If you are strictly looking for "free" gun-based games rather than this specific 2005 title, there are several high-quality, legitimate options:

Modern Gun: War Shooting Games | Microsoft Store - Microsoft

You can manually edit the Gun.exe file using a hex editor like HxD to unlock the frame rate. For example, replacing specific hex values with "3C" will enable 60 FPS , while "90" will allow for 144 FPS . Modern Alternatives for Free Gun Games

Because was released nearly two decades ago, players often use community-driven technical fixes to make the game look and run better on contemporary monitors: