The provisions of this paragraph shall not apply to the possession of a large capacity weapon or large capacity feeding device by (i) any officer, agent or employee of the commonwealth or any other state or the United States, including any federal, state or local law enforcement personnel; (ii) any member of the military or other service of any state or the United States; (iii) any duly authorized law enforcement officer, agent or employee of any municipality of the commonwealth; (iv) any federal, state or local historical society, museum or institutional collection open to the public; provided, however, that any such person described in clauses (i) to (iii), inclusive, is authorized by a competent authority to acquire, possess or carry a large capacity semiautomatic weapon and is acting within the scope of his duties; or (v) any gunsmith duly licensed under the applicable federal law.
(n) Whoever violates paragraph (a) or paragraph (c), by means of a loaded firearm, loaded sawed off shotgun or loaded machine gun shall be further punished by imprisonment in the house of correction for not more than 2 1/2 years, which sentence shall begin from and after the expiration of the sentence for the violation of paragraph (a) or paragraph (c).
(o) For purposes of this section, “loaded” shall mean that ammunition is contained in the weapon or within a feeding device attached thereto.
For purposes of this section, “ammunition” shall mean cartridges or cartridge cases, primers (igniter), bullets or propellant powder designed for use in any firearm, rifle or shotgun.
Source - http://www.mass.gov/legis/laws/mgl/269-10.htm