-accessories/cerrosafe-chamber-casting-alloy-prod384.aspx)

Note, this alloy contains cadmium, so ingestion and breathing of fumes needs to be avoided.

How it works: https://www.brownells.com/guntech/cerrosafe-sup-sup-the-gunsmith-s-secret-weapon/detail.htm?lid=10667 (https://www.brownells.com/guntech/cerrosafe-sup-sup-the-gunsmith-s-secret-weapon/detail.htm?lid=10667)

How to measure a cast: https://www.brownells.com/guntech/cerrosafe/detail.htm?lid=10614 (https://www.brownells.com/guntech/cerrosafe/detail.htm?lid=10614)

Hints for use: https://www.brownells.com/guntech/hints-for-using-cerrosafe/detail.htm?lid=10387 (https://www.brownells.com/guntech/hints-for-using-cerrosafe/detail.htm?lid=10387)



Title: Re: How much jump before pellet hits the grooves?
Post by: Greg_E on November 10, 2021, 05:35:30 PM
Thanks for the tips, I'll read those later tonight.
Title: Re: How much jump before pellet hits the grooves?
Post by: subscriber on November 10, 2021, 08:42:34 PM
It helps to preheat the barrel to get the cast to fill out.  Considering that the casting metal melts at about 70 degrees C, the idea is not to make the barrel glow.   The barrel should be hot enough to burn you at the area you want to cast, but anything over boiling point of water is too hot.

An industrial hot air gun may be a better tool than a MAP gas torch for this.  So, use your judgement.

I just had a look at the instructions, and they advice a clean dry chamber.  My thinking is that as long as you don't scorch or boil the oil by heating the Cerrosafe over 200 C, having a very thin film of oil helps get the cast out.  I wiped the barrel with an almost dry patch  and it worked; perhaps because I was impatient and did not wait a full 30 minutes after casting to remove it.   One might watch people making chamfer casts on youtube and listen to those who make the best looking casts.


https://www.youtube.com/watch?v=KgRp3r9VPE0 (https://www.youtube.com/watch?v=KgRp3r9VPE0)


https://www.youtube.com/watch?v=2AbOhlZVwAU (https://www.youtube.com/watch?v=2AbOhlZVwAU)



To the point on a very long video:
https://youtu.be/c9stuhaYuj4?t=819 (https://youtu.be/c9stuhaYuj4?t=819)
Title: Re: How much jump before pellet hits the grooves?
Post by: lennyk on November 11, 2021, 11:57:15 AM
a bit off topic, but with the original fx smooth twist where only the last few inches of barrel was grooved, how does “jump” before engaging grooves work out in this case?
Title: Re: How much jump before pellet hits the grooves?
Post by: Nvreloader on November 11, 2021, 02:21:15 PM
For filling in the TP hole, to get a chamber cast,
used a turned alum/PET/Oak wood plug to fill the hole, to just inside the chamber walls,
pour in the Cerrosafe, let harden, pull plug from the TP hole, tap out the Cerrosafe cast.

Then see what you have..... ;).....LOL
And post a photo for our site.
Mine is coming.

ps,
Wood burns at 750* approximately, not sure on PET material.

Tia,
Don
Title: Re: How much jump before pellet hits the grooves?
Post by: subscriber on November 11, 2021, 04:16:19 PM
a bit off topic, but with the original fx smooth twist where only the last few inches of barrel was grooved, how does “jump” before engaging grooves work out in this case?

Lennard,

That is not off topic; it is spot on.  My overly detailed discussion might be taking it off topic...

The original FX smooth twist barrels had around 18" of "jump", depending on barrel length.  They shot well with specific JSB pellets, but not always with other kinds.  Pellets recovered from these barrels by shooting into water showed sever skidding in the rifling.  Slow motion video showed that pellets fired from these barrels rotated at only about 1:48" equivalent twist at the muzzle, despite the choke appearing to have a helix angle equivalent to 1:16.  So, a lot of slip going on - as one might expect due to the abruptness of encountering the lands.  Land shape was polygonal, so slip did not shave the pellet, as slip in traditional spline-type rifling might do. Pellet skidding evident here:  https://youtu.be/w2w1rrq-80s?t=119 (https://youtu.be/w2w1rrq-80s?t=119)

The smooth twist barrel had a reputation for long range accuracy when pellets were pushed over 900 FPS at the muzzle, because they did not over-spin pellets (which tends to result in spiraling at ranges over 50 yards). 

Waisted pellets are aerodynamically stable and rotating them just fast enough to null out anomalies in form provided the best long range results.  Here, the strategy was much like the slow twist used on arrow fletching; just enough rotation to average out the direction of a slightly bent shaft.

A higher twist rate favors pellet accuracy at 10 to 50 yards.  As waisted pellets came about to support low power smooth bore airguns, and rifling came later for short range competition, air rifles got a twist  rate that worked well for such use (also probably shared available rifling tooling used for rimfire rifles - hence how common 1:16 twist airgun barrels were for 100 years). 

Remember, that projectile rotational speed is function of barrel twist times muzzle velocity.  So, what appears to be a "fast twist barrel" at 900 FPS is not "too fast" at muzzle velocity of 550 FPS.

Back to "slugs".  They are much heavier than pellets (the point; to increase BC).  Thus slugs will be more prone to skidding in the lands after accelerating in long free-bore.  The original smooth twist FX barrels have the "ultimate" in free-bore and are not likely to do very well with slugs.  The counter point is that an 1/8" of free bore hardly seems "too much" by comparison.  Because, while slug acceleration will be high, its velocity at "impact" with the lands isn't.


FX smooth twist choke:
Title: Re: How much jump before pellet hits the grooves?
Post by: lennyk on November 11, 2021, 05:21:06 PM
good info, didnt realize those original barrels were well studied.

I would guess the jsb pellets which seem to use a softer lead would also be less affected by skidding.

Has this spiraling of those barrels used over 900fps been well documented ?
Title: Re: How much jump before pellet hits the grooves?
Post by: subscriber on November 11, 2021, 06:25:30 PM
Lennard,

FX used the soft JSB pellets to work with the rifled chokes.  I think the soft lead skids more, but without spoiling the aerodynamic shape.

I will send you a private message with videos that document pellets spiraling at ranges over 50 yards, when shot over 900 FPS at the muzzle from conventionally rifled air rifles.  Else, we are drifting off topic a bit too much :) .  There are many arguments about the exact cause on this forum, so I don't want to open that can of worms here.


EDIT:  Lennard,  Your PM inbox is full, so sent videos by email.
Title: Re: How much jump before pellet hits the grooves?
Post by: Nvreloader on November 11, 2021, 10:44:50 PM
Sub
A question for you,
I realize that you are talking/describing a big bore barrel/chambers etc,
Would a smaller caliber ie; 177 - 20 - 22 - and 25 cal barrels/chambers have the same length for the lands,
If using the same reamers as described, 2*degree and taper pin degree taper?

Someone, FIRR, maybe Bob?, had a chamber drawing for pellets?

Tia,
Don

The CAD models below show how deep the breech cone is for a 2 degree included angle, and for a standard taper pin reamer included angle of 1.194 degrees.   The other factor that directly affects the length of the cone is the rifling depth.  In this case, the land height (or groove depth) is 0.004". This is typical for calibers in the .357 to .45 range.  For PB and airguns.

As such, the fact that the model depicts a groove diameter of .452" does not detract from a discussion about a .357 caliber airgun barrel. The cylindrical section behind the cone is depicted as 0.001" larger than groove diameter.  The TP diameter is show as 70% of the groove diameter, with the TP placement somewhat arbitrary, based on the assumed length of the projectile and loading depth.

I happened to have a barrel design in .45 that I then modified to show the breech cone depth.  As a reference point, the breech cone spec for the .45 ACP pistol caliber has a 4.7 degree included angle at the start of the lands.  Very abrupt, compared to the typical 2 degrees, popular for PCP air rifles on this forum.  But then the pistol bullet has some jump, from the case to the lands.  Too shallow and angle would result in the bullet skidding excessively in the rifling.  (Slight skidding can normally be observed in fired bullets, just at the leading edge of rifling impressions of gently captured round nose bullets). 

