Shoot what is stamped on the barrel. Since different uppers can be pinned to a standard lower, what's stamped there (multi, 5.56, .223) doesn't mean much.
The biggest issue with assembling an AR isn't so much getting the parts together right - it's getting the right parts together. You'll remember the quality of a gun long after you forget how much you paid for it.
US Army 1966-69, VFW Life Member, Retired Geek