Going too heavy can affect the slide return. When the slide kicks to the rear, the barrel angles upwards just like on a real glock. However we don't have the return spring found on real glocks, simply because there isn't enough recoil to warrant it. So with that being said, when you hang something heavy onto the barrel such as a heavy compensator, the outer barrel meets resistance with the slide opening and may not fully cycle as you begin to run out of gas. Depending on the weight, you can probably expect going from 50 shots with a full green gas mag to drop down to about 30 shots before the slide won't fully cycle.
