Section 21A. Whoever, with intent to steal a motor vehicle, assaults, confines, maims or puts any person in fear for the purpose of stealing a motor vehicle shall, whether he succeeds or fails in the perpetration of stealing the motor vehicle be punished by imprisonment in the state prison for not more than fifteen years or in a jail or house of correction for not more than two and one-half years and a fine of not less than one thousand nor more than fifteen thousand dollars; provided, however, that any person who commits any offense described herein while being armed with a dangerous weapon shall be punished by imprisonment in the state prison for not more than twenty years or in a jail or house of correction for not less than one year nor more than two and one-half years and a fine of not less than five nor more than fifteen thousand dollars. Whoever commits any offense described in this section while armed with a firearm, rifle, shotgun, machine gun or assault weapon, shall be punished by imprisonment in the state prison for not less than 7 years.