Switch to your secondary weapon(Best would be Anaconda, M79, Flare Gun, Desert Eagle.) press and hold R, while holding R switch to your primary, now your primary fires your secondary's shot.(Like using the minigun and the flare gun, the minigun now shoots the fire from the flare gun)
I think it's the only cheat, but there is so many combinations, FN2000 is the only weapon with M79 that does splash damage, any gun that has an alternative attack(use with space) can use flare gun's alternative attack(napalm rocket) using the w/e your main gun, hold R, switch to knife(always key 3) and it automatically does 80 damage, shoots straight and has no kickback, get any gun with a bayonet and have your secondary as the flare gun, hold R, switch back to main gun WITH BAYONET, and hold space, you have infinite napalm rockets.
