The downside to the HK UMP Gas Blowback SMG being so fun to shoot is that the rounds in your 30-round magazine go quick. Better grab a few extra mags to stay in the game until you're the last one standing!
It is a universal fact that MP5 submachine guns are a blast to shoot, so making sure you've got enough mags to satisfy your thirst for MP5 trigger time seems like the only logical thing to do.
