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.

  • 3271
    MEMBERS
  • 185642
    MESSAGES
  • 0
    POSTS TODAY

Discussions

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

Started 28-Aug by VPMudde; 2965 views.
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. 

QuintusO

From: QuintusO

30-Aug

VPMudde said:

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.

Correct. If your projectile needs a 1/7 to stabilize with a 5.56 bore, it will also need a 1:7 7.62 bore if using a sabot. This is a tighter twist as you point out.

VPMudde said:

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

Separation creates interesting turbulent conditions which help knock the bullet around in complex ways. The best way to get over this is to make sure your projectile is well stabilized, and that there is reasonable muzzle pressure on uncorking (you do this via using a suppressor). A taper-fit cup sabot helps too.
 

VPMudde said:

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).

Last century is a long time; we know a lot more now that they didn't then.

 

VPMudde

From: VPMudde

30-Aug

Please excuse me while I get the hang of this forum's quotation thing

EmericD said:

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).

How does the sabot cause this dispersion? Wouldn't the same thing happen with light full caliber bullets al well? (say, 78gr Lehigh .308 close quarters).

EmericD said:

The second issue is that it's more difficult to achieve a perfect bullet / sabot concentricity. The more importa... [snip]

I didn't think concentricity would be that much of a problem. Good molds and a properly homogenized feed into the molder seemed enough TLC to overcome this. You could, of course, turn sabots on a lathe. Or do away with the bullet jackets altogether. No need for them if you've got a sabot to take the rifling.

EmericD said:

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 "lar... [snipp]

Perhaps there are alternatives to just clamping down on the bullet hard. A cannelure maybe, to have something for the sabot to hold on to by fit rather than compression.

Edit: getting the hang of this forum's quotation thing.

  • Edited 30 August 2021 20:17  by  VPMudde
VPMudde

From: VPMudde

30-Aug

QuintusO said:

Separation creates interesting turbulent conditions which help knock the bullet around in complex ways. The best way to get over this is to make sure your projectile is well stabilized, and that there is reasonable muzzle pressure on uncorking (you do this via using a suppressor). A taper-fit cup sabot helps too. 

You and Emeric have different ideas on what causes a saboted bullet's inaccuracy issue. I'm curious to see what you two would come up with for a solution. Me, I'm just grasping at straws with limited, but not what I believe is entirely incorrect understanding. (Not to toot my own horn or anything wink.)

QuintusO said:

Last century is a long time; we know a lot more now that they didn't then.

So what do we know now? Small diameter rods affected by otherwise non-barriers such as rain or light foliage are still small diameter rods affected by otherwise non-barriers such as rain or light foliage. 

VPMudde

From: VPMudde

30-Aug

gatnerd said:

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. 

In this thread I'm hoping to get towards that perfected -DS part. Though Aeroshell (and polymer jackets in general) are a nice and relevant development for sure, and provide some of the same benefits. The polymers and manufacturing techniques used there could be used in making a proper discarding sabot for small arms.

I just also really want that extra velocity and ultra sleek aerodynamics to happen no_mouth

gatnerd

From: gatnerd

30-Aug

VPMudde said:

I just also really want that extra velocity and ultra sleek aerodynamics to happen

Yep, the pairing of sabots with VLD projectiles is pretty much the holy grail for small arms development. 

Its a substantial challenge. On the plus side, theres never been an easier time to experiment with this.

-3D printing would allow the rapid prototyping of different sabot configurations

-.300 Blackout provides an ideal off the shelf sabot launching cartridge (plenty of ogive space + 1/7 barrel twist for .224 vld)

Off the top of my head, configuring a 2 piece sabot that it 'clips into' a groove in the bullet, and also closely follows the contours of the bullet, may be promising. And while that would be a bitch to machine it would be straightforward for a good 3D printer. 

Cutting Edge's line of rifle bullets have a groove already that could serve as a off the shelf jumping off point.

On the much more extreme side, Elite and Vanguards 5.7x28 projectile is very groovy in terms of where a sabot could clamp on:

TOP