Get the grenade launcher. Change to grenade launcher mode. Fire one shot and do
not reload. Switch back to single fire mode. Press H or [Semicolon] (or other 
re-defined key) to switch between the grenade and single fire mode 5 times. 
Then, very quickly press F to fix jam. Your ammunition should go to "-1". Switch
back to grenade mode and reload. You will now have one shot in your grennade 
launcher. Fire that off, then reload. You will now have 255 grenade ammunition 
in the grenade launcher and can fire off grenades when you fire regularly. 
Note: You cannot reload your regular magazine after you waste all of your 
ammunition. You must switch back to grenade mode and use those shots.