Paul: Maybe I do not explain correctly how I use the rubber gasket.
I bought in a harware store for US 10 cents a piece; for a 12 ga the 1" diameter adjust perfect to the outer magazine tube.
I trim the outside diameter of the gasket to fit the contour of the barrel guide.
To assamble, you put the spring and all the friction rings and conical washers as normal, then the barrel and, AFTER you put the barrel, the rubber gasket.
The rubber piece must be between the barrel and forend.
When the barrel returns at the final of the reloading cicle, it hits hard the forend, but this rubber gaskets dampen the smack to avoid craks. In my shotgun, that is a Breda semiauto, not a Browning A5, it preserves the integrity of the forend wood just nice.
I hope this new explanation clarify the use of the gasket.
Best,
