OK, I admit it. The title was just to get your attention. You and I are not going to get one of these with Title II tax stamps.

It's the Army's Next Generation Squad Automatic Rifle (NGSAR), intended to replace the M249 SAW. Now THIS thing has some potentially serious firepower! They expect chamber pressures to be on the order of a M1 Abrams tank.

Of course the caliber will be smaller, and curiously they're still searching for the right ammo. They think .308 has too much mass and not enough propellant, with .556 being the opposite.

At 60 rounds/minute and an effective range of 2,000 feet against body armor, THIS is an 'assault rifle'. Read. And drool.

What is the relationship between chamber pressure and muzzle velocity?  And more importantly, recoil?   Blasting like an M1A3 sounds great until you have to hold and aim the damned thing. 

Generally speaking the higher the chamber pressure the higher the velocity.

STANAG specs give about 75,000 psi for most rounds fired by the 120mm on an Abrams tank.  This gives the discarding sabot rounds (one of the lightest IIRC) a muzzle velocity of over 5000 fps.

There are several factors to calculate recoil for different purposes.  They all consider the total ejected mass (projectile and powder) and velocity of that mass.  Higher velocity equals more recoil.

The easiest way to deal with recoil is add more mass to the launcher.  Fire a 357 mag in a j frame and then in an N frame and you can feel the difference.

Recoil systems in artillery and tanks are another matter.

I love how AR/NATO calibers (for civilians) went from .308 being too large, 5.56 being too small, then coming up with all the .300 blackout and grendel and 6.5 or whatever are the hot new must-have calibers....Dude just get a 7.62x39  The answer was right there all along. 

Here's a note from my friend who is a former ballistics/explosives developer and tester:

"60-80,000 PSI in a rifle does not equal the same energy as a 120 mm tank gun.Plus the chamber pressure is going to cause rapid wear on the barrel. And I mean really rapid wear!"

I think I knew the energy part, even as a civilian. However the comparison to the previous rifle versions is, I think, impressive. And of course the users don't have to pay for the replacement barrels. We do!

You can't feel pressure by itself, you have to integrate pressure over the whole surface.

 

So as a gross approximation, a 120mm round is 17.5 in^2 across its base, times 60ksi is 1 million pounds of peak force, propelling the round forward and the gun backward (conservation of momentum aside for now).

 

A 7mm round is 0.06 in^2, times 60ksi is 3579 pounds. Still sounds like a big number but it's not even in the same league

 

To get MV you'd have to divide the propelling force by the projectile mass, subtracting bore resistance. But, again, it's an integral calculation because the propellant doesn't push consistently as it burns. The upshot is higher pressure will likely mean higher velocity but unless they're doing something crazy with the propellants or adding sound suppression they'll never get it to be hearing safe

M249 SAW is really cool to shoot.  Have done it indoors at 25 yards (frangible ammo).

"The Army would like the new weapon chamber pressure to be between 60 and 80 KSI (kilopound per square inch). To put that into context, the Army’s M1 Abrams main battle tank fires at that chamber pressure. Assault rifles tend to be around 45 KSI, so this will definitely be higher."

 

Uhm.. 5m855a1 is already at about 63k psi. 

 

People say stupid shit and then put it into articles. 

Thought for sure it was gonna be this, https://www.amazon.com/BUG-A-SALT-2-0-from-Skell-Inc/dp/B00STSZ77G