I'm wanting to build an upper for 300 BLK shooting super and sub, running suppressed and not suppressed. I've got my eyes on AAC's 12.5 inch barrel that I plan on having cut down to 10.5 inches. I also am wanting to get Adam Arm's piston conversion kit for the gas block and bolt carrier. But do I need the carbine or pistol version for this setup? In case it matters for some reason, I'm also running the Brigand Arms Atlas 9" handguard.

Pistol:
http://www.adamsarms.net/pistol-length-piston-kit-micro

Carbine:
http://www.adamsarms.net/carbine-length-piston-kit-micro

Usually the 300BLK uses a pistol gas tube since the subsonic ammo doesn't have much gas.

Pistol tube

I'd go with a pistol length gas system and just throw an adjustable gas block on it so you've got the extra pressure if you need it.

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