I've kicked around the idea of a pistol caliber semi-auto carbine. The Beretta Storm seemed interesting, but you are stuck with proprietary magazines, and the optics mounting didn't do much for me.
The Just Right takes Glock magazines, which are easily found and reasonably cheap. Optics are easily mounted
The Mech-Tech carbine conversions are also interesting. They are a little different concept, mounting on a pistol frame, either Glocks or 1911's.
What I have not heard is how well either of them, actually shoot
