How to equalize in-game sensitivity between two FPS games
Solution 1:
Mouse movement interpretation can be vastly different between different game-engines, and often even between different games using the same engine. There is no standard for mouse sensitivity, so the only way to measure it is to use our own impression.
When I start a new FPS game, the first thing I do to calibrate the mouse sensitivity is:
- close my eyes
- Perform what I feel is a 90° turn
- open my eyes and see how far my character actually turned
- adjust mouse sensitivity accordingly
- repeat until a "felt" 90° turn is an "actual" 90° turn.
Instead of a 90° turn you can also perform a flick-movement between two points.
- Take two points in the game worlds you can see on the same screen (like two sides of a door frame, or two edges of a crate)
- place your crosshair over one of them
- perform a fast mouse movement, which should place your crosshair exactly on the other one
- see how far you overshot or undershot and adjust sensitivity accordingly.
I could also imagine a third, more exact but more technical method.
- Place your mouse on a piece of paper
- start game A
- mark the position of your mouse on the paper by drawing a line at its edge
- perform an exact 90° turn
- mark the new position of your mouse and measure the distance with a ruler. You now have the exact distance you move your mouse to perform a 90° turn
- load game B and repeat steps 3-5
- adjust mouse sensitivity until the distances of a 90° turn in game A and game B are the same
Games use different FOV (field of view) angles. Maybe you feel better with calibrating your mouse sensitivity for the FOV and not for the game world. In that case replace the 90° turn with aiming at a point and moving your mouse until the point you aimed at is exactly at the border of your screen.
Solution 2:
All games use completely different interpretations of the sensitivity slider (if there even is a sensitivity slider). Battlefield 3 even, much to my annoyance, makes extreme differences in sensitivity between vehicle aiming and on-foot aiming.
You'll simply have to fuss with both (or more) games to try and get the sensitivities close enough. Easiest way to do this is play both games in windowed mode and attempt to move the same mouse the same distance in both games until they're close to matching. I recommend disabling mouse acceleration (in windows, and sometimes in games separately) to keep movements consistent as well (otherwise you might move your mouse quickly in panic and overshoot, or delicately aim a shot and find yourself not moving quite far enough before the dreaded edge of the mousepad).
However, aiming in any two FPS games requires lots of the same physical skill and muscle memory. As long as there's a 1:1 correlation between mouse and cursor movement with no acceleration or deceleration, you can train the act of aiming fairly well in any game. The main differences between games will be exactly how fast the weapon fires, how fast the projectiles hit, how close you need to be, etc. So training with a hitscan weapon in Call of Duty is not going to help your aim in a game with somewhat slow projectiles like Borderlands 2. The best you can do is train in the same game or at least something very similar in terms of game speed, player movement speed, projectile speed.
Solution 3:
Actually, there are drivers and dll files that will ignore ingame mouse sensitivity and only base it on your windows sensitivity. Thus it will keep the same mouse sens for ALL games since your mouse will be moving at a universal speed according to your windows sensitivity. I don't remember the program off the top of my head because I quit FPS some 5 years ago, but I'm sure you can find it in Google.
Programs might be detected as cheating from your game engine, but mouse drivers and dlls won't (given that you are using a mouse speed dll and not a hack). I recall I had the same problem you used to have, playing between CS 1.6, CS Source, and Osu, but that was eventually all fixed by one mouse driver :)