OK, so I broke down and bought some comm' subsonic ammo as a test of my build

[BrianK]

Test what? Testing supersonic ammo in a 300BLK is no test. ALL AR 300BLK firearms should reliably function with supers. But so far I've only tried S&B subsonics in mine and they didn't function the action, my reloads were built to function reliably so I stopped buying expensive factory ammo since I'm inherently cheap and I know I can make better ammo than I can buy. Anyone can make subsonic ammo for manually operated actions (I'll state that again in a bit.). FWIW, S&B supers work fine but again, that's no test.

Today was a nice day and the snowbanks were beckoning me to put some bullets into them for retrieval in a few weeks after they melt. (I can melt them down)

My LGS had all sorts of supersonic ammo but that's not a test of any semi-auto. They had AMMO Inc' /stelTH/ 220gr TMC subsonic @ $30/20 (specifically made of suppressors), and VELO 225gr subsonic poly coated @ $35/20. Yeah, that's why I reload! The stelTH states 1016 fps but no barrel length and the Velo 980 fps, again with no stated barrel length. Neither stated that they were made for semi-autos either (That should be stated one way or the other IMO, but that's one heck of a can of worms to open!). Heck, anyone can make subsonic ammo for a manually operated action!

I also got a mess of UTG mags in. The reviews for them were good and for $6 per I couldn't say no. On unpacking them I can state that they looked good, on a par with MagPul, but of course looks can be deceiving. Anyway, this was a first small test of one of them. They worked fine in this extremely limited test. 

My AR wears a AMTAC CQB can all the time. It has low back pressure so it's almost like no can at all. It does quiet the noise a bit, but it's main job is to attenuate blast. It is NOT ear safe but it's not at all painful to the ears indoors. What it is is short since >50% of it hangs back over the barrel. The 10" barrel with 10" can yields a 14" barrel assembly (approx').

The barrel has a pistol length gas port. I can't remember the port diameter.

The /stelTH/ ammo locked the action back and cycled reliably. The VELO didn't lock the action back so I stopped testing them. IMO it's either 100% function or it's not functioning. Would the VELO function with a more restrictive can in place? I didn't test that 'cause that's not how I'll use this 300Blk, but maybe that would make the difference.

My supers push a 85 gr copper bullet out of the 10.3" barrel @ 2300 fps for 1k ft/lbs, and my subsonic ammo pushes a 225gr cast poly coated bullet @ 1094fps. Both function reliably. Full disclosure, I also have a plinking load with a 180gr bullet that is supersonic @ roughly 1400fps if memory serves. I bought a few hundred of the bullets and the mfgr' shut its doors so I used them for plinking ammo. Like all supers should, yes, they function reliably. There's plenty of gas to work with when it's a supersonic load.