Start the game with the -edit command line parameter, then place the pointer in the game window and press SHIFT + Z, then press TAB to start game play.
Note: This procedure requires editing a game file; creating a backup copy of the file before proceeding is recommended. Using a text editor, edit the "lisp\options.lsp" file in the game folder. Enable mouse weapon switching in the file. Select any weapon other than the laser. Hold the Right Mouse Button when shooting that weapon and the ammo counted for that weapon will be taken from the weapon on the left side.
