locchau,

Here are the original Bren sizes for the gas system. You can quickly check the barrel hole, exit port & #4 port to see if the previous owner had drilled it out to Historic Arms spec.

The Bren barrel hole is .150" as is the #4 port on the regulator. The exit nozzle on the gas block is .161". The regulator #1 port is .101", #2 - .116", #3 - .136", #4 - .150"

Possibly the Historic Arms Bren required the gas port to be drilled out to function properly.

Joe