It was driving me crazy getting this to work with Gun4IR. I finally got it to work perfectly. Demulshooter was not the issue. The issue was that the Windows desktop resolution must be 1920x1080 AND refresh rate 60Hz AND Windows scaling 100%. I had scaling at 125% :(
With some Google search, you can make a AHK script to set your resolution and scaling and refresh rate on launch, and restore it back after you edit the game.