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

Started 28-Aug by VPMudde; 2831 views.
VPMudde

From: VPMudde

28-Aug

Hello fellow gun nerds, Long time lurker here, thought i'd try and stir up some interesting conversation. Apologies for the wall of text through.

Why isn't there more interest in using sabots in infantry rifles and machineguns? They can quite easily get you higher velocities than tradional bullet-to-the-bore cartidges because of several reasons, and the downsides don't seem insurmountable to me. In the NGSW thread Nate posted graphs with some ballistically excellent 5.56s. Loads like those from a conventional cartridge would be rather Eargesplitten Loudenboomer-like, so putting the bullets in a sabot would be a good way to prevent that AND keep your barrel short.

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.

Not being able to use a suppressor is another. Baffle strikes and all that. AAI with their ACR entry tried to get around this by widening the holes in the baffles as you down down the length of the suppressor, to allow the opening sabot through (I've seen pictures around, but can't seem to find them now, of course) Perhaps instead of that, one could design a suppressor wherein there is still contact with the sabot that keeps the petals closed. For example lining the "bore" with guiding surfaces matching the twist rate of the host barrel.

Feeding the cartridges into the chamber at high cyclic rates proved to be difficult. CBJ solves this using a sabot completely covering the projectile, so that it feeds like regular ammunition. Not ideal (more parasitic mass, CBJ sabot seems more complex and thus more expensive than an accelerator sabot) but definitely better than your gun jamming up every so many rounds. In the olden days of the late 19th/early 20th century, pistols feeding on hollow points would jam often, and in the years after they were designed to do better. Don't rightly know how, new feed ramps maybe? Could a similar solution be implemented in infantry rifles? Another solution would be going with CT cartridges, as in those the bullet isn't involved in chambering new cartridges, eliminating the problem altogether.

Then there is the issue of sending a small bit (or bits) of plastic downrange at high velocity. Catching one of those in the neck would not be a good experience. I got some very rough estimates out of some ballistics calculators, and the estimates say a 7.62mm 7 grain 0.01 G1 BC projectile at 1150m/s has only 100J left at 20 meters, and is subsonic after 35. Though that's not to say friendly fire is thusly not an issue, but probably only to team mates somewhere within a , say, 35m cone in front of the shooter, where team mate often are. I suggest figuring out exactly what size and shape this danger-zone has, and incorporating that knowledge into training. Or else have everyone don a fragmentation suit.

Sabots drive up costs. The sabot is an extra part to make and incorporate in the assembly process. To keep the mechanical accuracy penalty manageable you would need very tight tolerances and QA/QC, both of which are costly. But this is 2021, not 1991, and perhaps those 30 years of advances in manufacturing techniques and automation can alleviate some of the increased costs.

Now some things i can't come up with a solution for:

Fouling. I havent found much on this subject, but polymers are usually more fragile and less heat resistant than whatever metal a jacket is made out of, and even bullet jacket bits are a part of barrel fouling. I can imagine shooting saboted bullets all day really does a number on whatever gas operated system you've got going.

Impact resistance of the cartridges. Seems reasonable to say the interface between the projectile and the sabot and the case is less robust than the classic bullet to case. More mating surfaces, and there's materials less rigid than copper/brass involved in said surfaces. Box magazines protect the rounds pretty well, but a soft belt pouch not so much.

How do you all think these issues should be addressed? Sabots enable significant gains in muzzle velocity, and the importance of velocity in hit probability and terminal effects make them, in my opinion, very worth wile to investigate and discuss.

smg762

From: smg762

28-Aug

This has been my exact line of thinking since forever.

My desin is heavyily armour oriented, having watched a million YT vids, its always the faster rounds that defeat Ar500, such as he .17 rems, the 80grain 6mm magnums, and of course the 300win mag wih a sabot. 

Logically then a steel bullet at 5500fps has potential, because it wont desintegrate at that speed. Even at 300m it has 4500fps!

My design had a 4.2mm bullet in a very thin 6mm sabot.  You always want thinner sabots for more concentricity in the bore. Energy was 2300lbs, speed 5500fps. 

As for durability worries, u either want a CT case like the NGSW, or a case with a long neck to protect he bullet.

Fouling should be easy to solve but hot barrels are the real question...gun would need to have a huge emphasis on cooling....such as revolving multiple chambers.

the drawbacks you can never solve...sabots litter the firing range. And flash hiders will pose problems.

Lastly i dont think there are safety problems....the sabot flies straight

QuintusO

From: QuintusO

28-Aug

I haven't read your entire post and am just taking the post title as a QED question. The reason there's not more interest in sabots is due to an unwillingness to put the effort in to make them work. If the willingness was there to troubleshoot the concept, it would probably obsolete all current small arms ammunition.

Look up "Robert Truax", "Sea Dragon", and "SpaceX Starship", for an example of the same kind of thing.

QuintusO

From: QuintusO

28-Aug

Okay I've read your post in detail now. Going to take it basically paragraph by paragraph, starting with "Accuracy is one issue".

1. It is certainly true that sabot separation poses an obstacle to precision of the system. However, there are two things to note. The first is that most normal small arms rifle bullets for infantry rifles yaw quite a lot out of the muzzle (you can see it in this on video here), and sabot separation isn't really that big of a deal in that context. Even if we assume that an SPIW-like gun achieves 5 MOA, that's still only a little less accurate than a Tavor, for a gun that has a pH many many times higher at range.

2. Sabots handle gas ports just fine. Properly made "pepperpot" liners make suppressors practical for sabot rounds. You can see this in tank muzzle brakes:
 


3. A decently designed sabot should be basically a truncated cone, I don't foresee feeding issues unless the sabot itself isn't stiff enough, which shouldn't be a problem with modern plastics.

4. Catching a bullet in the neck is no joke either. But you make the point that a fragmenting petal type or disintegrating type sabot is a no-go. Cup type, of which CBJ's is a variant, is the way to go here.

5. Right and this is really the biggest obstacle. The will is not there to do the work to bring costs down. All of these problems are complicated, and they take focused, persistent effort to achieve realistically, something the government is bad at. The civilian industry is tiny and has zero incentive to make this happen.

6. The solution to the fouling problem is probably PPSU or a related medical thermoplastic, or just a light aluminum jacket.

7. This is no longer a problem. Modern plastics are extremely impact resistant.

8. These issues can all be addressed, but it will take a "small arms Elon Musk". No I'm not kidding.

  • Edited 28 August 2021 20:54  by  QuintusO
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.

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