Section 17. Whoever, being armed with a dangerous weapon, assaults another and robs, steals or takes from his person money or other property which may be the subject of larceny shall be punished by imprisonment in the state prison for life or for any term of years; provided, however, that any person who commits any offence described herein while masked or disguised or while having his features artificially distorted shall, for the first offence be sentenced to imprisonment for not less than five years and for any subsequent offence for not less than ten years. Whoever commits any offense described herein while armed with a firearm, shotgun, rifle, machine gun or assault weapon shall be punished by imprisonment in the state prison for not less than five years. Any person who commits a subsequent offense while armed with a firearm, shotgun, rifle, machine gun or assault weapon shall be punished by imprisonment in the state prison for not less than 15 years.