[PapaBear3625]

Unfortunately, that short barrel is going to seriously rob the cartridge of its power. You can only burn powder so fast, alot of the burn is going to happen as flash beyond the muzzle.

You can make ammo with faster-burning powder, intermediate between rifle and pistol powder.

[SampleMan]

You can make ammo with faster-burning powder, intermediate between rifle and pistol powder.

And why don't they always do that? Because burning all of the powder in a 7.62x51 cartridge in 14" would do two things:

1. Increase pressures to the point that you would need a 14 pound receiver to handle it.

2. Burn up the barrel in about 300 rounds.

That's why they don't pack cartridges full of C4.