Mass. Gen. Laws ch. 131, § 66: Shotgun shells, load

Section 66. A person shall not, except during the prescribed open season when deer may be hunted lawfully with a shotgun, have in his possession a shotgun shell loaded with a rifled slug, single ball, buckshot of any size, BB shot or air rifle shot in any place where birds or mammals might be found, except on a skeet, trap or target range between sunrise and sunset; provided, however, that the director shall promulgate rules and regulations prescribing the type and size of such BB shot.