r3d84

When you enter the same (or no) monitor size, the monitor size is completely ignored in the calculation. But as soon as there is a difference in the monitor size, it will be accounted for based on your conversion setup. So for 360 distance, nothing changes as it will stay the same regardless of monitor size. But for any other method, the FOV is used in the calculation to account for the monitor size difference, and since you don't have the exact same FOV in R6 and Warzone you get this difference.

This is wrong, the config file FOV is a multiplier of 70, while the in-game FOV scale of 70 to 110 corresponds to multiplier 1 to 1.55 in the config file, which in turn makes 110 in-game 108.5 in reality

You can just remove it, the game doesn't use it anymore. It doesn't automatically remove it from the file though.

Actually most scopes are like this, the game is calculating the sensitivity scaling based on the FOV you see around the scope, not the FOV in the scope. So technically all scopes needs an update to reflect this.

Hipfire is correct and the same as Warzone/Cold War. The FOV for the aims is not the same though, I will go through them all and add them today.

You only need to change to 116 fov, everything else can be the same. It will be the same as using your 24", except the extra 3" is providing extra fov.

For 2D sensitivity, your cursor moves slower on the 27", at 0.8x the speed.
For 3D sensitivity, your sensitivity will feel faster on the 27", 1.125x the speed.
This is because you have the same field of view on the new screen, even though it is a larger size.  You need to have a higher fov on the larger screen, maintaining the original 110 fov within the 24" portion of your new 27" screen.  Using 110 on the new 27" is the same as using ~103.5 on the old 24".  If you want to have identical sensitivity, you need to increase the field of view on the 27" to ~116.2. If you don't adjust the fov, you would have to reduce your sensitivity by 0.888x instead (cm/360 will increase by 1.125x), which will be identical to converting from 110 fov to 103.5 fov using monitor distance 0%.
 
tldr: You need to use 110 fov on the 24" and 116.2 fov on the 27".
 

I don't know how windows scaling works for games, but the principle as you described, if we take 1920x1080 and we have 400dpi, if we put 720 and stretch it, 400dpi x2 get 800. If we don't stretch 720 in the game and play in the black bars, the 400 dpi will stay the same on the monitor, that is, there will be no increase.
As for the multiplier in the game and dpi, the shooting is on a 2D monitor and the movement is on a sphere. There is a fixed distance of 360 and people write about it as "sens" in the game.
Let's say we have 20 cm, and these 20 we will feel on a 2D monitor at different FoV 120 and 60 in WZ, at 60 sharp and at 120 inert.
You can also measure the path in monitors and pixels:
360\120=3*1920=5760 total path in pixels
360\60=6*1920=11520 total path in pixels
S=c(monitors); t=l;

When the angle of view changes, the speed of the pointer changes, the path does not change, the path is a circle 360=20cm. When the viewing angle is 60, to cover the distance, you need to spend from edge to edge 6 monitors, but you need to match the path of 20cm on the mouse pad. At 120 it is already 3 monitors, and since the monitors are physically the same - one characteristic in length, in order to traverse a circle of 360 degrees around its axis, the cursor must move slower. Therefore, when setting FoV 120, there is a fisheye effect and the cursor becomes inert. If you set it to 60, the cursor will be very sharp, despite the fact that the character will move as if slowly, it will be difficult to aim. 120 the character will be fast, but the cursor will be inert (slow). This is all related to the multiplier "a", it starts to appear and affects the fact that the equality is fulfilled. Multiplier "a" depends on viewing angle, speed (speed depends on viewing angle, but in general they are complex)

c,l are length units (cm, m, inches, pixels, etc.)
C(the length of the circle in the game)=a(the product of some multiplier)*l(the length traveled by the mouse on the mouse pad). The a1 multiplier depends on the viewing angle(60), on the pointer movement speed, on the sensitivity in the game, not on dpi, sens. - is the speed of the pointer movement.
The "c, l" of the path depends on dpi. The speed of movement depends on "sens. 

let's hypothetically imagine that the path of the mouse from edge to edge on the mouse pad equals one monitor. For the equality 6=a60*1(figuratively, these numbers(6;1;) are not values, they include a complex of numbers) and 3=a120*1 for this equality to hold, a60≠a120. That is, the multipliers "a" are not equal, that with different viewing angle, different dependence of "v" speed, different dependence of "sens." sensitivity, we can call them synonymous v=sensitivity, sensitivity=v. 

1) PPI
2) size
3) scaling in windows changes the sensitivity, always when you start the game - should be 100%, when not playing - you can return to 125%
4) Mouse testing program. Mouse movement is physical X'Y movement, pointer movement is cursor movement. They should be strictly the same. https://drive.google.com/file/d/1DrAyu3fgnmuHlyAVvIH0lV0yA1Qi6EgB/view?usp=sharing
5) Enhance pointer precision - if enabled, it will also interfere and the program will show it. Also if needed, a file that cuts "enhance pointer precision" from the registry. Some games ignore the disabling (unchecking) and use from the registry. The file will remove the function completely and games can not use it in any way.
Afterwards you have to reboot
https://drive.google.com/file/d/1hSb5oYqu2FrnCe2YH_NuQVXiKjaVCNd8/view?usp=sharing