What about rebarreling a Remington R25? I don't know what it takes to rebarrel an AR style rifle, but I bet you could get a used R25 for a decent price
Only way I can figure to get there is either a very expensive and heavy AR10 or
Rebarreling is easy for the enduser but expensive to aquire. The have to be headspaced with a barrel adaptor and the gas hole has to be timed. No big deal for a AR smith but pricey. Sometimes you can only buy the whole upper.

http://www.usarmsinc.com/barrel-asse...arrel-assembly

http://www.parts4ar15.com/ar-15_acce..._Assembly.html

http://www.jsesurplus.com/dpms338fed...ycomplete.aspx