XCOM: Combat Mechanics

There’s no goddamn manual and the tutorial is two hours of pure suck. Help me figure out where the game pulls all its effing numbers from.

Stuff I’ve learned. Maybe common knowledge, maybe helpful. This is all in classic difficulty.

The basics of aiming:
Each unit has an aim stat. That = the base percentage to hit.
Light cover is -20% to hit, heavy cover -40%.
Flanking from the side negates all cover bonuses, but doesn’t add to the base aim. 50% critical chance, though.
Flying units are 20% harder to hit (always in light cover, essentially)
Close range adds an aim bonus. Shotguns (and other weapons, I’m sure) get an aim penalty at longer ranges. Need to do more tests to figure out how this works, exactly.
Elevation is a single state (I think!). As long as you are higher than your target, you get an aim bonus. I think it was 20%, but that seems too high. You’re also harder to hit. I’ll need to recheck these numbers.
A poisoned soldier has a -20% aiming penalty.

Mind-melded sectoids have +50% critical chance. Kill the sectoid that is sending out energy and you’ll kill the other guy as well.
Simply holding a medpack (having it in inventory) makes that soldier immune to poison.
Holotargeting (heavy ability) adds +10% to all allies aiming at that alien even in the heavy’s shot misses.
A unit that is dashing (using both moves at once) gets a dodge bonus against reaction/overwatch shots. Not sure how bug, though.

Question: weapons can do “up to” X damage. What determines how much it ends up doing? Pure random?

Still have plenty more to figure out.

Thanks Giaddon. This is super helpful.

It turns out there is a manual available online and unfortunately it’s fricken useless. Truly an awful manual for a strategy game like XCom. It’s here (hat tip to Blackjack at GT):


Does that count now as a manual?? It´s less than 10 pages long and half of them are legal stuff!

The original UFO manual wasn´t great, but…


I’ve learned that you can get reaction shots while the aliens are doing their ‘free’ move after being spotted.

I’ve noticed this too, and I find it really, really odd for some reason. It seems at odds with how a round in all other instances play out in terms of movement and interrupts.

That’s so you’ll buy this. Haven’t seen it, but probably what the manual should have been.

Yep. My question, though, is: Does that soldier still get to take a reaction shot during the following alien turn? I’ve been looking for this to happen, but I haven’t had the situation unfold on its own yet. Or if it has, I wasn’t paying close enough attention. Anyone know?

Sniper rifles definitely get a penalty at short range.

I think it’s the reverse: that sniper rifles get a bonus the farther way from the target the shooter is. I think the game says this somewhere in a loading screen tip or the item description or something similar. I can’t remember where.

One of the sniper perks implies that, at least.

Light cover is -20% to hit, heavy cover -40%.

Well, that’s a mystery solved, it explains all the cases of people having kills even being on heavy cover. The cover isn’t as important as it may seem at first.

Maybe, but they must have a max range. In the downed UFO forest map I have had a sniper at the landing zone be unable to target an alien outside near the saucer. Even though he had squad site and a closer squad member did have the option to shoot the alien, my sniper didn’t have the option to shoot the alien.

Oh yes, they do. Max range seems to be sight range for all weapons. But you can really up your chances to hit with Squad Sight. I’ve had Snipers across the map getting 100% to hit chances. Combined with Double Tab, it’s nearly unstoppable.

Edit: Your Sniper probably didn’t have direct line-of-fire, which you still need even with Squad Sight. Cover was probably blocking the shot.

The description text of the sniper rifle claims it gets a penalty at close range IIRC.

If that’s so, then I stand corrected. I seem to remember somewhere saying sniper rifles get a bonus a long ranges. Maybe it’s both.


It’s all relative. I thought the % protection would be even higher, something like -33%,-66%. Still, of course it’s better to have 20% protection than 0 :P

Something to note, the base chance to hit is NOT 100% - it’s your OFFENSE SCORE, which improves with rank. Some of my high ranking guys (same, MAX rank) have different scores, so they seem to go up at differing (random) ammounts. My best unit is an Assault with an 89 Offense, for example so if he’s firing at medium range against a target with Full Cover he’s only going to have a 49% chance to hit, so even in the very late game positioning and flanking makes a huge difference.

Against that same target, the rookie (I try to always bring one if I can) willl have like a 25% chance to hit from about the same distance, due to having a lower base Offense score.

Thanks Turin, I had thought you were going crazy for a second there! :)

Yeah full cover is a kick in the pants. The aliens will shoot at you regardless, and eventually they’re going to get the dice roll. I’ve started relying on both full cover and distance. There’ no combat log so I can’t tell where they start getting a range penalty. But it seems to help. On the next turn you can move multiple soldiers up closer and hopefully kill the enemy.

Another tip: when you see 3 full cover spots in a row, move to the middle one so they can’t shoot at you “around the corner.” Then maneuver into a firing position the following turn. But be sure you’re far enough away that the enemy can’t walk up to the corner and flank you. I still get surprised how far units can move. Plus the simultaneous reaction fire means you might not be covered like you think you are.

Something obvious to keep in mind-

If there are 3 tiles of heavy cover in a row, the middle one will hide your unit completely from non-flanked attacks:

…x <-- hidden from anything ^ the row of H H H

You won’t be able to attack, but the enemy won’t be able to spot you.

That’s pretty awesome, you guyst posted the same tip at the exact same time.

Yeah, staying completely out of the LOS of the enemy is ideal when you can swing it. It’s CRITICAL to only move one action at a time, so if your first action reveals something you can run back to a hidden position, which is normally possible.