While common with PCPs, loading bullets partially or fully engraved into the rifling would eliminate skidding, it would not be practical with PBs for a number of reasons (with few exceptions, such as schutzen rifles:  https://www.pyramydair.com/blog/2015/06/ruger-model-3-32-40-schuetzen-rifle-part-1/ (https://www.pyramydair.com/blog/2015/06/ruger-model-3-32-40-schuetzen-rifle-part-1/) ).


Title: Re: How much jump before pellet hits the grooves?
Post by: Spin on November 11, 2021, 11:49:32 PM
Agreed, I think the drill press will produce a better result.  Or do a hybrid…use the bench grinder/drill approach to remove the bulk of the material.  Then chuck the reamer into a drill press (or hand drill clamped to a surface) and spin it up, then slowly bring in a spinning emery wheel and just kiss the shoulder.  Be patient and continue to just barely let the wheel abrade the shoulder so it takes down only the high spots.  Eventually it will get to the point where it is touching all flutes equally for the full rotation.  Usually can tell by the way it sounds.  If uncertain, color the shoulder with a Sharpie (or blue layout dye) and spin it up again and briefly touch it with the emery wheel and stop to see if it removed the marking from all flutes.
Guys the spindle bearings on drill presses, moto tools, etc aren't meant to handle the side torque you're talking about applying. A Bridgeport or similar knee mill would be a better choice but even there slop from wear and inaccuracies in original machining can foul you up before you even get started. Take a precision ground pin and chuck it up in a Bridgeport collet some time and put a tenth or half thou indicator and rotate the spindle by hand. And grinding requires rigidity or that wheel or grinding drum will start bouncing around like Roy Rodgers on Trigger!
     Now clear things up a bit. Are you trying to put in a short free bored throat A La' Roy Weatherby effectively creating a very short smoothbore before the projectile engages the rifling, also was commonly done in large artillery pieces for quite a time. (sounds kind of like it and that does boost velocity) or are you trying to cut and grind in a shallow long angled throat that will gradually constrict down to meet the inside groove diameter?
     For skirted pellets the initial air blast and breech pressure assures maximum ID bore dia. seal. You might grind in a tiny angled lead in the face of the lands? Simple polishing will go a ways towards accomplishing that. Precision seating seems like it might serve you better and with less "metal wrecking" -( trade slang for Machinist - Metal Wrecker) some Carm magazines and single shot loaders employ internal O-rings that gently center the pellet or slug before final seating. Doesn't FX have internal
O=rings in their breeches that the probe pushes the round and itself through when it seats the projectile.
     It just seems to me you're going to great effort for something that may see minimal reward or go south and cost you for your efforts.
Title: Re: How much jump before pellet hits the grooves?
Post by: subscriber on November 11, 2021, 11:53:15 PM
Don,

The length of the ramp into the lands is independent of caliber.  Only the breech cone angle and land depth matter.  Because .177 and 22 barrels tend to have rifling that is half the depth of .357 and .45 barrels, this means their ramp lengths would be about half as long - assuming the same standard reamer angle was used.
Title: Re: How much jump before pellet hits the grooves?
Post by: Spin on November 12, 2021, 12:08:05 AM

Don,

The length of the ramp into the lands is independent of caliber.  Only the angle and land depth matter.  As such, .177 and 22 barrels tend to have rifling that is half the depth of .357 and .45 barrels.  This means the ramp length would be half as long - assuming the same standard reamer angle was used.
OK so if you only want to angle the faces of the lands use an oversize reamer and have it ground to the leading bore diameter minus .0006 to .0012.  It will serve as a guide just stone off their leading edges so they won't catch and dig or scar the bore. The angles front of the cutting flute will be the only cutting edges with back relief. Just grind them to whatever angle you want on the face of the lands. Do the actual cutting inside the barrel by hand, plenty of lube and stop and withdraw the reamer and blow out any chips and metal particulate frequently Layout dye and a good  gooseneck LED light. The centers should already be in both ends of the reamer. If they aren't buy a reamer that does have them. A tool grinder would be great but with the right tooling you could do a real precision job fabing just the kind of hand reamer you need.
Title: Re: How much jump before pellet hits the grooves?
Post by: subscriber on November 12, 2021, 12:40:31 AM
Jerry,

The focus of this thread is an optimal chamber shape for a .357 slug.  One that combines easy loading with peak muzzle energy and the tightest possible groups.  Pellet performance was discussed, but is assumed something that might need to be compromised to optimize slug performance.

Grinding the cutting edges off a taper reamer does not have to take a lot of force.  After all, while the reamer is hard, not much metal is removed.  The idea is to remove only a short section of cutting flutes behind the .358" section of the cone. 

To better illustrate what we are on about; I used Solidworks to modify a standard #7 taper pin 3D model I had downloaded from McMaster .  See attached images. 

If a less than rigid and controlled grinder is used then the relieved section may be cut as a reverse taper, rather than the cylinder shown in the first image below.  In that way the reverse taper section is just clearance for tool relief.  Thus the flutes in that relieved section are not sharp, and don't need to be.

I agree that a rigid setup for the grinder would make this easier; and reduce the risk of dulling the reamer flutes that form the cone up to the .358" section.

Title: Re: How much jump before pellet hits the grooves?
Post by: subscriber on November 12, 2021, 01:03:59 AM
Bob's DIY chamber reamers - click each link and scroll down for follow up posts by Bob in that thread: 
https://www.gatewaytoairguns.org/GTA/index.php?topic=132316.msg1313037#msg1313037 (https://www.gatewaytoairguns.org/GTA/index.php?topic=132316.msg1313037#msg1313037)
https://www.gatewaytoairguns.org/GTA/index.php?topic=99759.msg1192357#msg1192357 (https://www.gatewaytoairguns.org/GTA/index.php?topic=99759.msg1192357#msg1192357)
https://www.gatewaytoairguns.org/GTA/index.php?topic=95348.msg892334#msg892334 (https://www.gatewaytoairguns.org/GTA/index.php?topic=95348.msg892334#msg892334)
https://www.gatewaytoairguns.org/GTA/index.php?topic=99759.msg1192614#msg1192614 (https://www.gatewaytoairguns.org/GTA/index.php?topic=99759.msg1192614#msg1192614)

mackeral5's chamber reamers: 
https://www.gatewaytoairguns.org/GTA/index.php?topic=171866.msg155981211#msg155981211 (https://www.gatewaytoairguns.org/GTA/index.php?topic=171866.msg155981211#msg155981211)
https://www.gatewaytoairguns.org/GTA/index.php?topic=181490.msg156064772#msg156064772 (https://www.gatewaytoairguns.org/GTA/index.php?topic=181490.msg156064772#msg156064772)



For light relief;  a chamber in need of serious help (with a dull rough looking bore)

"These are not the lands you are looking for":

(https://i.imgur.com/IJI6Xa0.jpg)


Title: Re: How much jump before pellet hits the grooves?
Post by: subscriber on November 12, 2021, 01:08:38 AM
Jerry,

Are you suggesting modifying a standard diameter reamer by grinding in tapered cutting flutes at the front end?  Modifying a standard taper pin reamer to stop cutting at a given diameter seems easier...

While a pilot can help with alignment and prevent chatter (like Bob's DIY reamers), a badly made pilot will score the land tops.  At low speeds, by hand, even a pilotless modified taper pin reamer works - see makeral5's post links in the above message.
Title: Re: How much jump before pellet hits the grooves?
Post by: nervoustrigger on November 12, 2021, 02:11:11 AM
Guys the spindle bearings on drill presses, moto tools, etc aren't meant to handle the side torque you're talking about applying. A Bridgeport or similar knee mill would be a better choice but even there slop from wear and inaccuracies in original machining can foul you up before you even get started. Take a precision ground pin and chuck it up in a Bridgeport collet some time and put a tenth or half thou indicator and rotate the spindle by hand. And grinding requires rigidity or that wheel or grinding drum will start bouncing around like Roy Rodgers on Trigger!

