Glock Factory +2 Magazine Extensions for Glocks 17/22/31 This Glock Factory +2 (40 caliber) or +3 (9mm) magazine extension is combined with an extra power high quality spring to help in proper feeding. Includes everything you need to increase the capacity of your existing 15 round .40 or 17 round 9mm magazine. If you want more rounds in a factory extension….order this today! Please note: The included magazine insert does not have a button-like protrusion to lock into the magazine floorplate. You can confirm proper installation of this factory part by ensuring the floorplate latches onto the two plastic notches on the side of the magazine tube.