This is a GG&G, Inc. Shotgun Magazine Tube Components known as the Aluminum, model GGG-1546.
This 2 shot tube adds two additional rounds without increasing overall length of the shotgun. The end cap of the magazine extension tube can be removed separately to facilitate cleaning and the use of a magazine plug.
Features:
Fits 12ga Remington 870, 1100 and 1187
Type III hard anodized
Non-reflective matte black
