Let's deal with the conversion question first. A shotgun has a lower threshold than a rifle for being an Assault Weapon.
MGL Chapter 140: Section 121:
“Assault weapon”, shall have the same meaning as a semiautomatic assault weapon as defined in the federal Public Safety and Recreational Firearms Use Protection Act, 18 U.S.C. section 921(a)(30) as appearing in such section on September 13, 1994,
18 USC section 921(a)(30):
The term "semiautomatic assault weapon" means -
(B) a
semiautomatic rifle that has an ability to accept a
detachable magazine and has
at least 2 of -
(i) a folding or telescoping stock;
(ii) a pistol grip that protrudes conspicuously beneath the action of the weapon;
(iii) a bayonet mount;
(iv) a flash suppressor or threaded barrel designed to accommodate a flash suppressor; and
(v) a grenade launcher;
(D) a
semiautomatic shotgun that has
at least 2 of -
(i) a folding or telescoping stock;
(ii) a pistol grip that protrudes conspicuously beneath the action of the weapon;
(iii) a fixed magazine capacity in excess of 5 rounds; and
(iv) an ability to accept a detachable magazine.
Again, I think this is all crap and I don't wish any heat on anyone over it. It's just not for me and there were more than a few of us who thought the shotgun rules were the same as the rifle rules who would have been in the same leaky boat without a reread. Maybe someone wants to be the one who takes it all the way to the US SC.