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Why no sabots for infantry guns?   Small Arms <20mm

Started 28-Aug by VPMudde; 2835 views.
QuintusO

From: QuintusO

28-Aug

Great post, and great questions, btw.

EmericD

From: EmericD

29-Aug

VPMudde said:

Accuracy is one issue. The Swedes, however, disagree, as I hear their snipers shoot what looks to be M948. The higher velocity increases hit probability more than the lower mechanical accuracy decreases it. And Emeric in his neckless cartridge thread figured that, all else being equal, the hit probability with a 4 MOA rifle isn't that much lower than a 1 MOA rifle, especially at long range.

Well, I think the "accuracy issue" is more complex than that.

Yes, you could achieve 4.5 MoA with a sabot, but you need to use also a pretty short bullet (as found on the XM948 SLAP). Trying to launch a spin-stabilized bullet with a L/D higher than 4 with a sabot is a challenge because sabots don't like "short" twist (the L/D of the XM948 is 3.7, and it's a flat-base design).

A few years ago, I found some accuracy data of several bullets fired from sabots, and the accuracy was exponentially decreasing as the bullet length was increasing. The FAMAS MSD project used a bullet with a L/D of 4.1 and the accuracy was so low that you wouldn't expect to hit a man-sized torso beyond 100 m. Very similar results were achieved during the US ACR program with the AAI and Steyr ACR firing flechettes, the accuracy was so poor (we are talking about something like 20 MoA) that the hit probability above 75 m was lower than the one of the M16A2.

So, you're probably not going to be able to use a very sleek bullet, and will need high density material (tungsten) to achieve a high sectional density to balance for poor form factor.

The XM948 is working because it is using a short, high density bullet. Using a less dense material will decrease the BC to unpractical level, and increasing the bullet length to regain some sectional density will probably decrease the accuracy beyond practical level...

I'm not going to say that it can't be done (the US is working on it for the .50 APTC program), but at least you could expect the cost of your ammo to significantly increase.

smg762

From: smg762

29-Aug

Do you think there would be an accuracy improvement wih, say a .17 bullet in a 6mm sabot.....as opposed to a 7.62 sabot.

Also would sabots melt ina GPMG barrel....could his be solved by a CBj style sabot with a solid polymer beari surface.

Lastly could one have a sabot which is only about 0.5mm thick, for example to reduce the wear of a 22.250 in MGs.

https://m.youtube.com/watch?v=kD6niXoUvwM&t=2s

This guy got a reasonable group at 100m

Oh my last question....how would an infantry sabot cope with lots of mud or water in the gun? Thanks

smg762

From: smg762

29-Aug

Oh, regarding L/D ratio, i noticed the 50 slap uses long bullets, and CBJ 762 uses fairly long ones too. I think a more sturdy sabot design would work with fast twists

VPMudde

From: VPMudde

29-Aug

I figured the whole reason the sabots decrease accuracy is because they influence the bullet's just-out-of-barrel yaw. If I'm not wrong, in regular cartridges this effect is easy to compensate for as it is very consistent. If you introduce a sabot, it's separation causes an additional yaw motion which is far less consistent.

Yes, you could achieve 4.5 MoA with a sabot, but you need to use also a pretty short bullet (as found on the XM948 SLAP). Trying to launch a spin-stabilized bullet with a L/D higher than 4 with a sabot is a challenge because sabots don't like "short" twist (the L/D of the XM948 is 3.7, and it's a flat-base design).

That a sabot dislikes tight twist rates seems counter intuitive to me. I assumed that a faster twist causes faster sabot separation due to higher centripetal forces, and thus it has less time to impart additional yaw. But maybe I'm wrong about what causes the accuracy penalty. I'm a chemist, not a ballistician.

Also, this is the first I've heard of the APTC program, normally I'm quite up to date on these things. I can't find much more than an NDIA slideshow and a TFB article. Is there some more in-depth information available? How exactly does it address the accuracy problem? It does seem like a step in the (according to me) right direction, and should have been completed before the they decided on a 6.8mm magnum infantry rifle. (They could have gone back to 7.62 and admit 5.56 was just a fling no_mouth)

CC to EmericD
VPMudde

From: VPMudde

29-Aug

Making a sabot as thin as possible somewhat hampers their ability to get more velocity. There is of course less parasitic mass and the reduction of friction, but as you reduce the sabot's diameter you also reduce the otherwise increased area for the propellant gas to push on (after all, pressure × volume = force = mass × acceleration, and more acceleration = more good). It seems like there is a sweet spot to be found for each bullet, balancing the parasitic weight and velocity gain. And with modern manufacturing making perfectly concentric sabots isn't much harder than making sabots in general.

Sabots wouldn't really melt in hot barrels. While they are usually made of thermoformed polymers, these take time to heat up. Chambering a round and immediately firing it, as GPMGs are want to do, leaves very little time to heat up much of the sabot's bearing surface. Polymers are good insulators, so you really need plenty of time for the heat to be conducted deeper into the material.

QuintusO

From: QuintusO

29-Aug

Sabots do require tighter twists, although part of the problem is just making sure they are gripping properly. The other part is that effectively, twist rate is constant for projectile calibers traveled per rotation. Smaller projectile relatively to bore means proportionately tighter twist.

The ultimate solution to all of this is to rip the bandaid off and use flechettes.

  • Edited 29 August 2021 17:56  by  QuintusO
QuintusO

From: QuintusO

29-Aug

There's always an ideal sabot ratio and for this application it's about 1.5:1.

Farmplinker

From: Farmplinker

29-Aug

They need to go back to 11.4mm and admit these small bores are a joke! stuck_out_tongue_winking_eye

More seriously, rather than a sabot, perhaps a thick polymer jacket? Say, enough to take a 5.56 bullet up to 6mm. Maybe it would be easier than a sabot?

gatnerd

From: gatnerd

29-Aug

Farmplinker said:

polymer

A sort of non discarding sabot / polymer jacket is being looked into by SOCOM with the Aeroshell projectile:

http://forums.delphiforums.com/autogun/messages/7521/1

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