On that M11 if you take off the forend you'll find 3 rings between the barrel and the recoil spring (no O rings) brass friction piece, steel friction spring and a steel friction ring. For light loads the friction ring isn't needed so it's stored at the bottom under the recoil spring. For heavy loads it sits on top of the spring with the concave part up so the brass friction piece sits in it and on firing it squeezes the brass ring tightly onto the mag tube slowing down the
recoiling barrel. I have my fathers 12 ga. I think he bought it the yr. I was born, I grew up getting beat black and blue by that gun. I still have it, still shoots good, still kicks.
H08