This could go on forever, so let me see if I can clarify a bit:
1. From time to time I had someone ask me, "Can I sue?" My answer was: "Wrong question; for $2 anyone can sue. [At one time, the civil action entry fee was $2.] The real question is: Can you win?" The real question entails a prediction of how the court will rule on a given question -- when, as and if that question is ever presented to it. (That is the essence of what lawyers do: predict the outcome of hypothesized future litigation.)
2. The issue of whether 140/122B precludes mail order ammo sales to Massachusetts addressees is actual two questions: A) is that what the statute intended? and B) if so, does it pass muster under the Commerce Clause?
3. I'd say 50-50 on the first question. On the one hand, if you assume the General Court intended to require licensing of ammo sales, it doesn't make a lot of sense to license local sales but say "OK" to mail orders. On the other hand, there is no prohibition on a Massachusetts denizen zipping up to NH and buying the same stuff and bringing it home, in which case all the statute does is penalize local dealers. How would a court rule? BTSOOM.
4. I pass on the second question, as it would require research. For those interested, though, see if what you can find on the statute that bars mail-order wine sales, which was the subject of some notoriety lately.
5. Now, none of this addresses the question of how, assuming the mail order sales of ammo were unlawful but some out-of-stater keeps doing it, a prosecuter gets jurisdiction over the out-of-stater. At one time the rules were simple and easy to administer: you could only get in personam jurisdiction on a person who (or whose agent) you could physically serve with process within the borders of the Commonwealth. More modern precepts are sort of convoluted and complicated, but I suspect that the Massachusetts courts would rule that 223A/3 extends to such cases.
6. Now you hypothesize that the prosecuter sues the out-of-stater (say, for a DJ and injunction) in a Massachusetts court, asserts in personam jurisdiction under ch. 223A, and serves process by mail. The out-ot-stater, back in his home state, utters a hortatory subjective in the prosecutor's direction and ignores the process. The Massachusetts action proceeds to default judgment. So what (so far)?
7. At that point, the Massachusetts prosecutor has to take his default judgment to the out-of-stater's jurisdiction and ask that it be enforced by the out-of-stater's state courts. This will be required if (A) ch. 223A does apply (a question on which the out-of-state court will likely defer to the Massachusetts court), and (B) the out-of-state court decides that so extending in personam jurisdiction is consonant with the mandate of the Due Process Clause of the federal constitution. That is sort of a crap shoot.
8. The bottom line is a pragmatic one. However a court might some day rule, the position asserted in favor of applying 140/122B to out-of-state mail order houses is credible enough that they have elected effectively to concede. As a pragmatic judgment, that is hard to fault.