Agreed that you should look into a 5.56/.223 AR-15. That is what they were natively designed for. That is standard issue for the military as well, and we shot them from as far as 500yds with the standard sights (no scope) with pretty decent accuracy.
I'm building an 18/20" Stainless barrel varmint style AR-15. I personally went to Bill's and bought a stripped lower ($120 I think), as that is the only thing you need a Permit to buy. Everything else I will be buying online and will slowly assemble it over time.
I've found a lot of good deals at
www.MidwayUSA.com on fully assembled AR's, or separately assembled uppers and lowers.