I think the way to do it is to roll your own.
I just built from scratch an Aero Precision AR15.
You pick and choose exactly the parts you want to use.

If you're into the 308AR/AR10 frame the absolute sweetest receiver set is made right here in Tumwater....(Mega also does the AR15 frame)
http://megamachineshop.com/armsnews.php
I feel like having a cigarette every time I look at one of those Mega Machine receiver sets and I don't smoke.
Put whatever barrel you want on it.
Careful though, Black Rifle is addictive.

Here's a great (the best) video series that calls out all the tools and details the assembly of an AR15.....
http://www.brownells.com/.aspx/lid=11004/learn/
For the large frame you will use a different method to hold the receiver.
If you can get your hands workin' it's not that difficult.