Basically, any ammo that converts it to a new projectile that is unique does not use ammo, therefore, it becomes infinitely usable. Here is a list:
Bee’s Knees
Silver Bullet (due to new thingy caused by 1.4.4)
Snowman Cannon
Blood Rain Bow (?)
Marrow(?)
Ice Bow(?)
Shadowflame Bow (?)
Celebration (?)
Celebrationmk2 (?)
Any sentry weapon in the OOA that uses Eternia Mana (all tavernkeep sentries only during event)
Anything with (?) means that I have not tested and therefore may or may not use ammo, and is unconfirmed.
Feel free to edit with any new ones that are confirmed and remove the (?) for any on the list which are confiremed/unaffected.
List them in the comments as well if you do not have regular.