You can use the same suppressor on multiple firearms. The tax is for the suppressor itself. They don’t have to be the same caliber. A .45 suppressor on a 9mm pistol doesn’t suppress as well as a 9mm suppressor, but it does reduce the decibels.
Actually, I have a Specwar762 for my 308 (and 5.56) and the listed sound rating is the same or lower than the Specwar556. I am guessing larger overall volume offsets the larger hole.