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Author Topic: How much jump before pellet hits the grooves?  (Read 3017 times))

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Re: How much jump before pellet hits the grooves?
« Reply #20 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 




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.



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Offline Greg_E

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Re: How much jump before pellet hits the grooves?
« Reply #21 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.
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Re: How much jump before pellet hits the grooves?
« Reply #22 on: November 09, 2021, 02:22:57 PM »
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Re: How much jump before pellet hits the grooves?
« Reply #23 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 rifleshttps://www.pyramydair.com/blog/2015/06/ruger-model-3-32-40-schuetzen-rifle-part-1/ ).

« Last Edit: November 09, 2021, 04:53:50 PM by subscriber »
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Offline Greg_E

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Re: How much jump before pellet hits the grooves?
« Reply #24 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.
« Last Edit: November 09, 2021, 11:35:39 PM by Greg_E »
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Re: How much jump before pellet hits the grooves?
« Reply #25 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.
« Last Edit: November 10, 2021, 12:21:01 AM by subscriber »
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Offline Nvreloader

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Re: How much jump before pellet hits the grooves?
« Reply #26 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

Very easy to take a cast of a chamber.

HTH's
Don
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Re: How much jump before pellet hits the grooves?
« Reply #27 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

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.
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Re: How much jump before pellet hits the grooves?
« Reply #28 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
« Last Edit: November 10, 2021, 02:37:03 PM by subscriber »
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Offline Greg_E

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Re: How much jump before pellet hits the grooves?
« Reply #29 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.
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Re: How much jump before pellet hits the grooves?
« Reply #30 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
« Last Edit: November 10, 2021, 02:43:21 PM by Greg_E »
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Re: How much jump before pellet hits the grooves?
« Reply #31 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

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

How to measure a cast: 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



« Last Edit: November 10, 2021, 02:53:57 PM by subscriber »
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Offline Greg_E

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Re: How much jump before pellet hits the grooves?
« Reply #32 on: November 10, 2021, 05:35:30 PM »
Thanks for the tips, I'll read those later tonight.
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Re: How much jump before pellet hits the grooves?
« Reply #33 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.









To the point on a very long video:
https://youtu.be/c9stuhaYuj4?t=819
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Re: How much jump before pellet hits the grooves?
« Reply #34 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?
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Offline Nvreloader

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Re: How much jump before pellet hits the grooves?
« Reply #35 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
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Re: How much jump before pellet hits the grooves?
« Reply #36 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

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:
« Last Edit: November 11, 2021, 04:36:15 PM by subscriber »
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Offline lennyk

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Re: How much jump before pellet hits the grooves?
« Reply #37 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 ?
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Re: How much jump before pellet hits the grooves?
« Reply #38 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.
« Last Edit: November 11, 2021, 06:38:41 PM by subscriber »
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Re: How much jump before pellet hits the grooves?
« Reply #39 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 rifleshttps://www.pyramydair.com/blog/2015/06/ruger-model-3-32-40-schuetzen-rifle-part-1/ ).


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Sheridan 64/67 yr models, 20 cal/Hammerli 850 Air Mag -17 cal x 2, + 22 cal /QB-79 - 22 cal /Guantlet - 22 cal / Crosman 150 - 22 cal, Second Variant Model /Crosman 160 - 1st Variant Model - 22 cal /MRod Varmint 22 cal /Sentry 705-2 - 22 cal /Sentry 705 - 9T - 25 cal / Dar 17 cal /22 cal Discovery
CF 4500 45 min SCBA tanks x 3, w/SB F-10 compressor

"Speeds fine, but Accuracy is final"

"We must reject the idea that every time a law's broken, society is guilty rather than the lawbreaker,
It is time to restore the American precept, that each individual is accountable for their actions."
Ronald Reagan