Carbine is a shorter gas tube than a middy. From what I understand, though I've never noticed this personally, the main difference between the two has to do with pressure in the bolt and running fast. For day to day plinking, either is fine. If you're running full-auto, I think carbine is preferred because it has more pressure which makes the bolt work faster. Mid-length is supposed to cut down on recoil a bit and not be as hard on the bolt. I think. This is all just how it's been explained to me.
But yes, in short, the Carbine is typically a shorter handguard and because it's mil-spec they often have the mounted front site. I like that rail because it goes around the site and still allows full-length rails.
