I have recently picked up the Reflex Sight from the mission store. The mod offers +14% accuracy.
Sadly this isn't the case when you look at the info on the item screen.
An Armalite with: 76.54 accuracy should gain according to my math...10.7156, we'll round it to 10.7 here.
But the accuracy with the mod is really 81.73, a gain of 5.19, less than a 7% increase.