Bushmaster sells 6.8SPC rifles now.
They also sell gas piston retrofit kits for $400.
I guess what I’m most concerned with is making sure an enemy combatant can’t get off another shot or two after being hit. With a 7.62 that isn’t much of a problem. A 6.8 would be a step in the right direction, but I’m not sure why it would be better than going all the way to a 7.62.