In Forge, enter Edit Mode and bring up the object menu by pressing X. Scroll to the Vehicles category, and move the cursor over whichever vehicle you wish to perform the glitch on. Get the party leader to start a new round. Just after "Round over" is heard, the screen will fade to black, and the menu will stay up for several seconds after "Round over" was uttered. If you start a new round, end the game, or save the map with the turretless vehicle and load it again in Forge or in a Custom Game, the vehicle will be in its normal state.