I have a Sub 2000 with a Beretta grip in 40 S&W and have wanted a way to mount a spare mag on the carbine but did not like the one offered by KT. I am not sure if they are available for the Beretta mags anyway. I felt the best place to place the mag was in line with the forearm along either the left or right side. After playing with it I found that the best place for me is the right hand side forward on the grip. I made one out of Kydex, it wraps around the forearm and is held in place with one of the forearm screws. It is not only extremely fast to reload from the position but the mag is held securely and not at all in the way. I did use PVC cleaner and cement after forming the Kydex with a heat gun and it seems to be solid. I can adjust the tension by applying the heat gun and tightening or loosening the fit of the mag. The pics show the holder with a 16 round mag in place (actually a 20 round 9mm Beretta mag that holds 16 rounds of 40 cal with no changes and feeds fine). The tape you see around the mag is not used to hold it to the carbine but used on the mag to make the cheap mag fit the mag well better.
http://www.nccops.com/ktrange/images/currie/magholder003.jpg
http://www.nccops.com/ktrange/images/currie/magholder004.jpg
http://www.nccops.com/ktrange/images/currie/magholder007.jpg
http://www.nccops.com/ktrange/images/currie/magholder008.jpg



http://www.nccops.com/ktrange/images/currie/magholder003.jpg
http://www.nccops.com/ktrange/images/currie/magholder004.jpg
http://www.nccops.com/ktrange/images/currie/magholder007.jpg
http://www.nccops.com/ktrange/images/currie/magholder008.jpg