I used the method posted by LitaOsiris (TheDiamondD's method for "A Hero's Duty" has been patched) but I have one important thing to add:
Unequip the weapons you don't want ammo for. If you hit level 5 on a weapon and can't replace it with a weapon that still needs XP then replace it with a gun you already have equipped
, this will turn it into an empty slot and keep you from getting unnecessary ammo. In fact, you could do this all the way down to one gun and level up one gun at a time if you wanted - if it's the only gun equipped it's the only ammo you'll get.
Here's me using the method: