Recommended barrels for new build

[Drillboss]

There are more knowledgeable folks than me on this, but I'll give you what I can. This tool will lap down the face of your upper receiver to give more solid contact between the receiver and the barrel extension:

http://www.midwayusa.com/product/821295/...tool-ar-15

Coating the outside of the barrel extension with blue loctite will fill up any gaps between the receiver and the extentsion.