For unlimited ammo, find the weapon's folder in
game guru\files\gamecore\guns
and then locate the gunspec.txt inside and open in notepad.
somewhere near the beginning you will find a setting for
reload=x where
x is the number of shots it can hold.
reloadqty = -1
will give unlimited ammo. and the player will never reload or run out of ammo. This is typically used for melee weapons. If you want the player to still need to reload but not run out of reserve ammo, you will need to script that up as smallg suggested.
Gigabyte P67A-UD4-B3, Intel Core i7 2600K, 16GB Corsair DDR3, EVGA GTX 970 SC, Win7 Pro 64-bit SP1, Primary monitor @ 1920x1080, secondary monitor @ 1024x1280