Full auto 300?

[82nd airborne]

That, sir, depends on many variables including but not limited to barrel material, surface finish, dimensions, buffer, spring, chamber, yada yada. However, I would guess it to be about 0.109" to 0.111" diameter for a 16" barrel with a carbine length gas system.

[300Blk]

Or even larger if you want to use an H2 buffer.

[rockrivershooter]

Integrally suppressed 300blackout

https://www.youtube.com/watch?v=UJbV-uA ... ata_player