This glitch, also known as "the paintbrush glitch," allows you to carry a detached forearm and strike enemies with it. You can exploit this glitch in singleplayer and multiplayer, with any character or player slot and any map where you can pick up the sniper rifle. The fewer guns that you have the simpler this trick will be... First, unarm yourself, with only your hand showing, and make sure you don't already have the sniper rifle in inventory. Now, find a sniper rifle and pick it up. If you had no weapons before picking up the sniper rifle, quickly tap the change weapon button (A-button by default) two times. If you had one or more weapons beforehand, the amount of times you will tap the (A) button depends on how many weapons you were carrying. If you had one weapon, tap three times; if you had two weapons, tap four times; and so on. (Think: number of weapons plus two.)... You can wait to start tapping although once you do start you'll need to be quick about it. If the trick doesn't work you'll have to die or restart the map then try again. Once successful you will be carrying a blue-suited forearm for use in melee combat. You will lose it after changing weapons.