The Glock 21SF Generation 3 has some minor exterior design changes as compared to the Glock 21. The Glock 21SF incorporates three changes. The first is the ambidextrous magazine release, the redesigned magazines will function in both designs, but the old magazines don't have the necessary cut out in the front of the magazines to function in the redesigned weaon. The magazine capacity didn't change: you still get 10 rounds of .45ACP in the mag, plus one more in your chamber. The second the size of the grip frame, it is noticeably thinner from the backstrap to the trigger (Short Frame) . The third is the Glock Rail.