Use the following trick to get the MG42 as an SMG, Colt handgun, or grenades. It depends on the gun you are using in the first place. Set up the MG42 anywhere. Then, press [Next Weapon] + the key to un-set the MG42 at the same time. You might get the Colt or grenades. If you want the Thompson SMG, drop either the hand gun or the grenades (pick them up later) and you should pull out an SMG. If you die, it disappears entirely and you cannot get it back.