I would like to build a 308 cal. AR.
What do I need besides the corect barrel?
I have a Billet Rec.

How about the correct upper & lower, bolt, bolt carrier...

a whole new rifle like an AR-10

The lower receiver has to be an AR10 receiver.  The .308 round is longer than a .223.  You will also need an AR10 upper, bolt and bolt carrier for the same reason.  About the only parts that are common between the AR-15 and the AR10 are some fire control parts, sites and stocks, aside from some pins and springs.

Go to the armalite web site and peruse the catalog.  The parts that are common are marked as such (I think).

I am gonna third or fourth what these other guys are saying. You can't attach a .308 upper to a .223 lower and vice versa. The magazine well and pin location is different. The bolt and bolt carrier are heavier. Buy an AR 10 and save the hassle.

Thanks guys this is why I am here!!!

Napier2 -

Like the others said, there is quite some difference between the AR-10 and AR-15.  This link will show some good pictures that help illustrate the point

[url]http://feistyrooster.com/500phantom/parts.html[/url]

Also, you said ".308" and while that typically refers to the .308 Winchester (7.62x51 NATO), there are a few .308 caliber rounds that will fit in the AR-15.  Most notable is the .300 Whisper(tm) or the 30-221 if you will, as well as the 30 Coyote (30-223) and there are a few .308 variants of the 7.62x39 out there (the Russian round is 0.311 caliber, but a .308 barrel could work in a pinch)

Hope that helps

Marty