locchau,

The link I posted will give you the specs for the piston head diameter and my drilling trial for the gas system.

Checking the head dia. of the piston will give you an indication if that is part of your problem. I did successfully drill out a regulator to the sizes I posted. The barrel hole and exit hole can easily be done with a hand electric drill. The regulator is another story. It is hardened and has to be annealed (heat to 1600 deg F and slow cool) before it can be drilled with HSS tooling. Each port on the regulator is 2 cross-drilled holes. I did the drilling in a mill and it was very tedious as each hole had to be lined up then drilled, 2 setups per port.

I'd be looking at the springs and any indication of binding before I did anything to the gas system. With no round in the chamber pull the cocking handle back slowly, see if you notice a change in force particularly at the location where the case is just fully out of the chamber.

I've never handled or seen a Historic Arms Bren so I really can't comment on how they should be set up.

Joe