If you don't want a lot of hassles then get a CA launcher. While most guns are mostly TM compatible, barrel-mounted m203 launchers are the exception. Pretty much every brand mounts the m203 differently. There are adapter kits available to allow cross-brand mounting but really, if you have a CA gun, get a CA launcher.
This is a pretty good thread for getting misc info about grenade launchers.
Be advised though, running an m203 rig isn't cheap, mostly because of the grenades rather then the launcher (though that isn't cheap either). Look at paying the cost of an upgraded higrade aeg to do it right.
Kuro_Neko