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.

  • 3361
    MEMBERS
  • 191244
    MESSAGES
  • 7
    POSTS TODAY

Discussions

NGSW Phase 2 Consolidation and info   Small Arms <20mm

Started 30/8/19 by gatnerd; 555557 views.
nincomp

From: nincomp

23/4/22

 TV has claimed that their tech can be used for an 80K or 85K psi cartridge (I forget which).  If indeed that is the case, it would be logical for them to demonstrate it.  Since many countries are taking a "wait and see" approach to the NGSW program, there is no reason for THEM to be locked into Sig's Hybrid case.   

roguetechie said:

We know from existing documentation that the sig belt fed is a 10 moa gun too, giving us another strong indication that what we're dealing with is a bad projectile design for some reason since it's not producing the kind of accuracy we really want in anything.

As Emeric noted, at this point we know few of the details of the testing.   This, of course, will not stop me from speculating.   I suspect that the combination of the XM1186 bullet and the weapon's unusual chamber design combine to produce poor bullet alignment.   I have spent several hours pouring over various chamber, cartridge and bullet designs and these are my current thoughts:

1) I suspect that the bullet is a large part of the problem.  Below is a rendering from a SIG video.   It is very likely that a high-BC EPR-style bullet (one with a steel or tungsten penetrator at the tip) has characteristics that will make it difficult to align in a barrel bore without centering and yaw problems.  A long ogive with a heavy weight at its tip and a long boat tail ends up with a relatively short full-diameter bearing surface (shank).  This shape and weight distribution is difficult to keep aligned with the bore as it encounters the rifling and begins to spin rapidly.  When compared to a HPBT match bullet like the 135 grain SMK, the forces from a slightly misaligned bullet are greater and when those forces are distributed over the smaller bearing surface cause deeper deformation of that surface, allowing more mis-alignment in the bore.

     a) Best accuracy would be achieved if a significant length of the full-diameter shank is exposed from the case mouth and chambered into a long freebore with minimal clearance.  You end up with a cartridge like the 6.5 Creedmoor, 338 Norma or 6.5 PRC.

     b) With a long section of exposed bullet, the case gets shorter and loses capacity for a cartridge of a given OAL.  To gain back velocity, the case diameter must get larger, chamber pressure must increase, or both.   This is the "no free lunch" problem with wanting a powerful, short, small diameter cartridge that also needs to be very accurate.

     c) Conversely, the accuracy of a high-BC EPR bullets would be expected to become more erratic if shot from a chamber with a short freebore.  A small increase in the diameter of the short freebore will cause significantly more alignment, yaw and balloting issues.

2). If the SIG Hybrid uses the chamber seen in the SAAMI drawings for the 277 SIG Fury:

     a)  The chamber has an unusually short freebore to assure initial bullet alignment

     b)  Very little of the full-diameter bullet shank extends beyond the case mouth

     c)  If the Automatic Rifle/LMG version uses a looser chamber to reduce malfunctions, accuracy with a high-BC EPR bullet would be expected to be poor.  This would explain the "10 MOA gun" issue.

...[Message truncated]
View Full Message
  • Edited 23 April 2022 16:19  by  nincomp
stancrist

From: stancrist

23/4/22

nincomp said:

I suspect that the bullet is a large part of the problem.  Below is a rendering from a SIG video.   It is very likely that a high-BC EPR-style bullet (one with a steel or tungsten penetrator at the tip) has characteristics that will make it difficult to align in a barrel bore without centering and yaw problems.  A long ogive with a heavy weight at its tip and a long boat tail ends up with a relatively short full-diameter bearing surface (shank).  This shape and weight distribution is difficult to keep aligned with the bore as it encounters the rifling and begins to spin rapidly.  When compared to a HPBT match bullet like the 135 grain SMK, the forces from a slightly misaligned bullet are greater and when those forces are distributed over the smaller bearing surface...

The drawing from the SIG video is inaccurate. 

The artist's illustration shows a bullet with substantially different proportions than the actual 6.8 GP.

The bearing surface, ogive and boattail lengths of the 6.8 GP look to be the same as on the 135 SMK.

Also, lead and copper are denser than steel. 

Even with the small air space in the SMK nose, there may be little or no difference in tip weights.

Below, right:  6.5mm 150gr SMK sectioned.  (I couldn't find a photo of a sectioned 6.8mm SMK.)

EmericD

From: EmericD

24/4/22

stancrist said:

The drawing from the SIG video is inaccurate.  The artist's illustration shows a bullet with substantially different proportions than the actual 6.8 GP. The bearing surface, ogive and boattail lengths of the 6.8 GP look to be the same as on the 135 SMK.

While I agree that the illustration is simply an "illustration", the 6.8 mm GP is really different from the 135 SMK, which is a pretty old design.

The ogive of the 6.8 mm GP is much longer than the SMK (in both actual dimensions, and proportionally to the bullet length) and the CoG is located at the junction between the ogive and the shank, while the CoG of the SMG is nearly in the middle of the shank.

The boat-tail is also significantly longer.

DavidPawley

From: DavidPawley

24/4/22

Big agree there.

Its not so much “preparation to fight the last war” (although it is to an extent) as “using the wrong tools”. The plan should not be for infantry to engage infantry at 600+ m with organic small arms, use the IFV AC for that.

NGSW would have been better if it aimed for ballistic improvement and lighter weight ammo for 5.56 equivalent. A neckless polymer .224-Valkyrie-like (or even .22 homologous) with a long olive allowance and an efficiently high pressure with a rifle engineered for that and ngfcs would be pretty useful out to what are currently impractical ranges without compromising the utility for the near fight.

Or, bring back the FABRL, neckless, in an IMR BLUE alike.

gatnerd

From: gatnerd

24/4/22

G&A has now put online their previous print story of the TV Bullpup and 6.8 ammo. While no longer relevant for NGSW, its a fantastic article on the system:

https://www.gunsandammo.com/editorial/lonestar-bullpup-true-velocity-68x51mm-tvc/460008

Apsyda

From: Apsyda

24/4/22

This is an interesting write-up, I didn't pay any attention to the exact details of the SAAMI chamber designs, not thinking much about it. But you're right that there's a significant difference in the amount of freebore between the designs. Good catch. I figure GD trying to 'double dip' with their basic rifle design as both a rifle and automatic rifle with interchangeable parts is the root of that. Like you wrote up, it helps with reliability at the cost of accuracy. Which is something that you'd do with an automatic rifle.

Mix the funky freebore with a barrel that is not fixed into the action, and that chunky gas system and you could have a lot of barrel whip going on. This is entirely speculation however. The GD engineers behind the initial design weren't dumb enough to miss most of this.

Apsyda

From: Apsyda

24/4/22

>G&A tested a commercial-version of the 6.8 TVC ammunition — machine-turned, 135-grain, all-copper projectile — flash signature was virtually non-existent when the barrel was fitted with a Delta P Design suppressor.

>During bench testing, I often printed three out of five shots in a single ragged hole, but two shots strayed outside of a 1 MOA circle.

So it was around a 2 MOA gun with a solid copper bullet off of probably a cold barrel . And assumedly a at least somewhat more than that with the more internally complicated military projectiles. Decent accuracy for a combat rifle, but I could see how with this silly '800 yard point target engagement' concept the Army was pursuing, that wouldn't be desirable.

TOP