This is the law M.G.L. c.140, § 131.
This is the CMR 501 CMR 7.00
Not to pick nits, but for the sake of accuracy, the statute that prohibits the sale by a FFL of non-approved handguns is MGL 140-123, Clauses Eighteenth to Twenty-first, inclusive.
The penalty for violation of such is found in MGL 140-128.