I said that I was going to post some rules from the zombie survival campaign, so here goes. These are the rules I use for firearms in my current campaign. They function pretty well at the moment.
TL;DR: 3d6+Skill vs. Target Number
Step 1: Stability
Start off with the shooters position. Their stability, or perhaps rather their instability, is rated on a scale around 2 to 6. I currently call this SHAKE in lack for a better term, but it really should have a better name. Lower numbers are more stable, and higher numbers more shaky. This accounts for both the stability of the shooter themselves, and the time spent lining up the weapon (i.e. aiming versus snap shots), and results in a multiplier.
|Very stable||2||Weapon mounted or on sandbags|
|Normal stability||3||Weapon properly braced with body|
|Unstable||4||Shooting while moving, or shooting from a vehicle on road|
|Very unstable||5||Sprinting and shooting, or shooting from a vehicle off road|
|Short aim time||+1||Fast shot, or multiple shots/burst fire|
|No aim time||+2||Reflexive shot, or fully automatic fire|
Step 2: Range
Now, this is conceptually very simple: each doubling of range adds SHAKE to the target number. If you don’t want to count, you can just use this table. Just find the row for the apporpriate SHAKE value, and the column which most closely represents the distance from shooter to target.
|SHAKE||1 m||2 m||4 m||8 m||16 m||32 m||64 m||128 m||x2|
See, it wasn’t that complicated. This procedure can obviously be extrapolated to any SHAKE value possible. If you are really fancy with this you can deal with partial SHAKE values (is 24 meters closer to 16 or 32? Split the difference and do 4.5xSHAKE) but I wouldn’t personally bother.
Step 2B: Target Size
All of the above assumes human-sized targets. Size adjustments effectively work as range adjustments. If the target is twice as big as a human, reduce the effective range by one step (i.e. if you are shooting hulk twice the size – target area that is – of a human at 128 meters, count it as a human-sized target at 64 meters).
Step 2C: Visibility
Visibility adjustments also work as effective range adjustments. Seeing a pattern? So if you are shooting a target at night, in starlight only, you would count it as four times as far away (i.e. 4 meters counts as 16 meters). These modifiers stack up to Completely Obscured if multiple apply.
|Lightly obscured||x2 (one step)||Sight obscured by fog, frosted glass, or dusk|
|Heavily obscured||x4 (two steps)||Sight obscured by heavy smoke, or night|
|Completely obscured||x8 (three steps)||Sight obscured by opaque wall or pitch-blackness|
Note that if you shoot at a Completely Obscured target you must know approximately where they are through other means (perhaps sound or prior detection), otherwise you can only shoot at random.
Note also that things like night vision goggles or thermal imaging can mitigate some of these issues.
Step 3: Movement
This isn’t too complicated either. Just take a look at the shooter and the target’s relative movement. This adds a flat modifier to the target number. For simplicty’s sake, I don’t usually take into account the direction of movement. Actually, I usually just brutally wing this. Something like this maybe:
|Human: fast jog or slow run||2-5 m/s | 7-18 km/h||+1|
|Human dashing||6-9 m/s | 21-32 km/h||+2|
|Human sprinter top speed||10-13 m/s | 36-45 km/h||+3|
|Wolf sprint||14-17 m/s | 50-60 km/h||+4|
|Greyhound sprint||20 m/s | 72 km/h||+5|
|Cheetah sprint||35 m/s | 130 km/h||+8|
Step 4: Roll To Hit
Your skill bonus, in my system, is +1 per rank in shooting (0 to 3 ranks), +2 per rank in the specific weapon class (pistols, rifle, support gun).
Roll 3d6+Skill. If it’s equal to or greater than the previously determined TN, you hit. Each 3 points you exceed the TN by improves the level of hit.
|Hits by||Hit Quality||Harm|
This damage system is up for some further review, because I don’t quite like how extremely much better hits are at close range, but I’m not quite sure what to do about it yet.
Step 5: Damage Roll
You didn’t think I’d forget that guns come in different sizes now did you? This also handles armor and cover.
Essentially, add up all damage modifiers, roll 1d6, and add. If it’s 7 or greater, increase the hit quality by one. If it’s 0 or lower, reduce the hit quality by one. This keeps going at 13+, 20+, etc. (though it should be impossible to get there with reasonable means).
|.22 LR pistol (~5.59 mm)||-1|
|9 mm pistol||+0|
|12.7 mm pistol (.50)||+2|
|8.58mm rifle (magnum)||+3|
|12.7mm “rifle” (.50 BMG)||+5|
I think that’s it, more or less. It might look like much, but when you get to running it it’s pretty easy and efficient. The only hiccup might be counting up range if you’re not used to that, perhaps. Most of it could probably be squeezed into one or half a page of quick reference, if necessary.