I have a Breda 710 12ga, which I believe is the same as a pre-MK II. My question is how to install the washers and friction ring(s) to 1.) operate lighter loads, and 2.) operate heavy loads. I have 2 barrels, one marked Magnum, and the other unmarked. Does the Magnum marking indicate a 3" chamber? This is an older gun, possibly 50's era. It looks very similar to a Browning A5 system of long recoil.