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Author Topic: How much jump before pellet hits the grooves?  (Read 2803 times))

Offline Greg_E

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How much jump before pellet hits the grooves?
« 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.
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Re: How much jump before pellet hits the grooves?
« Reply #1 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....



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
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Offline oldpro

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Re: How much jump before pellet hits the grooves?
« Reply #2 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
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Offline Greg_E

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Re: How much jump before pellet hits the grooves?
« Reply #3 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.
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Re: How much jump before pellet hits the grooves?
« Reply #4 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
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Airsenal:
1750 CO2 Carbine, .177 Uber-Pumper, .22 Uber-Carbine, .25 Discovery, 2260 PCP 8-shot Carbine, 2260 HPA (37 FPE), 2560 HPA (52 FPE), XS-60c HPA in .30 cal (90 FPE), .22 cal QB79 HPA, Disco Doubles in .22, .25 & .30 cal, "Hayabusa" Custom PCP Project (Mk.I is .22 & .25 cal regulated; Mk.II is .224, .257, 7mm, .308 & .357; Mk.III is .410 shotgun and .458 cal), .257 "Monocoque" Benchrest PCP, .172/6mm Regulated PCP and .224/.257 Unregulated, Three regulated BRods in .25 cal (70 FPE), .30 cal (100 FPE) & .35 cal (145 FPE), .257 Condor (180 FPE).

Offline Greg_E

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Re: How much jump before pellet hits the grooves?
« Reply #5 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.
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Online lennyk

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Re: How much jump before pellet hits the grooves?
« Reply #6 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
« Last Edit: November 05, 2021, 08:33:16 AM by lennyk »
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Offline mr007s

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Re: How much jump before pellet hits the grooves?
« Reply #7 on: November 05, 2021, 10:34:34 AM »
This may be of some help. https://conicalendmills.com/
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Re: How much jump before pellet hits the grooves?
« Reply #8 on: November 05, 2021, 11:16:45 AM »
.....................
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Offline nervoustrigger

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Re: How much jump before pellet hits the grooves?
« Reply #9 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.
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Offline Greg_E

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Re: How much jump before pellet hits the grooves?
« Reply #10 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.
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Offline JuryRigger

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Re: How much jump before pellet hits the grooves?
« Reply #11 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...

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Offline nervoustrigger

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Re: How much jump before pellet hits the grooves?
« Reply #12 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.
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Offline Greg_E

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Re: How much jump before pellet hits the grooves?
« Reply #13 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!
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Offline Paulemar

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Re: How much jump before pellet hits the grooves?
« Reply #14 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.
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Offline Greg_E

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Re: How much jump before pellet hits the grooves?
« Reply #15 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.
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Re: How much jump before pellet hits the grooves?
« Reply #16 on: November 06, 2021, 01:58:42 PM »
 ::)

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1750 CO2 Carbine, .177 Uber-Pumper, .22 Uber-Carbine, .25 Discovery, 2260 PCP 8-shot Carbine, 2260 HPA (37 FPE), 2560 HPA (52 FPE), XS-60c HPA in .30 cal (90 FPE), .22 cal QB79 HPA, Disco Doubles in .22, .25 & .30 cal, "Hayabusa" Custom PCP Project (Mk.I is .22 & .25 cal regulated; Mk.II is .224, .257, 7mm, .308 & .357; Mk.III is .410 shotgun and .458 cal), .257 "Monocoque" Benchrest PCP, .172/6mm Regulated PCP and .224/.257 Unregulated, Three regulated BRods in .25 cal (70 FPE), .30 cal (100 FPE) & .35 cal (145 FPE), .257 Condor (180 FPE).

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Re: How much jump before pellet hits the grooves?
« Reply #17 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....


« Last Edit: November 07, 2021, 04:58:22 PM by subscriber »
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Offline Nvreloader

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Re: How much jump before pellet hits the grooves?
« Reply #18 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,

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Offline Greg_E

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Re: How much jump before pellet hits the grooves?
« Reply #19 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.
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