Re: AR Builders?
Rainier Arms is building mine. I sourced most of the parts from them (upper/lower, barrel, lower kit, trigger, stock and stock guts). I'm doing a MATEN build as well and happen to be out of the country. At first I was going to put it together myself, but decided that I didn't want to muck with it when I got home and wanted it ready to go as soon as I got back. Rainier has been great to work with through the process and from what I've read other places their gunsmith shop really knows AR's.
Once I get back I'll put up some pictures as well as a review on the Rainier Ultramatch barrel.