Neat. 300 blackout can be hard to tune, since it's meant to be a CQB weapon that fires subsonic very quietly but can also run supersonic to reach out to 300 yards. A piston makes more sense, since tuning isn't an issue here. Of course, it's a Sig, so that's counterproductive to reliability.
A few points though:
1.) The HK416C project was cancelled. HK deemed the short buffer setup too unreliable.
2.) Honey Badger don't care.