Accuracy

Hit chance value is a huge number of factors all rolled into one single value.

You can know all these factors, though--they are both explained in detail in the manual, and you can use the combat log for a breakdown during firefights. For details see Interpreting the Combat Log.

Base hit chance before any modification is 60% It cannot be higher than 95% and cannot be lower than 10%.

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Volley Modifiers:

 +3%/cell if range < 6
 +attacker utility bonuses
 +10% if attacker didn't move for the last 2 actions
 +3% of defender heat
 +10%/+30% for large/huge targets
 +15% for hack-linked targets
 +5% w/robot analysis data
 -1~15% if defender moved last turn, where faster = harder to hit
 -10% if attacker moved last action
 -3% of attacker heat
 -10%/-30% for small/tiny targets
 -10% if target is flying
 -defender utility bonuses

Weapon-specific Modifiers:

 +utility bonuses
 -recoil (from other weapons)
 -10% while system corrupted (corruption% chance/shot)

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Source of information is this post by Kyzrati May 22 2015.