For anyone that is planning an SBR or pistol upper .....
I got a Model 1 Sales 300 barreled upper. It started out its life as a 16" barreled CAR. The first thing I did was cut down the barrel to 10.5" and thread the end to fit my can (1/2-28)
The first test fire was less than impressive. I learned that with a CAR length gas system and a barrel that ends just in front of the batonet lug, it fires, but will not extract.
I bought a carbine length free float tube, a 7" gas tube and a gas block. I turned down the barrel for the gas block, and drilled a new .110" gas port hole in a pistol length configuration.
I threaded the old gas port hole and plugged it.
I put the front sight tower back on.
It runs great now.