There are a number of enhancement's that do strongly lean towards Combat Sim usage. And that's cool, if your into that sort of thing.
This viewer may be just what you have been looking for.
Personally I am more of a fashion oriented person in SL.
Its all about fun though. And the Cubed team seem to have accomplished that goal.
Here are a couple of screen shots, and the DL site. Oh yeah BTW it does have very respectable FPS, about 21 on average and rock solid steady. A lil bit on the slow side at loading textures, but that may just be my ISP having a crappy day.
So for all the shoot em up lovers / have fun / enjoy / and please don't gun me down in a dark alley somewhere :_)