(a) Whoever commits an assault and battery upon another by discharging a firearm, as defined in section 121 of chapter 140, shall be punished by imprisonment in the state prison for not more than 20 years or by imprisonment in the house of correction for not more than 21/2 years or by a fine of not more than $10,000, or by both such fine and imprisonment.
[There is no subsection (b).]