When using Fire at Will the hit rate of the beams drops to the hit rate level of a common turret. The hit rate does not noticeably improve with [Acc] modifiers on the weapons.
I suspect that FAW either ignores [Acc] modifiers or uses a wrong magnitude (e.g. +1% bonus accuracy instead of +10%).
Some sample data (22,7% bonus accuracy vs 54,3% bonus defense):
Turret (common): 79% hit rate
Turret [Acc]x2: 91%
Beam array (common) with FAW2: 75%
Beam array [Acc]x2 with FAW2: 76%
This was a short test (~12min, ~470 shots per weapon), so there is some room for error, but it is already obvious that the [Acc] modifier is not being applied correctly.
How to reproduce:
1) Equip beam arrays and turrets with the same accuracy modifier.
2) Enable the combat log.
3) Shoot at a target with a fixed defense value (that is higher than your bonus accuracy) and use FAW. Keep doing this for a long time to gather useful data.
4) Look at the combat log and calculate the hit rates for beams with FAW and turrets.
5) Repeat steps 1-4 with different accuracy modifiers.
(To make it easier, you can also equip weapons of different energy types with different [Acc] modifiers, so you can test them simultaneously and still have useable combat log data since it distinguishes between different energy types (at least for FAW2).)
FAW hit rates with a given bonus accuracy level are equal to hit rates with other weapons with the same bonus accuracy.