This also has been covered many times in the Gun Laws forum. The long and the short of it is that if the firearm is operated by a bolt, lever or pump, it is not considered to be semi-auto, and therefore not covered under the Mass AWB. The ban clearly states semi-auto as a requisite for inclusion.
I bought the pistol grip and pump grip for my Mossy 500 a while ago, along with the barrel shroud. That's my home defense weapon. In 10 minutes, I can restore it to its original configuration for trap/skeet/hunting. It adds yet another dimension to the configurable Mossy 500.