I'm speculating now, but if they clarify that anything modern and over 200k in use is protected, a consequence could easily be that all "assault weapons" bans are unconstitutional. Hope they clarify that accessories to protected arms, like standard magazines, are also protected. Yes, I'm getting my hopes up!
Setting a number held cannot be a true threshold because then the states can simply ban any gun not in common use and not allow new weapons to attain the required sales.
The truth is that the standard is actually "commonly held for lawful purposes" therefore even a handful of a certain weapon would place it under protect unless those weapons were used for nefarious purposes more often than not.
From Heller:
"Miller stands only for the proposition that the Second Amendment right, whatever its nature, extends only to certain types of weapons."
"We may as well consider at this point (for we will have to consider eventually) what types of weapons Miller permits. Read in isolation, Miller’s phrase “part of ordinary military equipment” could mean that only those weapons useful in warfare are protected. That would be a startling reading of the opinion, since it would mean that the National Firearms Act’s restrictions on machineguns (not challenged in Miller) might be unconstitutional, machineguns being useful in warfare in 1939."
So, Scalia indicates that if one holds to Miller, then machine guns are very likely protected - which the court doesn't want.
It goes on to say:
"United States v. Miller, 307 U. S. 174, does not limit the right to keep and bear arms to militia purposes, but rather limits the type of weapon to which the right applies to those used by the militia, i. e., those in common use for lawful purposes."
But this again opens up machine guns to protected status since the vast majority held in private hands are held by law abiding citizens for lawful purpose. Even if the news promotes stories of inner cities being inundated with glock switches, those events don't come close to eclipsing the number of legally held NFA items (both FFL and transferable)
Where does this take us - Cargill is key and keeping the court for the next decade is of utmost importance if we are to keep and restore our 2a rights