I had read a bit about the .300, but had discounted it because it was stepping away from what was "common". One of my goals is to have a platform with relatively common parts and readily available caliber.
If you are looking for common parts, you probably do not want a piston gun since most of them have proprietary parts and every piston system is slightly different from the other guy's piston system.
As for a readily available caliber, it will probably be a while until you see .300 Blackout in WalMart. On the other hand, Federal, Remington, Hornady, etc. all sell the caliber online. I think the best price I've seen in a quick look is still $0.08 per round though.
And the only difference parts-wise between a .300 Blackout and 5.56mm is the barrel/barrel extension. It uses the same gas tubes, bolts, magazines, etc.
But I definitely understand the desire to stick with 5.56mm.... I just think you are sacrificing a lot of what makes 5.56mm useful once you get down into barrels under 11.5" though.