First thing you have to do is enable "Show hidden folders" and "Show file extensions. This is very IMPORTANT, because otherwise you won't find the files.

This is how you do it on Windows 7:

Click "Start/Windows Button" -> Computer -> Click the "alt" button on your keyboard -> Tools -> Folder Options -> Click "View" in the top left corner -> Check "show hidden files, folders or drives". Click Apply and OK

On Windows 10: Go to "Computer -> Click "View" in the upper left corner -> Check "File name extensions" and "Hidden files at the top middle part.

There is 2 possbilities to find your UserMP.ini from there on.

1. Click the Windows Button in the bottom left corner on your desktop, named "Start" and then Computer. In the Top right corner of computer there is a search option (I have it disabled on my PC). Simply type in: UserMP.ini and wait for the search to finish. IMPORTANT: Make sure you search from "Computer" location, so it searches your whole PC and not only one drive, that possibly doesn't have the UserMP.ini file on it.

2. Search for it manually. Go to "Start"->Computer->Your system drive (highly probably named: "(C:)" ) -> Users -> "Your Username" -> AppData -> Roaming -> Bioshock2Steam -> UserMP.ini

Basically everywhere it says:

"MouseX=IF_MOUSE_LOOK_ENABLED Count bXAxis | IF_MOUSE_LOOK_ENABLED Axis aMouseX Speed=2000.0
MouseY=IF_MOUSE_LOOK_ENABLED Count bYAxis | IF_MOUSE_LOOK_ENABLED Axis aMouseY Speed=2000.0"

you want to change the 2000.0 number to about 500.0 . IMPORTANT: There is more than 2 values for this, so make sure you go through the whole ini file and change ALL the values of "MOUSE_LOOK_ENABLED".

You can make this a bit easier by pressing CTRL+F on your keyboard when accessing the ini file and typing " mousex" or "mousey", checking "down" in the field that pops up and then clicking " OK " after changing both values of:

" MouseX=IF_MOUSE_LOOK_ENABLED Count bXAxis | IF_MOUSE_LOOK_ENABLED Axis aMouseX Speed=xxx " and
" MouseY=IF_MOUSE_LOOK_ENABLED Count bYAxis | IF_MOUSE_LOOK_ENABLED Axis aMouseY Speed=xxx "

In the UserMP.ini file search for your "Move forward" bind. It highly probably is "W".

Now add this:

| set Engine.PlayerInput MouseAccelThreshold 0.0 | set Engine.PlayerInput MouseSmoothingMode 0

to the line of "W=MoveForward" so it looks like this:

W=MoveForward | set Engine.PlayerInput MouseAccelThreshold 0.0 | set Engine.PlayerInput MouseSmoothingMode 0

It's very important that you add a space between "W=MoveForward" and "| set Engine.PlayerInput MouseAccelThreshold 0.0 | set Engine.PlayerInput MouseSmoothingMode 0"

Voila! Your mouse control is now acceleration and smoothing free.

IMPORTANT NOTE: Since this is a fix bound to your "W" key, it will only start working once you press the "W" key ingame. So make sure to press "W" as your first action when a match is started.

ALSO: You can add this line to any other key you like if "W" doesn't suit you. Under the "[Default]" Section of the UserMP.ini you can choose any other key bind and add that line as explained above.

I'm pretty sure the sensitivity formula for this game is: Source/Quake sensitivity x 200. So if you want to change your sensitivity to the same you are using in Source/Quake Engine games like CS GO, Quake Champions, Apex Legends etc, all you have to do is: multiply it by 200 and put the values for all "mousex=" and "mousey=" values I showed above.

You can even use your Overwatch or any other FPS sensitivity by using this website: mouse-sensitivity.com

Just select the game you want to convert from and select Counter-Strike Global Offensive or Quake Champions as Target sensitivity and then multiply the result of your target sensitivity by 200 and type it for all your mousex and mousey values in the UserMP.ini file in BS2 as showed above.