As we've seen the development, design and worldwide acceptance of every Glock model and caliber, one model has all but been ignored - the G18/G18C. These are Glocks that offer the option of fully-automatic fire. The Model 18 has three external changes from the original Model 17: a left side, slide-mounted fire control or selector switch; a barrel that extends past the front of the slide and three horizontal and diagonal cuts that run across the top of the barrel to act as a compensator (the same design used on the 17L and 24 compensated models).
The newest Glock 18, the G18C, has a keyhole opening cut into the forward portion of the slide, not unlike the opening on the Glock long-slide Models 17L and 24, although the G18 has a standard-length slide. The keyhole opening provides a venting area to allow the four, progressively-larger (from back to front) compensator cuts machined into the barrel to accomplish their job, which is to afford more control over the rapid-firing machine pistol.

The Glock 18 is a pretty rare gun, as far as I know there are less than 20 in the UK, and even worldwide they are rare due to the restrictions on fully-automatic firearms.