Dumb Question but Relevent

[grayfox]

If none of them sold, there would not even be a web page entry. Companies won't spend webpage space on non-saleable components. Parts are lots of times batch run, particularly the better quality ones. Mfrs have other barrels to do etc, same with ammo. Reloading components as well.
As the seasons go along stuff comes into stock. Also, with some states and their craziness (like Va), runs on parts do happen.
Quick search on ammoseek showed 423 entries for factory ammo.