this black mesh side rail brass catcher attaches easily to a forend picatinny / weaver side rail or quad guard, to catch shell cases as they are ejected allowing you to easily save the brass. Work on bolt action rifles by making sure the catcher bag opening is fitted just short of the closed bolt. * For wood or plastic stocks, just fit a picatinny/weaver rail to the side. (I have 4 and 5 slot rails listed separately) Slide length is adjustable from 0-45mm. Catcher opening is 115mm long x 48mm wide. The zipped mesh bag measures 150mm wide x 190mm long and the zip can be left open to act like a brass deflector and drop the brass in a controlled area or into a container keeping the brass clean.