The problems with the ammo tax idea are many. The first and most obvious one is reloading — many shooters collect their spent brass, then buy components (primers, powder, bullets) and make their own ammo. Heck, some people even make their own bullets using lead and molds (this is pretty easy).
Ultimately, ammo is a fungible, untraceable commodity that be mass-produced by a hobbyist in a garage. It would be like trying to prevent criminals from obtaining Cheerios by taxing them. At best, you’d just create a robust black market for untaxed ammo, along with all of the problems that such markets bring with them.