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Military Guns and Ammunition

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

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NGSW evaluation update   Small Arms <20mm

Started 31/7/20 by autogun; 16348 views.
JesseH1234

From: JesseH1234

5/2/21

The whole thing just seems insane to me.  Or maybe I am nuts, who knows. 

7.62x51 is already losing a fair bit of velocity shooting out of a 16" barrel.  Cranking the pressure to 80k psi means you are probably losing a crapton of velocity out of a 16" barrel.  Maybe I am doing the math wrong, but a 16"/40cm barrel of 6.8mm caliber is going to have an internal volume of roughly ~14.5cm3.  Assuming the same case capacity as .308, 3.6cm3, that is only 4x the volume of the case, 5x including the case itself.  So even assuming an ultra fast burning powder to achieve that 80kpsi, all the combustion happening in the case itself, you are still going to have an exit gas pressure of 1/5 of that, or 16,000psi.... which if I recall correctly is like a 6-8" barrel on a 5.56, and we know how efficient and pleasant that is. 

Plus that pressure has got to heat up barrels fast, and I bet the cookoff threshold is going to be super low.  It will be loud as hell, and yeah you can mount a suppressor but that only exacerbates the heating problem: there is much more internal surface area absorbing heat vs external surface area radiating it.  And how much suppressor would be required to get the sound down to the level of a naked 20" 5.56 exiting at 4-5k psi.....I'm tired of math though. 

All this for something not producing any more energy than than a 70 year old AR-10, with close ballistics to .308 (at least with good ammo), and saving maybe a pound-ish of ammunition weight per soldier load, maybe less vs. steel case .308.  It seems utterly bananas to me, other than as a way to spend a lot of money, then get some people killed, then time to spend some money again. 

gatnerd

From: gatnerd

6/2/21

JesseH1234 said:

7.62x51 is already losing a fair bit of velocity shooting out of a 16" barrel.  Cranking the pressure to 80k psi means you are probably losing a crapton of velocity out of a 16" barrel.

Per the SIG 6.8 Briefing awhile back:

SIG Rifle 13" = 2850fps

SIG LMG 16" = 3000fps 

A pretty big bump in velocity for 3"; +50fps per inch. 

Assuming that held true, we'd expect 3200fps from a 20" barrel. Probably closer to 3150fps as velocity increase would likely get lower as barrel length increases due to drop off in bore pressure.

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