A smaller-diameter bullet should penetrate heavy clothing more easily than a larger, although I don't think the small difference in diameter would be that affecting. At any rate, the current penetrator round, which needs work, will penetrate a Kevlar helmet at 500 yards or so, so penetration doesn't seem to be a problem.
The "SPC" was supposedly designed for "special operators." It was the idea, maybe, of someone who had done spec ops, but the AMU developed the round, along with Remington. I guess Remington had invested in the developement, and tried desperately to sell the concept through a bevy of magazine articles. None of them I read reported how the weapon did on full auto, which was a concern of mine.
And the Lt. Col. head of the AMU said compared to all the people using the M 16, the "Spec Ops" guys may number in a few hundred or a few thousand at most, and to introduce a round for them with all the ammo companies now operating a full capacity, 24-7, to make current ammo requirements, just not the time to introduce a new round. Anyway, the 7.62 NATO is a better round for penetration, and on an AR platform performs well with available stores.
Personally, I feel the 6.8 was a snow job which Remington tried to market, but for the most part, failed.
I fired one of the piston-type gas systems, I believe a FN type adapted to an AR. It had a considerable amount more recoil than I expected. Not that it had a LOT, just quite a bit more than I expected.
I don't have as much issue with the current system for the M 16, which does blow gas out, but blows most of it where it can be removed easily. It's a far less ammo-sensitive mode than a piston system is, although the piston system certainly works well, apparently.
Pistons don't eliminate grime, they just confine it. An M-14 dirty piston would freeze the gun up, and if the Inspecting Officer couldn't hear that piston move when he turned the rifle upside down, you got gigged for it. Blanks were the worst offenders.
