Mass. Gen. Laws ch. 147, § 8A: Carrying police weapons and equipment

Section 8A. A sheriff, any of his deputies, and any officer of any institution under the supervision of a sheriff if so authorized by him, the commissioner of correction, and the deputy commissioners and any other officer of the department of correction or of any institution under its supervision if so authorized by said commissioner, the chairman of the parole board and any other officer of the parole board if so authorized by said chairman, and the penal institutions commissioner of the city of Boston, his deputy, and any officer of any institution under the supervision of said penal institutions commissioner if so authorized by him, may carry revolvers, clubs, handcuffs and twisters and such other weapons as are necessary in the performance of their duties.