This comes in 6 shell, 5 shell and I believe 4 shell sizes. I bought the SIX shell model for a Mossberg 500 tactical. For the Mossberg 500 it mounts as a metal plate by replacing the screw in the back of the receiver and replacing the punch out pin for the trigger assembly and replacing it with a screw. These two screws are used to mount a metal plate to the receiver and then the plastic shot shell holder mounts to that metal plate. It holds shells tight enough, but not too tight.
ONE POSSIBLE ISSUE: If your pump action forend overlaps the receiver on the back stroke it will interfere with the Sidesaddle. You will have to modify or replace the forend to work with the Sidesaddle. For me it was not too bad - I removed the standard Mossberg forend and carefully trimmed the forend with a coping saw and utility knife, then carefully filed and sanded it to look near perfect. Looks great and works perfect with the sidesaddle.