Breaking in a new AR-556
I offer this in the case someone else has a similar issue with a new ar-15.
I recently purchased a new AR-556, and though I shoot pistols regularly, this was my first rifle. I took the rifle home, and cleaned it thoroughly. The next day I went to the range with .223 Remington UMC. Two shots, jam, two shots, jam, two shots, jam, etc. Changed mags, same problem. After about 30 rounds I gave up, took it home and cleaned it again, paying particular attention to the bolt carriage assembly. I made sure that the gas rings didn't line up, and the gas system wasn't blocked.
The next morning I cleaned the gun a second time, and headed to range. Same problem.Changed to Winchester gray box .55 gr .223. No difference. Called Ruger. Lady was very polite, but had no definitive advice.
I did a lot of reading of similar problems for the AR-556 and other AR-15s, and the typical issues didn't fix my gun, such as the gas system. I finally found a thread on another manf. forum that pointed to the buffer spring not being fully broke-in. It advised to check for proper cycling with a 5.56, since most of these guns are based on the 5.56. It also suggested of going to a higher grain .223. In this case, the OP trimmed his spring. I didn't want to do that at this point.
Today, based on that thread, I ran 20 rds of 5.56 through it, followed by 40 rounds of .62 .223 Hornaday Black. After this, I went through 40 rounds of Federal 55gr .223, followed by 40 rds of 55 gr of .223 UMC. Not one jam. Problem solved (I hope).
It was great fun today to go out and actually shoot, and not spend all my time freeing jams.