flipmonkey, he knows how to make an ammo system, I think what he's asking is how you can have multiple sets of ammo. josght, for the perfect ammo system, that's irrelevant (but he will eventually need to learn if there are some monsters that don't die in one hit, but that's not what he asked right now).
You should have two death counters for EACH DIFFERENT weapon. One for clips. One for ammo. So when the dude picks out a different weapon, you have to reset minerals and gas so each set of ammo is independant from each other. You will have some lag time in transitions unless you create mineral/gas replacements for each amount which will take up a lot of trigs. Or you can make it so only gas clips are remaining. So if you have a little left of minerals, those get thrown out while clips stay.
You can bug up the system a little by changing weapons while reloading, but I don't think the results will be too catastrophic.
If you are only going to use a gun and a knife, just have an extra condition that says Knife Switch is cleared for the minusing ammo trigger. That way, when the dude pulls out the knife set the switch. When he puts it back and starts using the gun again, clear the switch.