Was super curious to see how you did the game in Python and downloaded the source and found the source of the bug:
File "momentus-proto-master\Player.py", line 212, in get_keys if self.itemA.check == False: AttributeError: 'Pistol' object has no attribute 'check'
It seems "game.machinegunpick" is being checked in the player update before "self.itemA" is being set to the machine gun class in the main game update.
Hope this helps!