/sarcasm on
Just give that particular weapon (crossbow) two sets of skills to use:
1.) Bow skill to load it since you clearly do not load gunpowder-cased bullets into a crossbow and instead load it with arrows... then draw back the bow and lock the ammunition into place.
2.) Rifle skill (100%) to fire it.
If you don't want to train the IP to fire the weapon's ammunition, simply load it with your bow skill and then hand it to an agent to fire it for you.
/sarcasm off