That method might not work in every case. My first ever ASM hack did the same thing with HP in Pokémon, and what I found was if an attack wasn't strong enough to kill you, you wouldn't lose any health, but if it was, you'd still die, because the game would still calculate your resulting HP to be zero and run the death routine. If you kill the calculation instead (the ADDIU in this case), it works as expected (all attacks do 0 damage).
So I'd go with these instead (not tested, but I see no reason they wouldn't work):
81175AFC 2400 Infinite ammo
81175AFE ???? Amount added to ammo when shot (default -1)
Someone should look for some pointers to the item each player is holding, then we could make fun stuff like any ray gun player 1 holds has infinite ammo.

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