1) Model 1 sales has a dedicated upper (1:15 twist) that uses a modified Ciener conversoin kit. BDM full body 27 round can be had from several places online, M1S being one seller. 44mag.com being another.
2) Spike tactical also sells dedicated uppers (1:16), but dont offer the options that M1s does. They also use a midified Ciener conversoin, and therefore, the same mags.
3) KKF can build you an upper, or sell just a barrel. Just bbls are expensive, the price for the upper seems competetive compared to others, but I dont know if he includes the Ceiner conversoin. KKF bbls are 1:16 twist, but have a .223 chamber, so you could use any conversion you like (Ciener, M261, Oly). He uses a solid bbl extension so it wont accept a .223/5.56 bolt. If I had the money, this is the route I would take.
4) I'm gonna build my own. I'm going to use an 1:12 twist bbl that you can find on the EE at arfcom for anywhere between $125 and $200, a flattop 9mm upper from Garrison MFG ($125), and a ciener conversion kit. Sarco may have 1:12 bbls, I dont know if they do or what they ask for them though.
5) I hear Compass Lake brought up a lot. I also hear good things about them, however due to the cost of them I havent really looked into them.
6) Bushmaster offered a .22 rifle or just the upper and mag at one point, dont know if they still do. They're based on the carbon 15 rifle, and weight about 4 lbs
No mags available over 10 rounds, that I know of.
Someone at arfcom put a 16" M4 M1S 1:9 with a ciener bolt head to head with his Spikes Tactical .22 upper, and the M1S bbl was more accurate.