Today I installed some Arredondo basepads onto Lou’s Glock. These basepads add 5 rounds to the 15 round .40 factory Glock magazine. That’s 21 (20+1 in the chamber) rounds of .40 S&W firepower! The longer baseplate also makes the mag easier to seat into the magwell:

Additionally if the baseplates are are used on a 9mm G17 magazine, it adds six rounds to a 17 round magazine, giving you 23 rounds plus one in the chamber.