taken from another discussion
When a guy is pulling the trigger and sending lead down on his targets, any interruption, no matter how brief would be welcome. Am I wrong about that? When firing the next round requires only another trigger pull, that next round is almost guaranteed. But when it requires a magazine change, there's at least a momentary delay plus the chance of fumbling the mag, dropping it, anything could happen, all of which are better than another trigger pull and another round fired.