A mill with near-zero runout would be nice but not at all necessary for the operation we were talking about.  This is just knocking down the flutes at the fat end of the tapered reamer.  The part of the reamer that will be doing the actual cutting of the leade is still preserved in its OEM form.  And on top that, the cleanup pass of "just kiss the shoulder" with a light touch from a spinning emery wheel...that means there is practically zero side load.  Really, a drill press is perfectly acceptable for this operation.
Title: Re: How much jump before pellet hits the grooves?
Post by: rkr on November 12, 2021, 03:41:25 AM
You know, if you made a thimble for the actual "chamber" part - you wouldn't have to modify the reamer. Just push in the reamer until the lands are removed at the very end of the barrel and then attach it to a thimble.
Title: Re: How much jump before pellet hits the grooves?
Post by: subscriber on November 12, 2021, 04:16:16 AM
I very much like the thimble approach.  The question is if the Benjamin Bulldog can be adapted with a thimble.  That is for Greg to figure out.

You know, if you made a thimble for the actual "chamber" part - you wouldn't have to modify the reamer. Just push in the reamer until the lands are removed at the very end of the barrel and then attach it to a thimble.
Title: Re: How much jump before pellet hits the grooves?
Post by: lennyk on November 12, 2021, 08:13:52 AM
FX uses a brass section which has the transfer port which is then attached to the barrel.
Do they do the ramp transition in that brass piece ?
Title: Re: How much jump before pellet hits the grooves?
Post by: rkr on November 12, 2021, 08:45:07 AM
FX uses a brass section which has the transfer port which is then attached to the barrel.
Do they do the ramp transition in that brass piece ?


It's just a straight thimble with a barrel port they use. Leade in starts right at the beginning of the liner.
Title: Re: How much jump before pellet hits the grooves?
Post by: Greg_E on November 12, 2021, 10:27:01 AM
One of the reamers arrived yesterday and I started looking at the work ahead...

I want to grind the tip a little to install a brass tube to help center the small end.

Then there is a lot of stuff that may need to come off the big end to get it to slip inside the chamber. Need to pull an o ring and do some measuring. I may wait to do the grinding until the chamber cast material arrives and I can really see (and measure) what I need. While I do have a junk barrel, I still want to be successful with the first and second try.

I have a straight flute on the way, and a spiral flute in hand. Bought the spiral after the first reamer was shipped as the wrong part, still waiting for a replacement and hedged my bets and bought the spiral flute.

Both of these are HSS, would a carbide end mill cut these well enough that they would work after the cutting? I may set them up on my little mill with rotary table and tail stock to be more precise. Need to check my mills but I think I have a couple of solid carbide that should be able to cut this tool steel. Grinding without a proper grinding machine will be haphazard at best, no matter the amount of care given. Since I couldn't find any small (and decent) cup wheel stone or diamond tools to put in my little mill, a carbide might be best. I'll fall back on a bench grinder and taking a bunch of time, but not my first choice.
Title: Re: How much jump before pellet hits the grooves?
Post by: sb327 on November 12, 2021, 10:42:22 AM
Your not going to be cutting hss with carbide tooling.

Can you make up a makeshift toolpost grinder from a dremel? (Can’t remember if you have a lathe)

Dave
Title: Re: How much jump before pellet hits the grooves?
Post by: JuryRigger on November 12, 2021, 11:18:18 AM
Agreed, I think the drill press will produce a better result.  Or do a hybrid…use the bench grinder/drill approach to remove the bulk of the material.  Then chuck the reamer into a drill press (or hand drill clamped to a surface) and spin it up, then slowly bring in a spinning emery wheel and just kiss the shoulder.  Be patient and continue to just barely let the wheel abrade the shoulder so it takes down only the high spots.  Eventually it will get to the point where it is touching all flutes equally for the full rotation.  Usually can tell by the way it sounds.  If uncertain, color the shoulder with a Sharpie (or blue layout dye) and spin it up again and briefly touch it with the emery wheel and stop to see if it removed the marking from all flutes.
Guys the spindle bearings on drill presses, moto tools, etc aren't meant to handle the side torque you're talking about applying. A Bridgeport or similar knee mill would be a better choice but even there slop from wear and inaccuracies in original machining can foul you up before you even get started. Take a precision ground pin and chuck it up in a Bridgeport collet some time and put a tenth or half thou indicator and rotate the spindle by hand. And grinding requires rigidity or that wheel or grinding drum will start bouncing around like Roy Rodgers on Trigger!
     Now clear things up a bit. Are you trying to put in a short free bored throat A La' Roy Weatherby effectively creating a very short smoothbore before the projectile engages the rifling, also was commonly done in large artillery pieces for quite a time. (sounds kind of like it and that does boost velocity) or are you trying to cut and grind in a shallow long angled throat that will gradually constrict down to meet the inside groove diameter?
     For skirted pellets the initial air blast and breech pressure assures maximum ID bore dia. seal. You might grind in a tiny angled lead in the face of the lands? Simple polishing will go a ways towards accomplishing that. Precision seating seems like it might serve you better and with less "metal wrecking" -( trade slang for Machinist - Metal Wrecker) some Carm magazines and single shot loaders employ internal O-rings that gently center the pellet or slug before final seating. Doesn't FX have internal
O=rings in their breeches that the probe pushes the round and itself through when it seats the projectile.
     It just seems to me you're going to great effort for something that may see minimal reward or go south and cost you for your efforts.
While you're absolutely right concerning side loading on drill press spindles; I can tell you from experience that a passable (IE not exactly precision; but workable and certainly better that could be achieved by hand) result can be had... the key is in going SLOOOOOOOOOOOOOOWWWWW... It's all about the concept of sparkout; and the highest points of rotation... After all; we're only after getting some material out of the way; and keep the 'top end' of the taper to cut the throat at least fairly concentric/perpendicular...
I've done some grinding in a Bridgeport with a 1/4" die grinder in the vise BTW  ;)
Jesse
Title: Re: How much jump before pellet hits the grooves?
Post by: Greg_E on November 12, 2021, 03:25:13 PM
I don't have a lathe, but I might be able to rig something on my mill. Put the reamer in a collet and a rotary tool on the table. I'll have to look at that as an option since it would be far more accurate than a bench grinder. Height is my biggest issue here, only have a little desktop micro mill, but I've never let it stop me from doing projects bigger than it can handle.
Title: Re: How much jump before pellet hits the grooves?
Post by: subscriber on November 12, 2021, 04:01:08 PM
Greg,

I have removed a broken HSS steel tap from a threaded hole by drilling it out with a tungsten carbide endmill.  If you do this, the carbide endmill should be considered consumable - an overhead cost.

I have cut HSS and tungsten carbide with diamond coated disc, cylinders, burrs, files and laps.  You are not cutting cemented tungsten carbide, but if you did, you should avoid breathing the dust.  The cobalt binder is very bad for your lungs.  High cobalt HSS might carry a similar risk. 

Diamond coated burrs, discs and files are not expensive:
https://www.mcmaster.com/4490A66/ (https://www.mcmaster.com/4490A66/)
www.amazon.com/Diamond-Cutting-Coated-Mandrel-Dremel/dp/B07D9L8MNN (http://www.amazon.com/Diamond-Cutting-Coated-Mandrel-Dremel/dp/B07D9L8MNN)
www.amazon.com/gp/product/B01DFO18BC (http://www.amazon.com/gp/product/B01DFO18BC)
www.amazon.com/gp/product/B002RL83DQ/ (http://www.amazon.com/gp/product/B002RL83DQ/)


I made a "toolpost grinder" out of wood that I gripped in my mill vice, to hold a Proxon grinder (fancy Dremel).  The HSS part was in the mill spindle collet, while the Proxon provided the speed for the diamond grit to work. 

The images below show a taper reamer being relief ground.  The first image has the Proxon axis perpendicular to the mill spindle as an initial setup.  The next two images show the Proxon set to provide the desired relief angle.

Title: Re: How much jump before pellet hits the grooves?
Post by: Greg_E on November 12, 2021, 05:45:05 PM
I was looking at my 123 blocks and figuring out how to clamp the rotary tool hand piece to them, very similar to what you did. Going to stick some round stock through the holes, drill and tap for a clamping screw, and go for broke. Just need to measure the hand piece when I get home to make sure I can do everything I want. An angle plate would be nice, but don't have one.

