This unit adds capacity to Generation 4 Sub-Compact, Mid, and Full Size Glock high capacity magazines. This magazine extension replaces the original floor plate and internal locking plate and adds two rounds to a 9mm; two rounds to a 40SW and 357SIG, and one round to a 45GAP magazine. This unit will not fit the 10mm or 45ACP models. These units alter the capacity of High Capacity Full Metal Lined Glock magazines. They are made from High Impact Polymer and withstand drop tests at temperatures from -20 F to 350 F.