Accuracy formula

% Chance to Hit = % Accuracy Stat of soldier (eg 88 for Bob) x x % Accuracy of weapon/shot (eg 30% for rifle snap shot) (x 115%, if kneeling): Kneeling bonus (x 80%, if firing rifle without a free hand): One-handed penalty x % Health remaining: Wounds penalty (x 100% - (10% for each critical wound)): Critical wounds penalty

Note this is NOT affected by distance - if you have a 100% chance to hit, you will hit, at any range - but 'missed' shots still have a chance to hit if their randomly picked path takes them through the target tile. This is why burst shots are your best choice at close range - even if the calculated accuracy is bad, it's still hard to miss.