I do have some cheap HarborFreight diamond tools, they might work with light pressure. I think I need to take around 0.070 inch off the big end which might be a lot of travel.
Title: Re: How much jump before pellet hits the grooves?
Post by: subscriber on November 12, 2021, 06:15:08 PM
Greg,

What you describe is going to be easier than you imagine.  Even if it takes most of an hour to set it up carefully. 

All you need is a clearance angle.  You can eyeball it.  A goal of the same taper angle as the reamer, but tilted the other way is achievable by eye.

Grinding relief into the reamer is going to take 10 minutes, tops. The hardest part is figuring out where to stop in order to preserve the cutting flutes at the desired diameter.
Title: Re: How much jump before pellet hits the grooves?
Post by: Greg_E on November 12, 2021, 09:56:43 PM
I just need to knock the flutes down, the tip is around .320 and the big end .4xx so I don't need to worry at all about angles. If I didn't want to try and put some kind of pilot on the small end, I'd only need to reduce the big end to fit.
Title: Re: How much jump before pellet hits the grooves?
Post by: subscriber on November 12, 2021, 11:28:37 PM
Grinding a pilot onto the reamer nose is not trivial.  You should know the land diameter of the barrel to make the pilot very slightly smaller.  The pilot needs to be very smooth and very round, else it may do more harm than good.

What the pilot does is reduce the propensity for the reamer to chatter against the lands; even when the chambering job is run in a lathe.  It also help the reamer "cut to true size" when used in a lathe.   

An unpiloted chambering reamer must only be turned by hand or it is guaranteed to chatter.  Using a left hand spiral flute reamer in a righthand twist barrel will make the job much easier.  The same direction of flute spiral as the rifling, at the same "twist", with the same number of flutes as rifling grooves is the worst for catching on every land...

I know I posted a bunch of threads earlier.  The one by makeral5 shows what a modified unpiloted reamer can do.  Note that the finish obtained with that reamer is before any kind of polishing, after chambering.

Bob Sterne makes piloted chamber reamers and they well work for him.  I suspect he offer many pointers on making them and the pros and cons.  It may be that a D-section reamer must have a pilot to support the cutting loads; while a multi-flute reamer can support its cutting loads between flutes; even without a pilot.
Title: Re: How much jump before pellet hits the grooves?
Post by: rsterne on November 13, 2021, 12:30:24 AM
Here is a photo of one of my custom chamber reamers.... and no, I do not make them for sale....  ;)

(https://hosting.photobucket.com/albums/oo221/rsterne/6mm Sporter/.highres/Tempered Reamer_zps2bub7oat.jpg?width=1920&height=1080&fit=bounds) (https://app.photobucket.com/u/rsterne/a/2f8e7dd4-f82d-4776-a4ea-72f8a8d34c0a/p/709cee97-5f3a-4e1a-b494-c02f9d70deb2)

The pilot portion is a thou or two smaller than the land diameter and the tapered cutting flutes start smaller than the lands and taper up to the chamber diameter.... Behind that point the diameter is constant.... Here is a drawing of a .22 cal version....

(https://hosting.photobucket.com/albums/oo221/rsterne/22 cal Disco Double/.highres/Reamer Side View_zpsiiqcmig8.jpg?width=1920&height=1080&fit=bounds) (https://app.photobucket.com/u/rsterne/a/d8768b6a-4979-4d94-aef4-f723fe8fbdb2/p/df530402-a554-43f6-9cbf-74a0df876f94)

The step down behind the pilot is about 0.010" to where the flutes start.... Here is a generic drawing with the dimensions of the flutes in calibers....

(https://hosting.photobucket.com/albums/oo221/rsterne/22 cal Disco Double/.highres/Reamer_zpsywtq5w0p.jpg?width=1920&height=1080&fit=bounds) (https://app.photobucket.com/u/rsterne/a/d8768b6a-4979-4d94-aef4-f723fe8fbdb2/p/41dc9d7e-b591-4166-a143-99b9fa90105b)

The dotted blue lines are for a single flute, the solid blue lines for a three flute.... The depth that you plunge the mill past the centerline provides an angle to the cutting edge.... The deeper it is, the faster the reamer cuts, while the closer it is to the centerline the finer the cut and the smoother the finish.... I use 5% of the caliber, as shown in the drawing....

I have not used a modified pin reamer, but others have, and very successfully.... I understand that the flutes are simply ground away where the taper reaches the desired chamber diameter, and do not touch the barrel at all behind the tapered part.... As such, that could be done with a hand grinder....

Bob
Title: Re: How much jump before pellet hits the grooves?
Post by: subscriber on November 13, 2021, 01:53:10 AM
Thank you, Bob
Title: Re: How much jump before pellet hits the grooves?
Post by: lennyk on November 13, 2021, 01:05:48 PM
Will a straight flute be much harder to use than a spiral flute due to the "biting/chatter" ?
Title: Re: How much jump before pellet hits the grooves?
Post by: Greg_E on November 13, 2021, 01:34:20 PM
Will a straight flute be much harder to use than a spiral flute due to the "biting/chatter" ?

That's a good question and one that I can't answer yet. And since the company that shipped the wrong reamer still hasn't got the correct one to me, I'm glad I bought a second. I have a spiral flute in hand and will start making my grinding rig next week. That will give me time to cast the chamber and double check sizes.
Title: Re: How much jump before pellet hits the grooves?
Post by: nervoustrigger on November 13, 2021, 01:55:34 PM
Will a straight flute be much harder to use than a spiral flute due to the "biting/chatter" ?

If you intend to do the operation by hand, a left-hand spiral reamer will help immensely because it resists self-feeding.

The number of flutes will also have a big influence on how it behaves.  For example, the worst case would be a reamer with exactly the same number of flutes as there are lands in the barrel.  That sets up a repeated grab then release.

If instead you can use a suitably rigid setup in a lathe, it's not nearly so much of a problem.  Feed really slowly and don't let the flutes clog up.
Title: Re: How much jump before pellet hits the grooves?
Post by: rsterne on November 13, 2021, 02:31:26 PM
I run my lathe at about 50 RPM and feed VERY slowly, removing the reamer every 0.1" of feed depth to blow off the chips and relube it with cutting (tapping) oil.... The chips are so fine they are basically dust....

(https://hosting.photobucket.com/albums/oo221/rsterne/Parts for Sale/.highres/Cutting the 224 Chamber_zpsauomqj0b.jpg?width=1920&height=1080&fit=bounds) (https://app.photobucket.com/u/rsterne/a/ec7c3593-4578-4151-a54b-04e884d967df/p/bd75f163-4025-4d0c-af4a-7f6f0513b55f)

Photo above showing reamer withdrawn for cleaning/lubing.... Note that all the cutting occurs in the tapered middle of the flute length, the front is smaller than the lands, and the back is the parallel chamber section.... You can click on the photo to enlarge it.... Here is a photo of a reamer after use.... You can see the polished area in the middle where the cutting occurs....

(https://hosting.photobucket.com/albums/oo221/rsterne/22 cal Disco Double/.highres/Reeamer After Use_zpsgc1frer6.jpg?width=1920&height=1080&fit=bounds) (https://app.photobucket.com/u/rsterne/a/d8768b6a-4979-4d94-aef4-f723fe8fbdb2/p/2f6d5ff5-069c-4cba-bce2-274591272baa)

Bob
Title: Re: How much jump before pellet hits the grooves?
Post by: subscriber on November 13, 2021, 06:55:56 PM
Having a different number of reamer flutes to the number of lands in the barrel helps break the resonance, when reaming using a piloted reamer.  However, if the reamer is unpiloted, then the cutting force can spring the front end of the reamer and set up a vibration that may be worse than with a matched flute and land count (even if the latter has no pilot).  This deflection is worse with small calibers because the reamers are much more flexible.

Some reamer manufacturers space their cutting flutes slightly unevenly to break chatter producing resonance.   Whereas the lands in a 6 groove barrel should all be spaced evenly.

A lefthand spiral flute, right hand cut reamer is preferred if you can get one in your size, because they do not "screw in" when cutting (as noted above), and produce the smoothest, most controllable cutting action.  Righthand spiral flute reamers cut aggressively at low axial force, thus do not produce as smooth a finish as a lefthand spiral flute reamers.  A straight flute reamer is most prone to chatter, or cutting lobed holes, but can work fine when the user knows how to prevent chatter.  This starts by turning the reamer very slowly and feeding it gently and smoothly.


