Stanton opined that a ban on M-16s can stand under scrutiny of District of Columbia v. Heller (2008), because such guns are “outside the scope of the Second Amendment.” She then turned to a ban on AR-15s, saying, “Thus, if a weapon is essentially the same as the M-16, it is not protected by the Second Amendment merely because gun manufacturers have given it a different model number and dubbed it a ‘civilian rifle.’”Somewhere in the 2nd there must be a "scope clause" that defines the limits as to what sorts of armaments a person may own. I'm looking at the word "people" and not seeing much of a limitation there, and the word "arms" and seeing no definitions or limits there either. Back when the amendment was written, people owned wall guns and field artillery so if we must invoke limits, allowing for modernization that would cover up to 155mm artillery and rifles presumably up to 20 or 30mm.
This looks to me like a good candidate for the Supremes, although even with todays court, giving them an opportunity to make such a ruling strikes me as rather dangerous.