Im wanting to build my first SBR in the near future and was wanted to make sure I am not going to end up with an ammo-picky, high maintenance item. I don't mind running quality shells and do in my other rifles. A friend has a registered M-4 and M-16 (the M-16 has a 12 1/2" bbl). Neither of them run through a full magazine on burst or full auto without problems. He is quick to blame the short gas tube and seems to be a fan of piston-driven guns, although I don't believe he owns one. I've never seen his weapons taken down to see how clean they were, so I can't comment on his maintenance.
I'm wanting to maybe put together a MK 18 Mod 0/CQBR. I don't know if I care if it has a 10 or 10.3" bbl.
http://img13.imageshack.us/i/mk18s.jpg/