The Big Red Die
In Battletech, most weapons just deal a big heap of damage all at once. A class 10 Autocannon (AC/10) will deal, yeah, 10 damage all in once place. That's pretty good. Other weapons, like missiles hit in clusters. 5 is the most common, but there are weapons that deal clusters of 2 or 1.
Example: You're shooting a class 20 Long Range Missile Rack (LRM 20). You roll 2d6 to see if you hit. Then you roll 2D6 and consult a Cluster Hits Table to see how many of your 20 missiles hit. You roll a '7', an average roll, and 12 of your missiles hit.
Cluster Hits Table
2 3 4 5 6 9 10 12 15 20
2 1 1 1 1 2 3 3 4 5 6
3 1 1 2 2 2 3 3 4 5 6
4 1 1 2 2 3 4 4 5 6 9
5 1 2 2 3 3 5 6 8 9 12
6 1 2 2 3 4 5 6 8 9 12
7 1 2 3 3 4 5 6 8 9 12
8 2 2 3 3 4 5 6 8 9 12
9 2 2 3 4 5 7 8 10 12 16
10 2 3 3 4 5 7 8 10 12 16
11 2 3 4 5 6 9 10 12 15 20
12 2 3 4 5 6 9 10 12 15 20
Numbers courtesy of Sarna's "CBT Tables" page.
Sarna.net is a great resource.
You then roll 2d6 for hit location 3 times, twice for clusters of 5 damage and then once more for a cluster of 2 damage (5 + 5 + 2 = 12).
Since most the damage is already divided into groups that may or may not hit, why roll again to see if the clusters from the weapon that hits actually hit? Why "hit" someone with twenty missiles at optimal range then only have six of them deal any damage? Why should an opponent that gets a lucky hit on you at extreme range hit with every. last. missile?
So under The Big Red Die System, each lesser cluster of damage (every 5 points for LRMs, each point for LBX Autocannon's, every 2 for SRMs) gets one normal-sized white die. You roll those with a Big Red Die. Add the Big Red Die to each white die to see which ones hit. Take out the white die that miss and roll again for locations, adding the big Red Die to each white die for where each cluster lands.
Example: An Atlas ASK-P0 fires off an LBX-20 Cluster into the face of a charging Timberwolf. It needs 9's to hit (including the -1 bonus for using cluster rounds). The player rolls one Big Red Die and 20--yes twenty--white die. On a side note, my local Dollar Store sells 12 D6's for $1.09.
The player gets a 4 on the Big Red Die and 1, 1, 1, 1, 2, 3, 3, 3, 3, 3, 3, 3, 4, 4, 4, 5, 5, 5, 6, 6. Since only 5's or better on the white die can hit(The target number of 9 minus the Big Red Die result of 4), then only five of the cluster rounds land on target.
The player collects those dice and the Big Red Die, moving the others to the side, and rolls again for hit location. They roll a 3 on the Big Red Die and 1,2,3,4,6. The Timberwolf suffers one point of damage to the Right Arm, Right Leg, Right Torso, and Left Leg.
This assumes you have a lot of D6's. I think that's a fair assumption.
But that's it. That's the system. I hope you enjoy a future of rolling handfuls of dice and burning cluster hits tables.
Corner Cases and Options
ATMs: The Clans Advanced Tactical Missiles and their strange damage per missile versus damage allocation complicate things somewhat. The roll is done as for clusters of five, with remaining damage being rolled with a third die, distinguishable from the red and white dice. Alternatively, the remainder die could just be another white die that's rolled later, if you don't want to worry about three different dice types.
If it helps, think about it as rolling an LRM volley of a size equal to the maximum possible damage for the missile type being used (ATM rack class * damage per missile). The ATM's integrated Artemis IV provides the standard Artemis IV bonus (below).
Example: An ATM 9 is firing HE rounds(3 damage per missile). The to-hit and damage rolls are made with a number of dice appropriate for an LRM 27 (ATM 9 * 3 = 27 maximum damage). The firing player rolls one big, red die, five white dice for five damage each, and a third die distinguishable from the rest for the remaining two points of damage (5 + 5 + 5 + 5 + 5 + 2 = 27).
Cluster Hit Table Modifiers: Artemis IV systems, NARC bonuses, and the HAG system convert Cluster Hits Table modifiers into To-Hit Number Modifiers at a 2:-1 ratio.
Example: An AMS's -4 on the Cluster Hits Table into a +2 to-hit, and an Artemis IV's +2 on the Cluster Hits Table into a -1 to hit. The HAG's ranged targeting modifiers effectively become: -1/+2/+5.
Pros & Cons
Advantages: This tends to clump damage and make it easier to hit with a scatterfire weapon, it also allows more missiles/munitions to hit when you're closer, and fewer to hit when you're further away. The hit locations distribution is clustered in the short term, but evens out in the long term, and I haven't seen a Cluster Hits Table in a long, long time.
Disadvantages: The effectiveness of LRMs at range is sharply diminished. Sure, you're more likely to hit with something, but the average damage per attack will diminish. Instead of hitting with 66% of your firepower times the percent chance of hitting, you're hitting with your full power times the percent chance of hitting. That seems nice until you realize that per the normal rules, an LRM 15 is putting a smackdown of about 10 damage when it hits--either at long or short range--while under The Big Red Die, it will do significantly less per hit(at TNs of 7+), but will hit more often.
One solution--untested--is to simply give LRMs a new Range Modifier Set: +2/+2/+2 and no minimums, or tabled as a modifier to existing Range Modifiers: +2/+0/-2.
Headshots are another issue. If you shoot off a spread of shots that hit a 'mech and roll a '6' on the Big Red Die when you're determining locations, then you might be scoring multiple headshots on a single target. While it evens out over time, in the short term, that's swings the luck of a lucky shot up really far.
The same is true for shots which roll a Through-Armor Crit (TAC) on a Big Red Die result of '1', but it's a bit worse because of multiple rolls to check for critical hits and the necessity of rerolling the Big Red Die and all TAC white dice for a new set of locations.