I see it as a means of backing into a pure "AW" ban. If ammo can be used in a regulated firearm, then it will be. Thus, if you own an evil "AW," you can only shoot special ammo through it. If you own an evil "AW," and a hundred other firearms (oops, I mean weapons) that take the same cartridge, then you must only possess the special ammo. But, if you get rid of the evil "AW," then you may lawfully possess regular, non-serial numbered ammo.
I'll bet the authors wish that no such ammo is ever produced.
Also, I wonder if any other state has an "AW" list that looks like this, or has Mass once again reinvented the turning disk device?