Getting back to Greg's reamer grinding; to create a pilot on a standard taper pin reamer will require adding metal to the reamer nose, not just grinding it away:  If you look at Bob's reamers, the pilot matches land diameter (minus 0.001"), then the tapering section behind the pilot is actually smaller than land diameter.  Some distance away, the taper in the reamer gently meets land diameter, and cutting starts.

How to convert a taper pin reamer to a piloted reamer then?  Easy:   Grind a sub-land diameter cylindrical section on the reamer nose, to look like an undersize pilot.  Then add what looks like a short section of brass tube on that HHS mini-pilot tp form the actual pilot. 

The brass pilot OD is sized 0.001" below land diameter to guide the reamer.  It is not essential that the pilot be free to rotate on the reamer nose, because then you need to be devise a means to prevent the brass pilot from falling off the end of the reamer, each time you remove the reamer from the barrel to clear chips.  The brass pilot can be a light friction fit to the sub-pilot on the reamer nose.

Rather than use 10,000 words to describe such a piloted reamer conversion, I figured pictures might help - attached below.   For reference, the sub-pilot diameter and length given are arbitrary, but will work in practice.   Likewise, the relief angle grind beyond the max .358" diameter section of the reamer is 1 degree per side, with an arbitrary but reasonable clearance length.


Images below start by comparing the modified taper pin reamer next to the stock one:
Title: Re: How much jump before pellet hits the grooves?
Post by: Greg_E on November 13, 2021, 08:42:47 PM
That's exactly what I'm going to do to make a pilot. I could only get stock sizes of tubing, so around .340 diameter which still leaves a bit of room to move around. To minimize this I'll extend the length of tube and might even try to flare the end a bit. A bit of epoxy should hold the tube in place.
Title: Re: How much jump before pellet hits the grooves?
Post by: Greg_E on November 13, 2021, 08:45:12 PM
By the way, those drawings are great!
Title: Re: How much jump before pellet hits the grooves?
Post by: subscriber on November 13, 2021, 08:57:49 PM
You could use you mill as a lathe, after attaching the section of brass tubing to the ground down reamer nose.  This 3/8" diameter brass tube could work for that:  https://www.mcmaster.com/7782T431/ (https://www.mcmaster.com/7782T431/)

I would not have a long section of tube hang off a short "boss" on the reamer nose.  The chances of angular error between the tube and reamer are significant.  Also, on drawn tubing intended for fluid flow, the ID and OD may not be concentric to 0.001".  The diameters, unless measured are nominal for any given tubing; and may be larger or smaller - also out of round....

You really need the reamer pilot OD to be close to land diameter for it to offer any useful support. 

One aspect that you need to evaluate is if simply gripping the reamer stem in your mill collet has the reamer run true; in terms of run-out and angularity (within reason).   If the reamer does not run true, first regrip it in the collet with a 90 (and then 180 and 270) degree rotation relative to the collet, to see if errors are stacking, or can cancel out.  Place the reamer stem as deep as it can go into the collet, without actually stopping on the collet holder.

Else, you may need to use the center dimple on the end of your reamer to force the front end of the reamer to run true, while spinning the reamer vertically in the mill.  If you have to support a wobbling reamer via a center attached to the table, then you will need to use a hand file to "turn" the pilot OD down. 

If the reamer runs true without supporting the small end, then use the mill vice to hold a sharp lathe tool, to turn the brass pilot to a known size, reasonably coaxial to the section of reamer flutes that will do the chamber cutting. 

You can use a single flute of a stationery endmill ideally positioned, and clamped in a vice (by its shank) as a lathe tool.  This assumes you have no lathe tools such as a boring bar - something that can also be used in a mill to bore holes.  If you have a broken endmill you can grind that into a lathe tool to cut brass.

You could also use your Dremel with the diamond wheel / cutter to abrade the brass pilot just like you plan to for the reamer.  This should be OK for a small amount of brass removal, as long as diamond grit does not shed off the wheel and imbed in the brass pilot OD.
Title: Re: How much jump before pellet hits the grooves?
Post by: subscriber on November 13, 2021, 09:01:22 PM
By the way, those drawings are great!

Thanks.  It helps to start with a free 3D model of a reamer :)

I can think in 3D, but without real scale, can easily imagine the equivalent of an Escher sketch, where anything seems possible.  Seeing a scale correct representation helps me think more clearly, and I enjoy creating that to help others.
Title: Re: How much jump before pellet hits the grooves?
Post by: Greg_E on November 13, 2021, 11:42:46 PM
All of the brass tube I found was 0.014 wall thickness so the next larger size is too thin. But I'll look again because I had to buy a pack of many sizes, price was the same as one piece of the .0340 tube.

Maybe I can solder a larger tube to the 0.340 tube and cut it down as you mention. I have a few dull or broken tools that I saved that I can grind into other things.

Could I silver solder the 0.340 tube to increase diameter and then cut it down? Or is silver solder approaching the "too hard" range? Talking pipe silver brite type solder, but I also have electronics silver solder too.

My thinking with a long but undersize pilot is that the drift is an angular issue. A long pilot would reduce the drift angle at the cutting location by acting at the far end. It does assume a straight tube, which might be a big assumption. Also assumes the tube will not bend. But even something 0.010 undersize is better than the 0.030 undersize of the cutter, and also prevents unwanted damage from the end of that cutter swinging over into the barrel.

Maybe I should rethink drilling a hole in some Delrin and cutting it down to proper size for a pilot. The tubing seemed like a quicker way to get finished. Or a piece of hardwood, I have some 3/8 oak dowel that would be easy to work with and for one or two uses it would probably be fine. Just need to grind down about the first quarter inch from the end of the reamer to make a spot to fit.
Title: Re: How much jump before pellet hits the grooves?
Post by: Greg_E on November 14, 2021, 12:21:11 AM
I just measured the brass tube again, it is a telescopic set of tubes and fits most of the way down the next size up with zero binding, so they must be straight. Size is 0.345 inches or somewhere in the 0.005 to 0.007 undersize. The error that I might get from this amount of difference probably isn't enough to worry about. Need to hit 0.315 on the small end grind to install it and epoxy in place, the reamer starts at 0.322 so not a lot to remove making the job easier. If I get to 0.317 I can probably heat the tube to shrink it in place, and make up another 0.002 in the process.

If I can get a good fit on the chamber end, I think errors will be small enough to be fine.
Title: Re: How much jump before pellet hits the grooves?
Post by: subscriber on November 14, 2021, 12:31:53 AM
Bright silver solder is 90+ % tin.  So, not too hard.  Not sure you can "build up" a brass tube with it by more than perhaps 0.004" in diameter.  Maybe not that much.

I think you want the reamer to find its own center on the conical cutting flutes, with the pilot used to help the other end center in the bore, and prevent any rocking moments.   The drive end should be allowed to float, by turning it with a symmetrical handle by hand; concentrating on not applying unbalanced side loads.

You may be able to make a long pilot, but my instinct says that it is not going to allow the reamer flutes to act as the other center of rotation.

If you have Delrin and can use the mill as a lathe, that might be even better pilot material than brass - as long as you don't drop the reamer on a concrete floor and imbed grit in the Delrin.

My suggestion is to study chamber reamers with pilots.  Pilot lengths seems to fall into a narrow range.  It seems to me that they vary in length from 1/4" to 3/8"; regardless of caliber.  Bob may have more insight into this, but I almost feel like a barrel shaped pilot would be perfect.  It would stabilize the front end of the reamer concentric with the bore, then allow the rear end of the reamer to self align in the cone it cuts, based on the average position of the lands and grooves.  That way, all of the rotational torque you feel will be from cutting, rather than from friction due to the pilot applying angular moments to the reamer.  A counter argument is that free spinning pilots exist for this exact reason.
Title: Re: How much jump before pellet hits the grooves?
Post by: subscriber on November 14, 2021, 12:44:17 AM
I suspect there are many ways to make this work.  You could solder the tubes together if you need to increase the diameter. 

