gatnerd

Military Guns and Ammunition

Hosted by gatnerd

This is intended for people interested in the subject of military guns and their ammunition, with emphasis on automatic weapons.

  • 3257
    MEMBERS
  • 184811
    MESSAGES
  • 11
    POSTS TODAY

Discussions

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

Started 28-Aug by VPMudde; 2841 views.
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

VPMudde

From: VPMudde

29-Aug

QuintusO said:

twist rate is constant for projectile calibers traveled per rotation. Smaller projectile relatively to bore means proportionately tighter twist.

Don't rightly know what you mean by this. A 1-in-7" twist in a 7.62 barrel is the same number of rotations per distance as a 1-in-7" 5.56. And small caliber ammunition generally has a tighter twist rate than a larger caliber to being with. The angle of the rifling will be different of course.

What I think you're getting at is the need to overcome the projectile's rotational inertia to get it to spin, yes? And in order to do so, you need a way to get a sufficiently tight grip on the bullet. I imagine that as the bullet and sabot are forced into the bore, the sabot clamping down would provide the necessary grip. And if not, increase the sabot diameter slightly, so there's a bit more material clamping down on the bullet.

But all of this still doesn't explain by what mechanism the sabot affects accuracy. That happens after exiting the barrel, no?

Flechettes have their own issues, unless you know things about them that the people at AAI didn't last century. So what are you suggesting? L/D 12 bullets, but with the trailing ¾ths ground away to become fins? (there's a fancy word for that that eludes me).

Regarding costs, I thought of a corner to cut: since the bullet doesn't touch the barrel, you could just load up 1-piece hardened steel VLDs, cut a slit all around in some place to make it fragment. It's no EPR, but it's cheaper?

EmericD

From: EmericD

30-Aug

VPMudde said:

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.

That's one part of the issue.

The first issue with a sabot is that the force needed to extract the bullet from the case and push it into the forcing cone is more dispersed, so the internal ballistic cycle is more dispersed (more pressure and velocity deviation).

The second issue is that it's more difficult to achieve a perfect bullet / sabot concentricity. The more important is the difference between material density, the more difficult it is to avoid in-bore unbalance. People manufacturing full bore jacketed bullets already found that the difference in density of the jacket and the lead core is sufficient to give them accuracy problems if the thickness of the jacket is not tightly controlled, now imagine adding a organic polymer compound in the equation!

The third issue is that, for proper accuracy, you need to avoid the "slippery soap" effect when the bullet leaves the barrel, so you want plastic deformation and not (reversible) elastic deformation. Using a "large" sabot in a tight bore to maximize the grip between the sabot and the bullet is also maximizing elastic deformation, because in the case of a full plastic deformation of the sabot, there would be no grip between the sabot and the bullet. When the bullet exit the barrel, you will have an elastic relaxation of the sabot that will give the bullet a lateral throw-off, even if the bullet-sabot separation is perfect.

The, finally, you have the possibility of bullet-sabot interaction just after leaving the barrel, but it's not the main source of bullet inaccuracy.

EDIT : spelling corrections.

  • Edited 30 August 2021 5:06  by  EmericD
17thfabn

From: 17thfabn

30-Aug

gatnerd said:

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

Sounds like the World War II era 76 mm HVAP and other similar anti=tank rounds. A standard AP projectile weighed approximately  15 pounds, the HVAP 7.5 . Velocity went from 2,600 fps for the standard projectile up to 3,400 for the HVAP. 

gatnerd

From: gatnerd

30-Aug

17thfabn said:

Sounds like the World War II era 76 mm HVAP and other similar anti=tank rounds

Thank you, I wasn't aware of those (more of a gun guy then a cannon guy) but thats very much the same concept:

https://en.wikipedia.org/wiki/Armor-piercing_ammunition#APCR_and_HVAP

And the reasons for their use - sabots/APDS hadn't been perfected - is pretty much the same reason we'd be interested in Aeroshell for small arms today. It could provide a bridge between conventional copper jacketed AP rounds and future APDS sabots for small arms. 

TOP