The Joker uses a Glock 17 that has been converted to full-auto, as the gun lacks the Glock 18's firing selector switch on the side of the slide. His Glock is fitted with a stainless slide and a G18's standard 33 round magazine. He is seen using it as his main weapon several times in the film. For someone as bizarre as The Joker, the gun fits him well as it is very rare, destructive, stylish although in some ways impractical. The extremely high cyclic rate makes it difficult to control, although in the hands of an experienced shooter, it is all the more deadly. The armorer referred to the gun as a Glock 18, but this is less of a mouthful then "Glock 17 converted to full-auto to resemble a Glock 18". Not to mention, he likely assumed no one would know the difference anyway.