In the appearance and weapons customization menu, if you have the heavy body-type selected, and you select a heavy machine gun as your primary and an assault rifle as secondary, then change your body type to Light or medium, you still have the heavy machine gun and assault rifle. So you can run around doing parkour and jumping from wall to wall while killing with a heavy machine fun XD

Though this is fun to play with this, it's still a mistake and should probably be fixed