So, looking at "Handheld Weapons" from TacOps (single volume: bottom of page 314 for the construction rules; TO: AU&E: bottom of page 126), it seems a 'Mech Mortar/1, a half ton of ammo for 12 shots (even when Fractional Accounting is not otherwise in play), and a half ton of armor (8 points) to protect it is a thing. The Inner Sphere version would be three tons total, while the clan version would be two (enabling Stingers to take advantage). BV for both appears to be 27 (10 for the weapon, 16 for the armor, 1 (rounded up) for the ammo)
A Commando could make use of an IS version with 5 points of armor (to keep Medium Lasers from breaking it) with 4 shots (BV 21? Maybe 20...). You could also argue that a Stinger that leaves half its MG ammo at home could use one... ^-^