This is intended for people interested in the subject of military guns and their ammunition, with emphasis on automatic weapons.
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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.
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.
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
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
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 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 )
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.
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.
There's always an ideal sabot ratio and for this application it's about 1.5:1.
They need to go back to 11.4mm and admit these small bores are a joke!
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?
A sort of non discarding sabot / polymer jacket is being looked into by SOCOM with the Aeroshell projectile:
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?