This glitch will not work in local games. Pick up a weapon in player mode (Spartan Laser, Shotgun, and sniper rifles are the only weapons that have been tested and known to work so far) then fire your weapon and go to Forge Mode at nearly the same time (only a fraction of a second should be necessary). If done correctly, you will have lost some ammo, but then in about a second it will return, as if you never fired.