This one’s a bit tricky - type “XR31TURBONUTTERBASTARD” while the game is in progress. (Try doing that without getting killed off!) You should now have a large supply of shields, cannon and lasers. They will run out eventually so you have to type it in again.
Another cheat file lists the code with spaces between words! Type “XR31 TURBO NUTTER BASTARD” and hit keys 1-6 to select a level. Then press RETURN for extra weapons.
The 1 may actually be an I.