The fine details of how the game determines the percentage chance for a shot to hit.
How Accuracy Works
Disclaimer: This is a quick and dirty guide written because I couldn't find the information elsewhere and had to go figure it out for myself. I do not 100% vouch for its accuracy.
When you fire a shot, the percentage chance to hit is determined by three things.
- The base hit chance of 75%.
- Bonus of 2.5% per point of gunnery skill.
- Potential penalties from evasion, indirect fire, etc.
Let's look at them individually.
The base chance to hit is 75%. That is, if you have a pilot with no gunnery skill firing a weapon with no bonuses at a mech with no evasion and no other modifiers, it will hit 75% of the time.
Next up is the gunnery skill. This is just a simple increase of 2.5% per point of gunnery skill which is added onto the base hit chance. So, 2 points of gunnery give 5% bonus and an 80% overall hit chance, 6 points give a 15% bonus (90% hit chance), up to a 25% bonus (100% hit chance) once you're at gunnery of 10. So far, so simple.
Then for the third point, which is where things start to get more complicated. All other bonuses and penalties to accuracy are combined into a single number called shot modifier, which you can see if you mouseover a weapons chance to hit when taking a shot (in the bottom-right corner of the screen). A higher shot modifier means a more difficult shot, and a resultantly bigger penalty.
Positive shot modifier (i.e. a penalty to hit) can come from enemy evasion pips (+2 per pip), firing at light mechs, firing a weapon outside of its optimal range, indirect fire for LRMs, recoil for autocannons, etc.
Negative shot modifiers (i.e. a bonus to hit) can come from weapon accuracy bonuses (all energy weapons, or weapon+ with accuracy bonus), TTS mods, arm-mounted weapons, being inspired (morale above 50%), taking a precision strike, or being at higher elevation than your opponent.
However, overall shot modifier can never be negative. Even if you have multiple bonuses and no penalties, you will still end up with a total shot modifier of 0, which means no change to the overall accuracy.
To put it another way, your maximum possible chance to hit is determined exclusively by your gunnery stat. For example, if you have a gunnery skill of 6, then you will never see an overall chance to hit that's above 90%, no matter how many bonuses you stack. All other accuracy bonuses can be used to offset penalties but will not increase your hit chance above your max.
On the other hand, if your shot modifier is positive, then that gives you a penalty to hit, with a bigger shot modifier equating to a bigger penalty. To be specific, 1-10 points of shot modifier give 5% reduced chance to hit per point, with each point of shot modifier beyond 10 giving a 2.5% reduced chance to hit per point.
As examples: a shot modifier of +6 reduces your hit chance by 30%, a shot modifier of +10 does so by 50% and a shot modifier of +14 does so by 60% (10x5 + 4x2.5).
This is not a full tactical guide, but just a brief overview of some of the consequences of the accuracy mechanics, especially the non-intuitive ones.
- Gunnery stat is the single most valuable accuracy boost because it always applies. Regardless of the circumstance, having a higher gunnery stat always makes you more likely to hit. All other accuracy bonuses are situational, and can only serve to counteract penalties.
- Be careful of stacking accuracy boosts. Having an effective shot modifier of -10 gives no benefit at all over a shot modifier of 0, so make sure it's really worth before investing in any other bonus.
- When trying to evade enemy fire, making the shot just a little bit more difficult will sometimes have no effect. For example: you are facing an enemy mech which has an arm-mounted PPC. That's -1 shot modifier from being arm-mounted and another -1 from being an energy weapon. Then you run far enough to gain 1 pip of evasion, which gives +2 shot modifier. This gives a total of 0, which is no improvement on the -2 you would have anyway. In this situation you'd have been better just getting yourself into a better position and ignoring the evasion.
- At the same time, there are diminishing returns on adding too much evasion. If a shot is already very difficult, adding more shot modifier only has half the effect it otherwise would. Whether this is worth it depends heavily on the situation, but it is something worth considering.
Disclaimers, Caveats and Anomalies
All the information in this guide was gained purely by playing the game, paying attention to all the different numbers and trying to make sense of them. I haven't actually checked the code and I absolutely don't have access to any privileged information. I am a fallible human being capable of making mistakes, and would welcome any (polite) corrections or additions.
I have also not attempted to verify that the percentage hit chances given by the game are accurate. I am assuming that if the game says 80% then that actually means 80%. If anyone actually wants to run a statistically significant test of this then I'd love to see the results, but I've no interest in trying that myself.
However, one thing of note is that the game only ever reports accuracy to the nearest 5%, which makes it harder to be sure about some things. Does the gunnery skill actually add 2.5% chance to hit every level as it claims, or does it add 5% every second level? Who knows? With the game only reporting accuracy in 5% intervals, it isn't obvious.
Finally, sometimes, the numbers just don't make sense. I have had shots (images below) with the same mech, pilot, weapon and shot modifier on the same turn have different % chance to hit, so there is obviously something else going on inside the game, though I don't know what. Possibly it's a hidden range modifier? Maybe a hidden luck modifier? I don't know. What I can say is that I have never seen the numbers deviate by more than 5% from the predicted value.