[Bosco]

6.8 is currently used by some U.S. Special Forces using the M-16 platform. It's a known quantity but it's an improvement as compared to the 5.56's stopping power.

Bushmaster sells 6.8SPC rifles now.

They also sell gas piston retrofit kits for $400.

[DoughtyOne]

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.