From a thread on a poorly received autocannon rework:
Early BattleTech weapons do damage either to one location or clusters. You can't really do something different with ACs from either of those two.
Actually, I think I can do something different from one location or clusters, though I would prefer to use it to unify the mechanics for some other weapons instead.
Single fire and short burst weapons are well represented by firing a single shot. Missile and other explosive weapons are well enough represented by the cluster hits rules. Weapons that fire for long enough to be walked onto a target are poorly represented. Pulse lasers never spread across multiple locations and have increased accuracy. RACs go all over and lack the accuracy benefit I would expect in comparison to pulse lasers. Machineguns see no mechanics for their many bullets except against infantry. These weapons should reasonably do damage in a line.
I propose that pulse lasers, machineguns, and any autocannon capable of multi-firing in the base rules operate as follows:
Roll to hit twice. If either hits roll two hit locations. If exactly one hit roll on the cluster table for damage.
Trace the shortest path on the location graph for the unit between the two locations. If there is ambiguity the attacker chooses between paths and the defender distributes the damage such that no location differs from another by more than 1 point. If a location is unarmored or was rolled as a crit on the hit location table it must be allocated at least 1 point of damage. If a low damage weapon crosses more unarmored locations or rolled crits than its damage value it can deal damage exceeding its nominal damage value. Most units will probably require different location graphs for different angles.
For example, the forward mech graph has the HD connected to the CT, the CT connected to the side torsos and legs, each side torso connected to the corresponding arm and leg, and the legs connected to each other. The rear mech graph differs in that the rear torsos are used. I'm not going to claim it's a practical way to simulate lines of damage, and I certainly wouldn't want to play a game where every autocannon used it. Applying it to machineguns matches the fluff, but may also slow the game too much. The Piranha would have to be banned for requiring as many die rolls as a whole company of normal light mechs and all the gratuitous machineguns in 3SW play would add up.
Other weapons probably need tweaking as well. The two rolls to hit represent the same thing as the pulse laser accuracy bonus. I think the ranges are so poor not for fluff reasons but to offset the accuracy bonus so they might go back up.
Ultra autocannons and RACs would enjoy a huge effective damage increase. Ultras might need to be replaced with a different concept, but RACs have room to reduce the maximum fire rate without losing their character.