Heat shrinking the tube on should work.  If the tiny thermal mass of the tube is enough to get it on before it cools.   If it cools before it is home, I think you can tap it the rest of the way: 

You could also leave the reamer "boss" 0.002" larger than the brass tube ID; then tap the tube with a very light mallet (or screw driver handle) to seat it on the reamer nose.  The friction will hold it in place and swell the tube OD by about 0.002".  If driving the tube on, break the sharp front corners on the pilot "boss".  I showed them sharp in the 3D model, but if you are using imbedded diamond cutters, you could break those edges to make getting the brass tube on easier. 

The fact that a lot of the original reamer flutes are still present need not matter - unless the tube stretched over it is lobed by more than 0.001" - or enough to ride up and down on the lands in an obvious manner.  You might argue that a looser fit held with epoxy avoids making the brass tube lobed.  I would concede that.
Title: Re: How much jump before pellet hits the grooves?
Post by: Spin on November 14, 2021, 07:39:49 PM
I don't have a lathe, but I might be able to rig something on my mill. Put the reamer in a collet and a rotary tool on the table. I'll have to look at that as an option since it would be far more accurate than a bench grinder. Height is my biggest issue here, only have a little desktop micro mill, but I've never let it stop me from doing projects bigger than it can handle.

LMBO  You work in enough job shops and doing too big jobs on too little equipment becomes second nature. You know how it goes,"we expect you to walk on water and do the impossible and we want it done yesterday!"
Here is a photo of one of my custom chamber reamers.... and no, I do not make them for sale....  ;)

(https://hosting.photobucket.com/albums/oo221/rsterne/6mm Sporter/.highres/Tempered Reamer_zps2bub7oat.jpg?width=1920&height=1080&fit=bounds) (https://app.photobucket.com/u/rsterne/a/2f8e7dd4-f82d-4776-a4ea-72f8a8d34c0a/p/709cee97-5f3a-4e1a-b494-c02f9d70deb2)

The pilot portion is a thou or two smaller than the land diameter and the tapered cutting flutes start smaller than the lands and taper up to the chamber diameter.... Behind that point the diameter is constant.... Here is a drawing of a .22 cal version....

(https://hosting.photobucket.com/albums/oo221/rsterne/22 cal Disco Double/.highres/Reamer Side View_zpsiiqcmig8.jpg?width=1920&height=1080&fit=bounds) (https://app.photobucket.com/u/rsterne/a/d8768b6a-4979-4d94-aef4-f723fe8fbdb2/p/df530402-a554-43f6-9cbf-74a0df876f94)

The step down behind the pilot is about 0.010" to where the flutes start.... Here is a generic drawing with the dimensions of the flutes in calibers....

(https://hosting.photobucket.com/albums/oo221/rsterne/22 cal Disco Double/.highres/Reamer_zpsywtq5w0p.jpg?width=1920&height=1080&fit=bounds) (https://app.photobucket.com/u/rsterne/a/d8768b6a-4979-4d94-aef4-f723fe8fbdb2/p/41dc9d7e-b591-4166-a143-99b9fa90105b)

The dotted blue lines are for a single flute, the solid blue lines for a three flute.... The depth that you plunge the mill past the centerline provides an angle to the cutting edge.... The deeper it is, the faster the reamer cuts, while the closer it is to the centerline the finer the cut and the smoother the finish.... I use 5% of the caliber, as shown in the drawing....

I have not used a modified pin reamer, but others have, and very successfully.... I understand that the flutes are simply ground away where the taper reaches the desired chamber diameter, and do not touch the barrel at all behind the tapered part.... As such, that could be done with a hand grinder....

Bob
  Very smart, in effect you've created a single flute, front piloted gun drill, That and having the leading angle going below bore size to eliminating shoulder trouble and insuring a clean edge. Did you rotate the tool under power or slowly by hand? Sometimes hand rotation prevents a cutting edge from biting or chatter .
Title: Re: How much jump before pellet hits the grooves?
Post by: subscriber on November 14, 2021, 07:53:23 PM
Did you rotate the tool under power or slowly by hand? Sometimes hand rotation prevents a cutting edge from biting or chatter .

I believe Bob already clarified that, quoted below for convenience:

I run my lathe at about 50 RPM and feed VERY slowly....
Title: Re: How much jump before pellet hits the grooves?
Post by: Greg_E on November 15, 2021, 04:46:12 PM
Couldn't get it to fit in a collet, I only have little MT2 collets in end mill sizes. The shank is 0.410 which doesn't fit anything I have. So I fell back on putting it in my rotary table and 3 jaw chuck. After much messing around, my DTI reads total run out of around 0.0005 to 0.0007 on the big end, and closer to 0.001 on the small end. Small end is difficult since I'm measuring on the cutting edges and jumping off and back on the flutes could introduce errors.

I'll have to put the diamond grinding wheels in an 1/8 collet and hope for the best.

This reamer is the longest object I can do this way, the little table is out of room for clamps!
Title: Re: How much jump before pellet hits the grooves?
Post by: subscriber on November 15, 2021, 06:31:40 PM
Greg,

That looks OK.  0.001" total runout at the small end means the reamer will cut more on some flutes than others.  While less than ideal, remember that a single flute reamer works.  You just have to proceed more slowly. 

The eccentricity also means that the tapering reamed hole might end up 0.001" larger in diameter than the reamer measures at the corresponding distance from the pilot.  This wider cutting effect would be biased towards the pilot, and diminish towards the wide end of the reamer.   The overall effect is to cut a slightly shallower angle than the taper pin reamer has from the factory.   No big deal.   

Why my assumptions stated above?  The pilot to max cutting OD distance is a little over an inch - image below.  This is based on my assumed pilot dimensions; rather than yours.   

My advice is to break the front and rear corners of the pilot OD at a shallow angle or radius, so that the pilot does not tend to bind against the lands, due to the slight angularity the unavoidable eccentricity might induce.  The image below shows a tangent radius at the pilot OD, to help the pilot slip into the bore and follow the cutting action, without hindering either axial or rotary motion.

I think your setup will work fine.  The important question is what is this barrel's groove diameter at the breech?  Determining that with the Cerrosafe, before you grind the relief at max functional reamer OD is required.  Knowing the land diameter will let you make an appropriate pilot OD.

From the bad barrel; you already have some idea if a .358" cylindrical cut will clean up the lands and leave a chamber that the projectile diameter you intend to shoot loads into easily, yet without obvious slop.  Part of this question is if you plan to size projectiles to exactly groove diameter; a little over, or a little under.   

If the parallel chamber ends up 0.002" larger than the projectile, instead of 0.001", is there any real loss?  Somehow I think that the long taper into the lands after the chamber will center the projectile either way.  Also, a diametral clearance of 0.002" on .357 chamber is less significant, than on a .177.  Partly because of the absolute diameter; and partly due to the longer projectile bearing length in .357.

You no doubt will polish the chamber and tapering lead after reaming.  That will open up the parallel chamber by perhaps 0.001" to clean up the finish.  How to polish the chamber is easy, but also easy to mess up.  So will require some thought and some planning - if you have not done it before.

Title: Re: How much jump before pellet hits the grooves?
Post by: Greg_E on November 15, 2021, 07:54:02 PM
Once again, your diagram provides great info and cements what I thought I was seeing.

The Cerro safe arrived today so I'll have to get messing with it tonight. If it goes well in the junk barrel, I'll pull the good one and measure it too.

The straight reamer arrived today too, I'm thinking I may want to try grinding it first because I think it may chatter, 6 flutes against 6 barrel grooves. The reamer above is 7 flutes so it should chatter less and I'll want to do my best job on it. Also harder to measure because I don't have opposed parts to measure across like I do on the straight flutes.
Title: Re: How much jump before pellet hits the grooves?
Post by: subscriber on November 15, 2021, 08:30:36 PM
Greg,

One way to locate a specific diameter on an odd flute taper reamer is to drop "washers" onto it.  If you measure the washer ID first and see where it settles on the reamer that should provide useful info.

If extrapolating from the reamer angle to find the location for the .358" diameter is too difficult and inaccurate, then ream out the washer with the reamer first.  Measure the tightest diameter in the washer to verify it is .358", then drop the washer onto the reamer to mark the start of the relief taper.  This reaming and measuring may require a few tries; and perhaps more than one washer...

