Who has expierence with using the AR-15 7.62x39 upper on a colt lower?

                     1. Does it work well?  ie. Problems....
                     2. What mags do I use?
                     3. Who makes the best ?
                     4. If it is a short barrelled upper, what are the legalites of buying it and making it into a pistol.. in other words do I have to have a class 3 to make a pistol with a short barrelled upper?