Not really, guns part would be easy bullets - simple...
I would do the bullets and guns for you in no time, however that wouldn't be the hardest part.
Hardest part would be somehow allowing players to host their own map (or game)
Not saying it would even need to be done that way, could simple do it like sony did with infantry online

which is just an awsome game.
http://archive.gamespy.com/previews/march01/infantry/
We can get rid of the stats window, and most of the other windows for that matter, no p2p trading, no guilds, no groups... Maybe some type of clan since that's popular...
I know your going to say bullet time how to do that?!
In MLO I added a fishing line right, straight line.. same concept can be used for bullets cept if it hits a player that player was shot, if it hits a wall no more bullet. But in the end it would be alot better then it sounds.
Grenades? ooo yah vbGore is perfect for that type of thing, wouldn't require much "modding"
Take a look at:
http://www.cs2d.com/
Addicting, and Fun...
A shooter would be alot and I seriously mean alot easier to finish then a RPG for one RPGs have skils upon skills unless its about pokemon.. true ORPG (EQ,WoW,UO) all have tons of skills and are very in depth, you need this that and the next thing plus this that can only be found there. orpgs are very time consuming doing each skill, each spell, each ability and perfecting them..
A shooter we would just perfect bullets

then add guns pretty simple eh?
Maybe add tanks, or dare a say add a engineer class to make sentry guns like TF... Oooo it would require a small a bit of "modding" though
