I know both of these systems have very useful or good features. But I would like an opinion, is it better to use Carbon Engine or Advanced Combat System [ACS] for First Person Shooter game, or does anyone have any suggestions beyond these two systems?

The default third person roblox gun system is quiet good. For fps try to make ur own guns as these systems are very old

Any good resources for FPS? I learn FastCast, is it good?

It’s good as resource for making gun system. There are many like it as they use raycast