All you need are some brass or copper washers, although mild steel is hardly going to hurt the reamer.  You can leave the pre-sized washer on the reamer while you relief grind the reamer; then use the washer as a comparator (after wiping off the bulk of the dust). 

You can even have two (or more washers) at .359 and .357 as GO and NOGO gauges.  The GO needs to be able to pass the max OD at the end of the relief grind; while the NOGO should not.  By also having a .360" sized washer on the reamer, you can use that to know when you are close and need to start paying attention with your relief grinding.

Just another idea...
Title: Re: How much jump before pellet hits the grooves?
Post by: subscriber on November 15, 2021, 08:45:18 PM
One recurring thought I have about taking an accurate Cerrosafe bore impression:  Before you take a chamber cast; check out the new barrel's factory chambering job with your bore scope first.  Convince yourself that there is no burr where the factory cone meets the bore.  If there is a burr there, that will make removing the chamber cast harder; and more significantly, scrape or swage down the cast to a smaller than actual diameter.  A 0.0005" error may not matter; but I think a burr or displaced steel 0.002" tall might.

If you see a burr or ridge where the new barrel's cone meets the bore, a very light polishing may be indicated to remove it.  Then your land and groove diameter info will be more accurate, to determine better pilot and max chamber OD dimensions.
Title: Re: How much jump before pellet hits the grooves?
Post by: Greg_E on November 15, 2021, 11:03:23 PM
I already had to clean up that new barrel... It even had some light rust in there. I have a little car wax on it now, should help prevent rust from returning.
Title: Re: How much jump before pellet hits the grooves?
Post by: Spin on November 16, 2021, 12:09:10 AM
At some point in the not too distant future I'm going to sell off some of my tools and I can tell you there are some things you guys would likely be interested in.
Title: Re: How much jump before pellet hits the grooves?
Post by: subscriber on November 16, 2021, 12:25:46 AM
Lathe and mill, Jerry?
Title: Re: How much jump before pellet hits the grooves?
Post by: Greg_E on November 19, 2021, 10:14:28 PM
Not pretty, but should be functional. I still need to knock the corners off the bigger end so they can't cause any damage, that will get the size where I need it. I was originally going to grind the entire large end on my mill contraption, but it was obvious that the amount of work was not worth it for a part that will never go into the barrel. Some tape to protect things from the cutting edges until I can grind those on a bigger grinder. I'll do things a little differently if I decide to change the spiral flute reamer, it was a lot of work to get here and I can reduce that amount of work with some alternate thinking.
Title: Re: How much jump before pellet hits the grooves?
Post by: subscriber on November 19, 2021, 11:11:16 PM
Looks like it should work, Greg

I am not sure what else needs to be ground?

The relief at the large end only needs to be a little deeper than the chamber you want to cut. 

As with most things, it is more work and more time consuming than the simple nature of the project would suggest.

I was tempted to PM you yesterday for an update, then decided that doing so might be annoying, and just to wait and see.   :)
Title: Re: How much jump before pellet hits the grooves?
Post by: Greg_E on November 20, 2021, 12:47:42 AM
My mill lives at work right now, so time is limited. Really need to make a space for it at home.

I'll grind a lot less on the spiral reamer next time, then bench grind the excess since it doesn't need to be as good that far back.
Title: Re: How much jump before pellet hits the grooves?
Post by: darkcharisma on November 20, 2021, 01:13:32 AM
So this is the reason why my .172 Impact went from 1030fps down to 950fps. all because i cut the breech shorter and cut the leade where the rear band enages the lands upon chambering. 28.15” down to 27.5” barrel length too.

 i thought my Impact broke! The Jump leade gave 2 inch wide prints at 100 yards.
Title: Re: How much jump before pellet hits the grooves?
Post by: subscriber on November 20, 2021, 01:15:33 AM
Duy,

That sounds like something else changed.  Not sure what.
Title: Re: How much jump before pellet hits the grooves?
Post by: darkcharisma on November 20, 2021, 01:47:34 AM
besides the 3/4 barrel chop and the no jump leade, i am sure there is nothing else unless the gun's reg just went haywire.

Going with Bobs formula I should get 63FPE, which is 1030fps with 26.6 grains. Now it's not. Only 940FPS at best
Title: Re: How much jump before pellet hits the grooves?
Post by: subscriber on November 20, 2021, 01:54:39 AM
Is there any way for the thimble to rotate (or be installed back to front), to partially obscure the TP?

Springers use "no jump" leades and work better that way.  I presume you are shooting slugs.  Perhaps the leade angle is too steep and too rough, thus generating a lot more friction.  Now, a running start does operate at lower friction (dynamic friction VS static friction), but the difference in efficiency seems over the top.

Not that you want to keep cutting the barrel shorter, but what about engaging only the front drive band on loading, and not to full depth on the lands - so still on the "ramp"?  On the other hand, if the driving bands are fully engraved on loading, most of that work is done during loading.  For a split second the static friction would be high, but less work would be done than engraving the lands via air pressure.

The rear of the loaded projectile should not obstruct the TP either...
Title: Re: How much jump before pellet hits the grooves?
Post by: darkcharisma on November 20, 2021, 02:59:48 AM
The rear of the slug does stick out about 1mm obstructing the TP . and It was going 1030fps when it as just at the edge of the TP.

So I am guessing 3 changes since 1030fps.

I decided to cut down the barrel to start a fresh leade to get the best accuracy. Might not have been the best choice
Title: Re: How much jump before pellet hits the grooves?
Post by: subscriber on November 20, 2021, 05:27:31 AM
One inch off 28" won't even drop you to 27/28 of the original FPE; unless the valve was held open till the projectile left the muzzle.  You have lost about 18% FPE.  The barrel length reduction is only about 4%, so something else is going on.
Title: Re: How much jump before pellet hits the grooves?
Post by: rkr on November 20, 2021, 11:54:16 AM
So this is the reason why my .172 Impact went from 1030fps down to 950fps. all because i cut the breech shorter and cut the leade where the rear band enages the lands upon chambering. 28.15” down to 27.5” barrel length too.

 i thought my Impact broke! The Jump leade gave 2 inch wide prints at 100 yards.

Did the accuracy improve with increased contact to rifling?
Title: Re: How much jump before pellet hits the grooves?
Post by: Greg_E on November 21, 2021, 12:19:06 AM
The straight flute reamer is a fail. It keeps falling into the grooves and trying to take a big bite. I'll have to get the spiral flute cut down and try again.
Title: Re: How much jump before pellet hits the grooves?
Post by: subscriber on November 21, 2021, 12:33:31 AM
The straight flute reamer is a fail. It keeps falling into the grooves and trying to take a big bite. I'll have to get the spiral flute cut down and try again.


It is a left hand spiral, right hand cut, right?  With 7 flutes, should not grab onto the lands like the straight cut reamer.

A right hand spiral reamer may be worse that a straight flute one.  Especially if the flute count matches the number of lands or a multiple thereof...
Title: Re: How much jump before pellet hits the grooves?
Post by: Greg_E on November 21, 2021, 01:54:20 AM
I counted wrong, it is 6 flutes too. Left spiral, right cut.
Title: Re: How much jump before pellet hits the grooves?
Post by: subscriber on November 21, 2021, 02:01:12 AM
The left hand spiral creates a force trying to "spit the reamer out".  That helps reduce its tendency to grab.  You just have to apply very little force pushing the reamer in to cut; especially at the start.

I think having the barrel vertical with the reamer at a comfortable height is the way to approach this.  The force you apply is almost just enough to keep it stable, more than showing it in to cut.

If the barrel is held in a vice, have the jaws at least 2 inches away from the reamer pilot, because a thin walled barrel will spring out of round, and you want to cut it far away from the temporary oval section.  Else, you will cut an oval into it - seen after removal from the vice.  This ovality can be reduced by using close fitting padded vice jaws.  And / or staying away from between the jaws.

It might help to use grease (or Vaseline) as the cutting lube, until more contact area is established.

