My melonite barrels have been fine, no chamber issues.
My plinker is a CMMG SBN 16"...eats anything I can get in the mag. (5.56 chamber, 1/7)
My SPR is an 18" AR15Performance fluted...shoots XM855 well out to 700 yards. (.223 Wylde chamber, 1/8, 5r rifling)
The other is a 7.62x39, but have not shot it yet, though it will chamber and manually eject a round just fine.
Remember that melonite (or QPQ, nitriding etc) does not add to the chamber and bore walls, whatever its milled to, it stays at...its a chemical treatment at the molecular level, not a coating.
I recommend the ARP in either a 16" mid, or 18" rifle.
http://ar15performance.com/barrels
Its not stainless, but still a very high quality barrel for the $$$, and will last much longer than an untreated stainless barrel.