How do you get nifenet emow for a gun in Resident Evil 5

well first you learn how to spell.
then you completely upgrade the gun you want unlimited ammo for.
then you finish the game.
then, from the same menu used to unlock the figurines, you will be able to 'purchase' unlimited ammo for the fully upgraded weapon (and any duplicates you have)

you will also have to turn it on in the 'bonus features' section, or wherever you alter costumes and game filters.

then as you start up gameplay, you will have the option of enabling unlimited ammo or not.