You must be in forge to do this, and you can't be party leader if you wan't to do the glitch.

1.Go to vehicles in the forge list.

2.Pick a vehicle with a turrent(I prefer a warthog) don't spawn it yet but keep it highlighted.

3.Then the party leader must start a new round, press A when the screen goes black.

4.Find the vehicle then, if you did it right, the vehicle should have no turrent(It's more obvious if it's a warthog).

That's the Moding Glitch!