It struck me there could be a middle ground between the Thumper Artillery Cannon and its full size version. Basically, instead of using the on board artillery rules at 17 hexes and less, just slightly increase the long range of the TAC. Of course, reduce the damage to that of the TAC, increase tonnage, crits and heat, and reduce the artillery reach by about half. I think alternative ammunition (beyond just thermobarics) should be an option (albeit with reduced damage). Consider a Jaegermech or Rifleman with two of these in place of their regular weapons...

Thumper Integrated Artillery System
Tech: Inner Sphere Experimental (3020)
Type: DB
Damage: 5 AOE
Heat: 6
Range: 3 minimum, 4/9/17, and 10 map boards
Tons: 13
Criticals: 10
Shots per ton: 20
Cost: 250,000 C-Bills
Ammo Cost: 10,000 per ton, or per type