The huge Mechblaster is an oversized recoilless rifle that also uses elements of a missile launcher. It fires a huge grenade that is propelled via three extremely powerfull rocket engines that cover the aft of the projectile like a sabot and fall of the second the projectile leaves the barrel. This combination gives the weapon an incredible range and firepower, and it only weighs half as much as a standard autocannon of comparable size.
However, the MechBlaster has some serious drawbacks: the ammunition is prone to ammo explosions, and with every shell carrying far more propellant than usual, an exploding shell deals twice the damage of a comparable round. Furthermore, as it is impossible to ignite all three rocket boosters at exactly the same time, the weapon is somewhat inaccurate. The warhead uses both an impact and a proximity fuze to compensate for this and make the grenade detonate even if it just closely passes by the target, but in this case, the grenade deals only 2/3 of it's normal damage.

Game rules (so far):
Weight: 10 tons
slots: 9
Heat: 10
shots per ton of ammo: 5

Range: -/7/17/25
Damage:
15, with a +1 modifier on the attack roll
10 with no modifier on the attack roll.

Pic is just a WIP, later for scraps.