[Grizzman]

The hybrid stainless/brass cases are already available to civilians. Someone on m4carbine.net bought some, but without a rifle to use them in, I have no idea why he made the purchase. Maybe he has deep pockets and plans to buy an MC- Spear with a 6.8x51mm barrel.

 

[Grizzman]

The .277 Fury is already available in complete upper assemblies, and likely separate barrels also As far as I know, all that is required to turn a .277 Fury chambered upper into 6.8x51 badassery is either factory 6.8x51 ammo, or the correct hybrid brass and the stones to put enough of the correct powder to reach the desired pressure level.

 

[Grizzman]

At what point does the redesigning of the rifle's barrel extension, bolt, upper receiver, bolt carrier, etc cause it to no longer be a large frame AR?

It's very likely that an AR-like rifle can be created to fire the 6.8x51mm cartridge, but i's doubtful that it could be accurately be called an AR-10, or whatever model name the design came from.

 

[Grizzman]

Large frame AR uppers have been available in Winchester Short Magnum (WSM) and Remington Short Action Ultra Magnum (RSAUM) cartridges for years. Even at 80K psi chamber pressure, the 6.8x51 isn't going to generate more recoil than a full power 300 RSAUM driving a 165 grain bullet at 3100 fps.

Large frame, and even regular, AR lowers aren't fragile. If a person's worried about a 7075 carbine receiver extension failing, they can use a 7075 rifle receiver extension surrounded by a foam reinforced A2 stock....or a UBR.

 

[Grizzman]

300 RSAUM 65k psi vs 80k psi

Joe

Yep, but it's driving a 165 grain bullet at 3100 fps from roughly 65 grains of powder. The 6.8x51 drives a ~140 grain bullet at 3000 fps, likely from about 45 grains of powder....so which is going to generate more recoil? Don't forget that the weight of the powder charge is a factor in recoil.

The 80,000 psi isn't used to generate high velocity, which creates recoil. It's used to generate 3000 fps from only 13 inches of barrel.