Baseline (Widescreen) Solution & Issues
Enabling widescreen settings in AC-130: Operation Devastation requires some simple registry edits.
Make sure the game isn't running. Now open Regedit and go to HKEY_CURRENT_USER\Software\Frozen Codebase LLC\AC-130\Video and set the Height and Width keys to your resolution. First, make sure you're not mixing up height and width or the game will fail to start with a DirectX error. Second, when you click on one of the keys to set it, make sure you select the Decimal radio button before entering in your resolution value.
Close regedit and enjoy your widescreen game. Why there isn't an interface setting for this is beyond me.
Ultra-Widescreen (21:9) Specific Solution & Issues
Eyefinity / Surround Specific Solution & Issues
4k UHD Specific Solution & Issues
Stereoscopic-3D Solution & Issues