Increased pressure stabilizes the follower and improves feeding, especially on high-capacity magazines. Glock - Hi-Cap 17 fits 17, 17L, 20 (15-rd), 22, 31 (15-rd), 34 (17- and 19-rd), and 35 (15-rd). Hi-Cap 19 fits 19 (15- and 17-rd), 21 (10- and 13-rd), 23 (10- and 13-rd), 26, 27, 29, 30, 33 (10-rd). Big Stick fits high capacity, 31- and 33-round magazines.