While playing solo, knife the entire 1st round and open the top door that leads to a possible mystery box. If it is there, throw a grenade under it and walk away. Wait for it to blow, then go to it but have your back facing the box. Buy it and stay looking away! When it says trade weapons,buy with your back facing it! By doing this, you get a Python, Thundergun, Commando, and AUG.