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I built a custom 300BLK SBR with a 10.5 inch barrel because the powder is fully burned at 9.5 inches. The 10.5 barrel leaves room for a suppressor when shooting subsonic rounds without being too long. Picking the right gas block position is also finicky. Supersonic rounds make plenty of gas for the carbine gas port, but the subsonics do better with a pistol gas port placement. I built with the carbine gas port since those barrels were easily sourced in 2015. I can buy an off the shelf upper today, so no need for a custom build.

Mine is a remington M700 300 blackout with 16.5” barrel. I use it to hunt deer and it being a bolt action means I can run faster burning powders which are quieter and not worry about port pressure in an AR platform. The AR’s can be particular about powders when running subsonic ammo because you’ve got to make sure you’ve got enough pressure at the gas port to cycle the action as you know. With the bolt action I don’t have to worry about any of that.