Accuracy is the statistic that determines how often a soldier can hit, as well as the tightness of their aiming circle. Accuracy can be affected by the chosen unit's level, their equipped weapon, and any buff or debuff effects on them such as Orders or Potentials.
How accuracy works in Valkyria Chronicles
Accuracy determines both a unit's ability to hit a target, and the size of the targeting circle. Innate Accuracy is the base accuracy a unit has, which increases as they level up. A tank's Innate Accuracy is its base accuracy (6 for Edelweiss, 10 for Shamrock) plus the effect of any accuracy-modifying Tank Upgrades.
Actual accuracy is determined as Accuracy = Innate Accuracy x (100% + Accuracy Modifiers) For example, Wendy Cheslock has an innate accuracy of 56.5% at level 20. With the Potential Kamikaze (+60% accuracy) active, she has a modified accuracy of 90.4% (160% of 56.5).
The base state for the weapon's targeting circle is the weapon's own accuracy stat; this is ranked in-game from A to E. The actual size of the weapon's targeting circle is determined by Targeting Circle Size = Weapon Circle Size x (100 - Accuracy). With the above example of Wendy, her innate accuracy would result in a targeting circle 43.5% of the weapon's normal targeting circle size, while with Kamikaze active it would be just 9.6% of normal size; in other words, four and a half times smaller. If the unit's accuracy is 100, their targeting circle size will be 0, which translates to the dead-centre of the crosshair being their entire aiming circle.
The final thing determined by accuracy is chance to hit. The modified Accuracy stat determines the soldier's chance to hit the centre of the aiming crosshairs; misses are randomly distributed in the remaining area covered by the circle. With the above example of Wendy, she will normally hit the centre of the crosshair 56.5% of the time, while 43.5% of her shots will end up somewhere else in the targeting circle. Modified, she only has a 9.6% chance of not hitting the centre of the circle, which is, as noted, also four and a half times smaller.
This should serve to explain the common observation of single-shot weapons either hitting bang-on or missing by a mile, with a tendency toward the latter when the user is low-level.