Statistically, with a traditional pistol caliber, unless its a head shot, you have a 90% survival chance given immediate medical attention. Not so with a rifle round. The higher velocity changes the damage mechanics. That is why a rifle is always preferred to a pistol, and the only reason you have a pistol is to fight your way back to the rifle you never should have dropped. Unless its impractical to have the rifle of course, like while walking the mall.
In a shoot out situation where you are likely to take one or perhaps two rounds, then certainly a rifle round is deadly due to increased muzzle velocity, just as that’s true of a pistol caliber carbine. In an ambush situation where you are quite likely to take 4 or more rounds the chances of survival drop dramatically. In that scenario dead is dead. I also do think, for me at least, a full stock isn’t really “much better” than a brace to shoot from the shoulder with this weapon. I’ve shot 5.56 from a rifle/carbine using a classic wooden stock, full Magpul stock (with cheek riser), a skeleton magpul stock and a hand brace. I shoot the same with each out to 100-150 yards. There’s not enough recoil with a 5.56 to make much, if any, difference. Once you step to 30 cal. that can change.