This guide is a subjective experience (it works for me), which means that for many other players, the method may not be working.
If something doesn't work for you, then I don't have to help with it.
Most likely, this method does not work for you.

Why might the full screen not work?
Simple speaking, cuz the old engine. Incorrect operation of the game is (usually) observed on new systems, because Blitz3D (engine) has not been supported by developers for a long time.

Full Screen is a consequence of the termination of support. It works poorly on new systems, which may cause the game to freeze completely.

Therefore, this guide will describe a way to fix the Full Screen using the program and its special DLL library. The method helped me and can help you.

This guide will also be useful for those people who have an integrated and high-performance graphics card at the same time (cuz Blitz3D sees only an integrated one).

The method can also work with other versions of SCP:CB.

What will the full screen give you?
Can give:

• More FPS;
  
• Ability to minimize the game window;
  
• Responsive mouse control (very).

But problem is:

• Possible incompatibility with OBS;
  
• Possible suspension;
  
• Unknown problems.

Download the archive from the off-site: http://dege.freeweb.hu/dgVoodoo2/dgVoodoo2

After the first two, copy the two files (dgVoodooCpl.exe, dgVoodoo.conf) to the root folder of the game (steamapps\common\SCP Containment Breach Multiplayer\).
(these files copying...)
(... and paste in the game folder)
After that, go back to the archive and go to the "MS" folder, then to "x86".
Copy all the files and paste them into the root folder of the game.
(copy all these files...)
(... and paste in the game folder)
Done! Now let's move on to the setup config.

Now that the last stage is over, we will need to configure it.

Run "dgVoodooCpl.exe ".
This window will open in front of you:
All you need is to set the settings like mine.
(In advance: if it does not work, then you will need to configure it individually for yourself).

In the General tab:
* In the Miscellanneous category, check Capture Mouse and Center App Window.
In the Glide tab:
* Remove the 3Dfx Watermark item from the Miscellaneous category.
In the DirectX tab:
* In the Videocard category, select "dbVooDoo Virtual 3D Accelerated Card" and also in VRAM "512Mb" (depending on the video memory you have available).
* In the Miscellanneous category, put two items on Force vSync (if there is a picture gap) and Fast Video Memory Access, also remove the item on dgVoodoo Watermark.
After all these actions - click on the Apply and / or OK button.

I won't explain what the other settings do, you'll be able to figure them out yourself.

Now you can run the game in full screen mode.
If everything works, I'm very happy for it, if not, then most likely you will need to configure the program manually, in extreme cases, the method does not work for you.

If I helped you anyway, then please like this guide.
Well, the reward for that...