Long post ahead, if you don't care about the methodology involved, feel free to skip to TL,DR below.
So someone was asking about how to set this up a few days ago and I did some testing to learn how to help them out. Thought some of you might be interested…
Several games out there use a 1:1 ratio for their aim controls and the reason is muscle memory. When the amount you have to move your mouse to drag the reticle to your target is the same whether aimed or not, your muscle memory doesn't have to work with what is essentially two different sensitivities. When scoped in, say 2x, the screen space to traverse will be double, but so will the size of your target, and this balances things out.
Overall, the goal is to achieve more proficient aiming.
Destiny has it, the Battlefield series calls it Universal Soldier Aiming, etc.
My testing setup was this:
-Factory Offline, big metal wall with a horizontal bar about chest level, located at Gate 3
-Crouch about two notches, about 10 feet away from bar
-Piece of paper under mouse
Then, start by pointing your rifle at the left most end of the bar and mark the paper where the left side of your mouse is.
Drag your mouse as closely along the bar as you can to the right until your reach the right end of the bar. Mark where the left side of your mouse is.
Now, repeat this process starting at the leftmost end of the bar, ensuring your mouse is in the original starting position, but do not ADS. By default, this distance should be shorter.
The goal is to get both distances to match by adjusting the two mouse sensitivities until you move the mouse the same distance between the two points, both in-game and on the paper.
My results showed that 0.19 (non-ads) and 0.27 (ads) matched about as closely as I could nail down.
This scaled to 0.38 and 0.54, respectively, indicating the scale is linear, not logarithmic.
To get this effect, take your preferred ADS and multiply it by .704 (70.4%) to get your matching non-ADS sensitivity.
Or, take your preferred non-ADS and multiply it 1.42 (142%) to get your matching ADS sensitivity.
It may take some getting used to, but it should improve your aim over a few days. Depending on your mouse, resolution, and DPI settings, your mileage may vary.