They don't block everything. You can suppress, blow up, and gas enemies on the other side without ever opening the door. This is by far the safest way to breach a UFO; opening the door before softening them up is generally suicide. The doors seem to block most bullets, some gas, and a very small amount of explosives and suppression. Almost all terrain can also be destroyed, but it takes a while.

It is higher than the game claims. If you shoot a shot from 5m away and have only 10% chance to hit, in reality you will hit maybe 30% of the time because the misses are fired randomly and have low spread; they will often "miss" at the same location as the enemy. Point-blank shots are actually guaranteed to hit for this reason if this source is to be believed.

Effectively, the game only shows the chance that you are guaranteed to hit. 10% CTH means that 10% of your shots will be automatically guided towards the target. The rest will fire randomly, and might hit the target.

Reaction fire is deterministic. If the enemy's TU%*reflexes*reactionmodifier >= your TU%*reflexes, reaction fire will occur. This has many important implications:

• If you're taking reaction fire and in danger, don't keep moving. That will decrease your TUs, allowing them to shoot at you. It's possible your enemy has not expended all their TUs, they simply stopped shooting because their TUs decreased to a level where they could not shoot at you at your current TU level. First have other squadmates kill/suppress the enemy who has reaction fire on you, then keep moving.
  
• Similarly when you open a UFO door taking one more action is generally enough to prompt massive reaction fire. But you can often get away with just opening the door and having the rest of your squad shoot / attract fire before you act more.
  
• Don't shoot at an enemy hoping you'll get lucky and they won't react. You literally can't get lucky.
  
• High reflexes are more important than they seem for assaults/shieldmen. Not only do they prevent an enemy from instantly gunning you down when you open a door (before you even take another action), but they also guarantee that you will instantly pump almost all your bullets into any enemy who opens a door.
  
• Avoid reaction fire vs reapers. In general your reaction shot will occur at long range with low accuracy, because it is deterministic. Then they will move into close range and kill you.

Armor is flat dmg reduction. Mitigation is flat armor reduction. If a bullet with 50 damage and 10 mitigation hits a character with 20 armor, it will do 40 damage.

Units lose armor equal to 15% of the damage they take before armor reductions. All sources of damage shred armor equally. Shotguns can kill an andron, it will just take more than 6x as long.

Effectively, mitigation is damage that is ignored if the enemy has next to no armor and does not contribute to shred.

Shotguns are bad at piercing armor primarily because they do lots of small attacks, not because of their low mitigation. This also means they're good after armor is shredded. Thus you should fire sniper rifles before shotguns. Unless the shotgun doesn't have enough power to do damage even after the sniper rifle shreds the enemy, then you should fire the shotgun first to effectively deal 15% of that damage to their HP each time a bullet which actually deals damage hits. Or don't fire at all, cuz 15% is really low.

Enemies can see you through walls within a certain radius. Exactly how is not clear.

Based entirely on my experience, it seems to be based off of "instinct". Androns hardly do it, Caesans do it pretty often, Sebilians and Beasts do it like crazy and will throw grenades over walls out of nowhere.

Smoke before you even see an enemy because it's very possible they already see you. Flanking enemies is only possible if you flank them before they get a turn within smell range.