As a quick point of comparison from a hotel's lobby computer, the L/44 120mm smoothbore gun mounted on an M1 Abrams is, canonically, a Light Rifle. 3 damage, -3 damage if used against armor with a BAR rating higher than 6.
Modern weapons simply do not damage BattleTech armor. Like, at all, unless you're using tremendous quantities of explosives, and even then the damage is minimal. Gotta go.