The M4 Benelli is, far and away, my favorite shotgun, and the best I've ever used. Here's a video on modifications I've made to it. The only other thing I've put on it since this was shot is a full length magazine tube, for a total of nine shell capacity. 7+1+1.
Tricking Out a Benelli Shotgun - Modifications to an M4 Benelli - Military M1014 - YouTube
Tricking Out a Benelli Shotgun - Modifications to an M4 Benelli - Military M1014 - YouTube