I haven't really worked on any big programming projects and i use Dark GDK, but i think DGDK and DBPro are very similar.
For AI, you could use DarkAI, but that would mean everybody would need to buy it (which means more profite for TGC
). Same for Dark Physics.
For bullet detection, you could use projectiles for snipers, rocket launchers, etc...
For machine-gun types (SMG's, AK47's, MP5's, etc...), you could use raycasting, or the pick object() command.
Somebody should get a working prototype up, that may be a good way to illustarte the concept. A prototype can just have some basics such as movement, shooting, different classes, etc...
OpenXDK is a homebrew kit for developing XBOX applications, but it requires a modded XBOX and is much harder then XNA.