I'd actually shoot Innovative an email and ask about WAR Uppers and 300BLK....their site says 5.56 but the demand for 300BLK has changed since they were released and it may not have been considered at the time.
I've got a 10.5" AR and I want a WAR for it because I'm sure the buffer and carrier are getting a workout as is
It's got three settings: unsuppressed, which allows a normal amount of gas through. Suppressed, which blocks about 80% or more of return gas, and then you can turn the switch in between those two and stop it entirely and single shot your AR if you desire.