This trick requires the Disarming Shot skill in multi-player mode. Have your partner equip and pull out his weapon. Use Disarming Shot to "steal" his weapon. Once the weapon has dropped, have your partner go to the Xbox Dashboard. This will not save his character, and you will have a "copy" of his weapon. This is very useful if you have many weapons you want to duplicate. Just have him equip it, shoot him, equip a new weapon, shoot him, and repeat.