Slug weight vs length for accuracy

[dens228]

I'll start by saying I'm using a Maverick VP, 600 mm barrel using a Standard Superior STX Liner.
Since I can't help myself from playing around with different slugs for fun I ordered the H&N variety pack, 28-36 grains.
This morning I went out and tried each slug, I did not adjust the regs but used the hammer wheel from 5 through 7. All of the slugs shot best at 7 but not much better than 6 other than the 34 and 36 grains, Much better at 7.
Here's the thing I'm wondering. I didn't bother with the chrono but figure as I went up in weight speed slowed down but the length of the slug was longer which I would think would give the liner grooves more of an effect.
As I went heavier/longer accuracy improved. 28's were pretty good, I'm assuming since they were really flying. My 34 gr pellets hit 907 fps for average.
30's were horrible. 32 was the same as the 28's, 34's were better than the 32's, the 36's were pretty almost as accurate as my pellets. Pellets at 30 are a ragged hole vs the slugs which were a slightly larger ragged hole.

Does slug length matter more than I thought before today? In my past dabbling I only thought about slug speed.

 

[elh0102]

It probably matters. In handloading precision rifle ammo, one variable that is sometimes critical, is the bullet position relative to the lands. The best load might be with the bullet .010" in the lands, just touching, or maybe .010" off the lands. Absent some machine work, the only way we can control that in an air rifle, is with the length and shape of the slug. Of course, that assumes all other variables equal, which is rarely the case. But, a certain length and weight will also be more, or less well matched to the twist rate of the barrel, and thus the stability of the flight.

 

[cfs916]

Seating depth is also important

 

[Hard2hurt223]

I don't know but I have a Maverick 700mm in 25cal shooting the nsa 38.5 gr slugs at 940 and they shoot great I also have a wildcat bt in 600mm shooting the same slugs at 900 fps and it also shoots great

 

[Vetmx]

It does. When you shoot without a choke, it really does. As far as your seating depth with different length slugs goes, FX has found a way to kind of cheat around that issue. So it’s not a priority concern when you shoot using one of their metal straws as long as you don’t have the back of your slug still sticking out the back of the liner when your cocking lever is closed. When you build a barrel or have a gun with a real barrel, the leade is your most major concern with slugs.

 

[Emu]

Larger projectile of same weight provide more penetration because less surface stops the travel.

More contact on barrel makes projectile to reduce speed.

 

[steve-l]

It does. When you shoot without a choke, it really does. As far as your seating depth with different length slugs goes, FX has found a way to kind of cheat around that issue. So it’s not a priority concern when you shoot using one of their metal straws as long as you don’t have the back of your slug still sticking out the back of the liner when your cocking lever is closed. When you build a barrel or have a gun with a real barrel, the leade is your most major concern with slugs.

Your comment is very misleading. You are correct in the case of PBs, but they use much, much more pressure and much more bullet swage than any air gun and therefore do not exert so much influence. The issue of length is all about cross sectional density and correct rpm for optimum stabilization. Further complicating this issue is that the rifling twist rate is the only stabilization source with a slug or bullet, but when using a diabolo pellet, you also have a competing stabilization source, skirt induced drag. This complicates the stabilization issue greatly. There are many books written about this and worth reading if you are so inclined. The result of these competing stabilization sources is the main cause of spiraling pellets.

 

[Vetmx]

Your post is kind of confusing me. What exactly is misleading in my post? We aren’t talking pellets and we aren’t talking powder burners. The question was slug length related to accuracy. I said yes. I just didn’t go into detail. You didn’t state anything we don’t already know. I just want to make sure I’m not missing something or giving bad advice.

 

[steve-l]

Your post is kind of confusing me. What exactly is misleading in my post? We aren’t talking pellets and we aren’t talking powder burners. The question was slug length related to accuracy. I said yes. I just didn’t go into detail. You didn’t state anything we don’t already know. I just want to make sure I’m not missing something or giving bad advice.

I mentioned PBs because you would be correct, but with slugs and airguns, you stated the most important factor is the leade and it isn't because a slug is simply not upset as a bullet is. In point of fact, I believe the leade has no measurable influence because both the driving pressure and the velocity is so much lower.

 

[qball]

Slug length in airgun is exactly the same as slug/bullet length in PB, it’s all about stability factor from slug length and barrel twist rate combo. After that it’s all about barrel quality/fit.

