I think a carbine would be pretty cool. If you have enough money you could even buy a laser from http://www.wickedlasers.com. Some of those lasers are powerful enough to pop balloons and burn through plastic, and they come in green so that would be kinda cool to hook up to a carbine.
I have every intention of making a carbine myself, and I will put in one of those bright green visible beam lasers. We'll be getting in a bunch at work soon (www.laserdome.com), so I'll probably be able to get it for song... or at least a couple notes...
I also intend on wiring up a working trigger and game captured firing sounds to every one of my weapons...except the sword unfortunately...because there isn't enough room in the handle for a sound chip/speaker...
Hehehe I have modeled the Spiker and have it textuerd in 3ds max. I modeled it the first day that the sweedish scans were posted online of Halo 3. I also have modeled the MA5K for the Halo CE CMT map making team which I am now part of.