If you decide on the x39mm AR I would check out Windham before building. If you are active Law Enforcement they give you a healthy discount. IIRC my SRC with discount was just about $700 shipped direct from Windham a few years ago.

Here is a video from their web site. Explains the importance of caliber specific mags and a few other modifications such as the feed ramps. Windhams x39mm AR's have a very solid reputation for reliability. It is a must watch if you are considering the x39mm AR purchase or build.

Good luck with your choice.
Klyde