This is literally decades if not century old topic.

 

[Vetmx]

I mentioned PBs because you would be correct, but with slugs and airguns, you stated the most important factor is the leade and it isn't because a slug is simply not upset as a bullet is. In point of fact, I believe the leade has no measurable influence because both the driving pressure and the velocity is so much lower.

I believe the leade is of utmost importance because I build slug barrels. Changing the jump a couple thousandths can make a no go slug a go slug.

 

[elh0102]

I believe the leade is of utmost importance because I build slug barrels. Changing the jump a couple thousandths can make a no go slug a go slug.

I'm not surprised that you have found this to be the case. When I was handloading CF ammo for BR, a few thousandths difference in seating depth mattered. Since most of us can't alter the seating depth of our pellets into the chamber, I would guess that the precise machining of the leade is important. It might not make a noticeable difference for casual target shooting or plinking, but at a level that demands precision accuracy, everything matters.

 

[Vetmx]

What takes place in a PB chamber vs an airgun chamber might as well be worlds apart. We don’t have 60,000cup behind a super tight projectile. We don’t have to add gas checks behind lead projectiles. We need a perfect fit. Guys wonder why this gun or that pellet gun seem to have a decent track record with slugs when other guns don’t. This or that gun doesn’t have a magic barrel or choke, it has a slug friendly pellet leade.

 

[steve-l]

I believe the leade is of utmost importance because I build slug barrels. Changing the jump a couple thousandths can make a no go slug a go slug.

OK, I do not have your experience, but if what you state is true, then the upset damage should be measurable on the slug. Please shoot a slug into a water filled pipe from an offending barrel and let's measure the distortion and depth of the styrations. Then shoot another slug the same way from a working barrel and measure the difference.

 

[elh0102]

I don't believe that would disclose what you need to know, which is the slug-to-chamber relationship after chambering, but before firing.

 

[steve-l]

I don't believe that would disclose what you need to know, which is the slug-to-chamber relationship after chambering, but before firing.

Why not? If a leade is too large the slug/bullet has the opportunity to accelerate without revolving then slamming into the rifling. This shock damages the slug and causes lead fouling and that damage is the primary cause of the loss of accuracy. This damage would be preserved if fired into a long tube of water. To further prove my point a 22RF could be also tested and styrations measured and compared to an air rifle slug. As a further reference to this theory, please also consider the case of PB revolvers where this jump is a necessary design feature that allows for slight misalignment between the cylinder and the barrel where you will find a tapered forcing cone. This comparison is relevant because velocities are similar and the pressures are also less than in a rifle. It is also relevant to consider the case of the Webley revolver because of the pure lead bullet (very necessary) and hollow base. This round actually upsets twice. Once in the cylinder and one more time in the forcing cone very similar to the typical air gun slug where the alloy is also pure lead and the thin wall in the hollow base which also easily allows the slug to conform to the barrel rifling easily with the propelling air pressure. If you perform this test you will discover that the slug styrations will only be at the slug base.

 

[shotmaniac]

I believe the leade is of utmost importance because I build slug barrels. Changing the jump a couple thousandths can make a no go slug a go slug.

wth is the jump

 

[steve-l]

wth is the jump

It is the distance the projectile must travel from its at rest position in the breech to the point of engagement with the start of the rifling. Freebore if you like. The greater this length, the greater the velocity at collision time and the greater the shock. You may note that the projectile is always slightly larger than the bore. This insures a tight seal between the projectile and bore as well as to provide enough projectile material so the rifling can bite into the projectile effectively swaging the projectile to size. This is also called upsetting.

 

[Scotty1]

It seems to me that there is a fair amount of info to be had just by chambering a round, and pushing it back out of the breech. Is the lead in forming the projectile, or peeling lead at the lands engagement? Does the transfer port leave any sign of impacting the projectile? This is probably way remedial as far as this discussion goes.

 

[elh0102]

It seems to me that there is a fair amount of info to be had just by chambering a round, and pushing it back out of the breech. Is the lead in forming the projectile, or peeling lead at the lands engagement? Does the transfer port leave any sign of impacting the projectile? This is probably way remedial as far as this discussion goes.

With most air rifles, this can present a safety issue. I there is air in the gun, I'd remove the barrel before pushing the pellet out from the muzzle to breech.