Those RRA NM AR's are very accurate and reliable right out of the box for a factory built rifle. They are a good value.

No break in is really needed or required on barrels these days. Some folks still do it, but most barrel builders will tell you to just listen to what the barrel is telling you. Some more frequest cleaning might be required for the first 1-100 rounds, but other than that just go out and shoot it! Any barrel break in is actually just helping to smooth any rough edges in the transition between the chamber/throat area and the rifling if these were not cut clean enough when the chamber/throat was first cut.

I shot NRA High Power rifle matches for a couple of years using a WOA built upper, and lots of guys on the line were using RRA uppers with very good results. So, go out and shoot it and provide some feedback with your results.