A buddy of mine built an AR-15 pistol in 45 ACP, using a Dalphon upper.  It was a pretty hefty device, when compared to a modern handgun, but it actually was built well and shot fairly straight, atleast as well as you could hold such a contraption.  The downside to the Dalphon project was the .45 ACP mags that were used to feed ammo into the weapon.  They were modified M3 Greasegun mags and by the monsterous spring tension we experienced, I'd say that they were probably produced about the same time that my Grandfather joined up for WWII.  Afer feeding rounds into those mags, I needed a physical therapist!  Magazine issues aside, Dalphon made an extremely robust .45 ACP upper and I enjoyed shooting it.

I also had the pleasure to rent a Wilson KZ-45 while at the range as well.  The pistol shot phenominally (-2"groups), but the plastic lower and grip frame took a while to get use to.  I noticed that I struggled a bit with keeping my site picture due to the gun weighing so much less than a standard 1911.  With each shot taken, I was readjusting a bit more than I do with my Kimber.  I think that with some time and more practice that I could compensate for this, besides the sites, trigger and accuracy were sensational.  If I have enough income tax money this Spring, I may just have to get me one on those new Wilson KZ-45s!  

Troy,
Thanks for the feedback.  I'll follow back-up with him to verify the particulars about the Dalphon upper.  He discussed the planned purchase with Dalphon at length before ordering the upper, so that he could make sure that what he wanted to do was legal and not otherwise.  What other particulars/specifications make an AR-15 "pistol" legal/illegal?  I'm seeing them more and more in Shotgun News, being sold by several of the popular AR manufacturers (Olympic Arms, American Spirit Arms, ...).  They have short buffer tubes, shortened barrels (I'm guessing about 7 inches in length) and take some type of modified SMG mags (Sten, Greasegun,...).  How can the Carbon-15 pistol be sold to the public?  I see them at many gun shows an from time to time, in gun shops.  How could these not be legal for a citizen to own, if they're being advertised so prevalently in the gun rags?  There's no mention of the need to be law enforcement or military to buy one of these units that I've read to date.  You mentioned 50 oz as the cut-off for the pistol versus assault weapon status, is that with the weapon empty or with a full magazine?  Feedback on this matter is greatly appreciated?