How about 6mm Creedmoor? Hornady is making properly head stamped brass available through G.A.Precision. He also has dies available.
G.A. Precision is building AR-10 style rifle in this caliber, so it should feed fine from a magazine.
Here's more info on the brass
http://www.snipershide.com/forum/ub...in=291084&Number=3302562#Post3302562
