I believe that everything in the game stacks as a percentage of the base value. I.E. If your original accuracy is 95 and you have a 25% boost then now you'll have 118.75 accuracy.
Now this does not mean that you have perfect accuracy. Instead it means that you have the ability to hit what you're aiming at out to 118% of the distance that your gun is designed to shoot at. In other words, if you could shoot something reliably at 100 yards before, you'll be able to shoot it at 118 yards now.