Rencounter's core mechanic is a skill for Fire and one for Melee, both rated on a 1 to 11 scale. Modify skill number by circumstances of range, cover, movement by shooter and target, etc, and roll less than or equal to it on a D12. There were some fiddly bits for handling the long tail if the adjusted number went negative.This works well enough, but linear added modifiers math gets funny over big ranges in how much a plus or minus one matters. I think the true relationship is probably closer to multiplicative. I fiddled with alternatives for a long time and in the last games I ran of Rencounter before my locals gave it the heave-ho for Necromunda variants, (late 90s?) I used this variant pretty successfully. Somewhere on a shelf I have the variant modifier tables from the mostly Old West games of Rencounter that used it. I'm thinking about either digging that out or recreating it in a new set of rules.
Modifiers become more coarse grained. The later table basically halves the net plus or minus for each factor that affects the attack. But instead of it being a die roll modifier its a DIE. If the net modifier is against you, throw that many D12s. All must roll skill or less to hit.
So, here's an off the top of my head shooting example.
Private Kranz is a middling shot, rated a 6 in Fire. He's firing at a guy with a big axe that just came around a corner that he was not overwatching so he's a bit surprised. The guy is running and jinking. He's in the middle range band for Private Kranz's musket.
We start at 1D12, add another for the range, one for appearing target, and one for the target running and dodging. That's 4D12, all must roll 6 or less, in order to hit, yielding a probability of 1/16.
Let's say Private Kranz decides to aim instead and wait until his next action round to shoot, since the axe guy has a lot of ground to close and no more cover to break LOS as he closes.
Then, Private Kranz's shot gets a lot better.
If axe guy is still in medium range, its up a die for the range, up one for running and dodging, down one for aimed shot. The surprise die no longer applies. That's a net +1 die, so 2D12 and both must hit, or 1/4 chance.
Skill matters a LOT under this system. Let's look at Sergeant York under the same set of circumstances. Yes, that Sgt York, who historically has to peg any ratings scale. Sgt York is an 11 in shooting, since a 12 always hits every shot.
In the first shot, he's rolling 4D12 needing to roll an 11 or less on each one. Chance to miss is 1/12 + 11/12(1/12) + 11/12(11/12)(1/12) + 11/12(11/12)(11/12)(1/12). I'm too lazy to resolve that right now, but 8% + 7% + 7% + 6% or 28% will be in the ballpark, so he'll have around a 72% chance to hit.
Since a few circumstances improve chances by dropping dice, how do you resolve the really sweet attacks where the dice sum is negative? Flip from ALL dice must hit the skill number to ANY die hits the skill number to succeed. So if I have a net -1, roll 2D12 and either one at skill or less makes the attack hit. So if Private Kranz is firing at optimal short range against a standing unaware target (-1d), and has time to aim (-1d), he's at -2d and will throw three dice ANY for a really good chance to hit.