Is the rifling a right hand spiral?  That should help with a left hand spiral reamer.
Title: Re: How much jump before pellet hits the grooves?
Post by: darkcharisma on November 21, 2021, 02:55:44 PM
So this is the reason why my .172 Impact went from 1030fps down to 950fps. all because i cut the breech shorter and cut the leade where the rear band enages the lands upon chambering. 28.15” down to 27.5” barrel length too.

 i thought my Impact broke! The Jump leade gave 2 inch wide prints at 100 yards.

Did the accuracy improve with increased contact to rifling?

I am trying to sort the tune out first. once i get it back to 1000fps i will shoot 100 yards and let you know.

Thanks Sub for reaching out via PM.
Title: Re: How much jump before pellet hits the grooves?
Post by: subscriber on November 21, 2021, 04:30:00 PM
So, Duy; did your groups shrink after cutting the shorter leade?


And for general speculation:  I wonder if smaller caliber slug guns are more sensitive to free-bore, because they have more projectile circumference in bore contact (friction), compared to the area that air pressure acts on to generate engraving force (and accelerate the projectile moving); because: 

Circumference is PI x D;
while area is PI x D X D / 4

As such, larger projectiles have engraving forces that increase with the diameter, but available air pressure forces that increase by the square of the diameter.  The inverse means that small calibers slugs with significant weight (flank length) may bog down more during land engraving. 

This obviously also depends on the how close the bearing diameter is to barrel groove diameter.  Dubber and his buddies have been shooting .22 cal slugs that are under groove diameter for a long time to good effect.  Members on this forum have recently noticed that large bore slugs sized 0.0005" under groove diameter shoot faster, with no loss in precision.  That approach seems perhaps better because it reduces friction all the way down the barrel, than slamming the slug into the rifling with a lot of free-bore. 

Of course, it depends how close the slug diameter is to the free-bore diameter.  If the free-bore is a gentle cone then it will align the slug with the barrel axis before the slug slams into the rifling.  What is the optimal free-bore for velocity, without degrading accuracy is the million dollar question.  Somehow, better precision seems more valuable than a few more FPE.  A trade-off one can't effectively speculate about, without absolute numbers.
Title: Re: How much jump before pellet hits the grooves?
Post by: Greg_E on November 21, 2021, 08:53:50 PM
In theory, pellets can be shot accurately out of smooth bore.
Title: Re: How much jump before pellet hits the grooves?
Post by: subscriber on November 21, 2021, 09:02:08 PM
In theory, pellets can be shot accurately out of smooth bore.

As long as the pellet head is adequately supported by the tube ID.

How do you know if a chamber is cut "too loose"?  The loaded pellet or slug falls out if you invert the open breech...  If the projectile fit is snug enough not to fall out, it should be pre-aligned adequately to shoot straight.
Title: Re: How much jump before pellet hits the grooves?
Post by: rsterne on November 21, 2021, 09:04:01 PM
Quote
In theory, pellets can be shot accurately out of smooth bore.

if the pellet is PERFECT (concenricity, balance).... Any imperfections will require at least a slow twist rate for accuracy....

Bob
Title: Re: How much jump before pellet hits the grooves?
Post by: lennyk on November 23, 2021, 09:17:23 PM
in my country we are only allowed smoothbore air guns in 177.

at 25m i would say they are very close

beyond 40m the key difference is stability in wind.
Title: Re: How much jump before pellet hits the grooves?
Post by: oldpro on November 30, 2021, 06:15:45 PM
 having tested this exact thing in the past(two years ago) I found pellets shot from a smooth bore had far less accuracy then ones shot with even the slightest twist. Modern pcp guns are getting slower and slower on twist rates as compared to years past though as we are finding you do need twist but not nearly as much as once thought. The fx smooth twist is a great example of a smooth bore with just a slight twist at the very end. Very accurate with pellets and horrible with any solids. 
Title: Re: How much jump before pellet hits the grooves?
Post by: lennyk on November 30, 2021, 09:01:31 PM
What smoothbore barrel did you use and what caliber?

having tested this exact thing in the past(two years ago) I found pellets shot from a smooth bore had far less accuracy then ones shot with even the slightest twist. Modern pcp guns are getting slower and slower on twist rates as compared to years past though as we are finding you do need twist but not nearly as much as once thought. The fx smooth twist is a great example of a smooth bore with just a slight twist at the very end. Very accurate with pellets and horrible with any solids.
Title: Re: How much jump before pellet hits the grooves?
Post by: Roboslug on December 11, 2021, 10:46:41 PM
In a past life before I had to use prescription glasses I would shoot shotgun slugs as a hobby.  My shotgun is a bone-stock Browning B-2000 auto with a smoothbore slug barrel.  Other than a .22 I don't have any rifles and never considered myself a rifleman, I was always just enamored with the idea of shooting slugs.  When I found a site that hosted postal matches I had to find a barrel.  I chased this for a couple of years trying countless slugs in the name of accuracy.  It was 18 months of shooting/searching (a slug) that grouped adequately before I adjusted the sights.  Though I never won, I did have fun. 

So yes, pellet/bore fit does matter.

50 yds
open sights
5 shots
12 ga smoothbore



Title: Re: How much jump before pellet hits the grooves?
Post by: Greg_E on December 27, 2021, 05:19:40 PM
I'm having a hard time finding motivation to work on this. I keep seeing videos on the new AEA Big 9 and wondering if I should just stop messing with this thing. It's going to take a large pile of work, and a bunch more money to get close to what you can get out of the box with many of these AEA products.  :-[

The spiral flute reamer is spiraled the wrong way, so it is worse. Still trying to make something to help me with the straight flute reamer. I have an idea to try, and after this I might quit.
Title: Re: How much jump before pellet hits the grooves?
Post by: subscriber on December 27, 2021, 10:17:35 PM
Finding motivation to get motivated can be difficult :)
Title: Re: How much jump before pellet hits the grooves?
Post by: Spin on December 30, 2021, 11:59:08 PM
having tested this exact thing in the past(two years ago) I found pellets shot from a smooth bore had far less accuracy then ones shot with even the slightest twist. Modern pcp guns are getting slower and slower on twist rates as compared to years past though as we are finding you do need twist but not nearly as much as once thought. The fx smooth twist is a great example of a smooth bore with just a slight twist at the very end. Very accurate with pellets and horrible with any solids.
     "  The fx smooth twist is a great example of a smooth bore with just a slight twist at the very end. Very accurate with pellets and horrible with any solids. "
        I'm Shooting NSA, FX, and ASA slugs out of a .25 cal. 600mm STX barrel and and I wouldn't call the results, horrible. I have no experience with any of the designated FX slug barrels but if extreme bench shooters are tagging bullseyes and silhouettes at 300 yrd.'s and out, I'm guessing they are doing OK with them. ;D
Title: Re: How much jump before pellet hits the grooves?
Post by: nervoustrigger on December 31, 2021, 12:26:08 AM
Quote
I'm Shooting NSA, FX, and ASA slugs out of a .25 cal. 600mm STX barrel and and I wouldn't call the results horrible.

FYI, Travis was describing the original Smooth Twist in which only a portion near the muzzle is rifled.

The Smooth Twist X (STX) is a different design in which the entire bore is rifled.

While their twist rates on paper may be comparable, the former produced an actual rate much, much slower on account of the pellet already having reached near max velocity before hitting the rifling.  It would therefore skid through the rifling and pick up only a small fraction of the twist rate...sufficient to stabilize a pellet but far too slight to stabilize slugs.
Title: Re: How much jump before pellet hits the grooves?
Post by: Spin on December 31, 2021, 05:20:52 PM
Quote
I'm Shooting NSA, FX, and ASA slugs out of a .25 cal. 600mm STX barrel and and I wouldn't call the results horrible.

FYI, Travis was describing the original Smooth Twist in which only a portion near the muzzle is rifled.

The Smooth Twist X (STX) is a different design in which the entire bore is rifled.

While their twist rates on paper may be comparable, the former produced an actual rate much, much slower on account of the pellet already having reached near max velocity before hitting the rifling.  It would therefore skid through the rifling and pick up only a small fraction of the twist rate...sufficient to stabilize a pellet but far too slight to stabilize slugs.
  Got It! Thanks Jason.