Choate Tool Mossberg Sa-20 + 3 Shot Magazine Extension. Includes Magazine tube Spring & Barrel Clamp. On some of the SA20 guns you will have to remove the crimp at the end of the magazine tube. This can be done by filing or using a grinder. DOES NOT COME WITH SHELL FOLLOWER. Specifications for Choate Tool Mossberg Sa-20 Shot Magazine Extension: Manufacturer: Choate Tool Gun Make: Mossberg Condition: New Country of Origin: USA Features of Choate Tool Mossberg Sa-20 Shot Magazine Extension steel construction. Includes mag extension mag tube spring and clamp. Package Contents: Choate Tool Mossberg Sa-20 Shot Magazine Extension Choate Tool CMT-02-04-43: Mossberg Sa-20 + 3 Shot Magazine Extension