Three Glocks in the most common, if not popular U.S calibers. The 19 is 9mm, the 23 is 40 S&W and the 21 is .45 ACP. Both the 19 and 23 are smaller, more compact versions for ease of concealed carry. Note the aftermarket slip-on grip. The grips on Glocks have long been a design problem, with almost a 50/50 split on people who love and hate them.