I happen to walk in on that conversation at LGS.What PappyM3 said, the AR10 can't be a copy or duplicate because it pre-dates the specifically named AR15. Now if an FFL would chime in that they are willing to sell it in MA, assuming it's feature compliant. Maybe someone should do a group buy? At least that way the legal costs can be spread out, I would hope Comm2a would get on board as well.
Legal status aside, the owner was telling the customer he is not going to sell any semi auto with detach mags that shares any trigger or bolt parts. He went on to say if and when the AG decides to bust LGS balls no matter what the law says it cost the shop money when the AG comes poking around.