only in horde, easy way to do it!
horde on checkout, sign in a guest account, friendly fire mutator on. when you are on the map BEFORE you build a command post give the money from the guest account to your character. then put your character standing in the corner, use the guest accout to kill yourself. then just keep spotting your own weapon as the guest account picks it up.
make sure one of the accounts has a retro and one a lancer, if you have the same weapon it wont work