One of the first things a lot of 3D artist models tackle are guns - I've chosen so far not to go with anything real, instead working on some fantasy designs: in this case, Newton Ewell's and Vince Martin's Rifts work. I designed these with a mind for minimizing curved surfaces, to see how close I could get to balancing quality with what could be rendered in real time in a game engine. One of things I've noticed is that games typically draw gun barrels as 6-sided cylinders, and rely on shading, textures, and distance to create the illusion of a smooth, round cylinder - great example of this is in Relic's Warhammer 40K Dawn of War. Looking at these, there's still plenty of room for optimization, reducing the poly count and such, but they're a start.