I'm trying to create an epic weapon for Dungeons & Dragons, but I need some help. Any tips?

Here's my dilemma. I'm trying to make a sort of ballistic longsword of sorts. I got the original idea partially from playing Call of Duty: Black Ops and being amazed at the versatility of the Ballistic Knife and I wondered how badass it would be to have a much larger, serrated version. I would imagine that the range of this weapon would be roughly 20 feet or so when fired. My friends and I are playing D&D version 3.5 with some elements of 4. Now we don't want to make any sort of weapons that would be too modern (so no lightsabers, guns, or stuff like that) but I'm having trouble figuring out how to make my sword, granted that the firing mechanism for a ballistic knife is either gas cartridge activated or spring loaded. (I'm fairly certain that in medieval times they didn't have that kind of stuff. I figure I could get away with the spring loaded version, but I'd like it to be powerful enough to actually work well with a projectile secondary function.) A friend of mine suggested some sort of magical rune explosion propulsion system. Another idea was a retractable chain so that I don't lose the blade itself. Any pointers and ideas of how it would function and damage?