Same here missed this thread. Built a 7SAUM 20" Carbon Six last year. The main reason I went with them over proof is because how "custom" it seemed you could go as far as length, threaded, twist, ect. I was able to have them chamber and extend the throat for my dummy round and I also sent in my action and had them put it on.
I have been really happy with it. I came up with two loads, 175gr eldx and 155gr hammers. Both accurate loads. I used the 175gr in a long range class in Aug and had no issued out to 1000+ with it.
Original 3 shot test group of the Hammer load. This grouped consistently out to 430yd.