The winning argument is not about mechanics or design; that is a debate that does not have a clear, undebatable answer, in which case the AG wins.
Rather the winning argument is this:
1) The AWB was enacted in 1998, based on a federal enactent dating from 1994. (Double check I have the dates correct.)
2) From the inception, the AWB was interpreted as not applying to AR-15 "clones" that did not include too many of the enumerated design features, rifles that became widely known as "Massachusetts Compliant ARs."
3) This consistent interpretation continued for 18 years after the Massachusetts General Court [ed. note: "General Court" is the technical name of the Massachusetts legislature] enacted the 1998 law. Millions of Massachusetts Compliant ARs were manufactured and sold to Massachusetts residents, and each and every one of these sales was (as required by law) registered with the Executive Office of Public Safety. [Double check that EOPS is the recipient of FA-10 forms.]
4) At no time did any executive official claim that the 1998 enactment was being incorrectly interpreted or applied. More importantly, at no time did the General Court enact legislation "correcting" the interpretation or application of its 1998 enactment.
5) It is well settled that long-standing consistent interpretation of legislative enactments that do not generate legislative correction usually become a part of the law itself. This is known as the "Doctrine of Legislative Acquiescence." E.g.,, Apex Hosiery Co. v. Leader, 310 U.S. 469 (1940). Where a long-standing interpretation is held to reflect the legislature's intent, it may not be amended by late-coming executive changes of mind, but only by duly enacted legislative action.
6) The foregoing is the legal reason why the AG is wrong.
7) Quite separately, as a political matter, when the legislature enacts a statute and for 18 years the executive interprets and applies it in a consistent way, and during that time the citizens rely on that consistent interpretation and application, it is fundamentally unfair for the executive, in the absence of legislative action, to reverse the interpretation in a way that makes those citizens unindicted (but plainly indictable) felons. (Yes, the AG says that her new interpretation will only be applied prospectively -- that is to say, she has said that such will be the case for a while, while reserving her "right" to change her mind -- the fact that the AG purports to decree a new, prospective-only rule demonstrates that she is really engaged in unconstitutional legislation, since prospective rule-making is a hallmark of an exercise in the legislative power.)
8) At this point, only the legislature can clean up the mess that the AG has created. It can -- and must -- do so by enacting legislation that (a) explicitly restores the 18-year long interpretation of the 1998 law, and (b), if the legislature deems it appropriate, sets forth with clarity what the rule should be going forward.