Beautiful work.
He added so much weight to the barrel, he needed a muzzle "booster" for reliable recoil operation.
One can keep the barrel weight to minimum and achieve a functioning 16" bbl, his is 12.4"; but then he does have a mounting rail for an optical sight on the barrel!
Interesting that the chose "pre-war" lugers for conversion/use; I guess he didn't think the Mauser lugers had that much better "metallurgy".
Michael Spohr does nice work for sure, that color case hardening is beautiful. I could find no mention of a price though.
I really like the case!
A couple pictures from his web sight, and my amateur self-made 16" carbine.
Mine probably costs less than his, maybe 1/10th ?