Like most customers of custom barrels I want to know everything about the product when I'm shopping. For the life of me I can't understand why it should be a secret. It raises suspicion on my part, simply because most businesses hire the cheapest supplier.
With that being said I have several of Rainier Arms ultra-match barrels and have nothing but praise for them. They aren't the only company that won't discuss their supply chain. Even utilitarian items like bolt carrier group's manufacturer won't be revealed which makes me even more sceptical.
I usually buy bolt carrier groups from Stag Arms because I can be reasonably certain that it was made by Continental Machine Tool, their parent company.
Many top barrels are contracted through Compass Lake Engineering because they have the machining capacity to produce in volume. They also have a stellar reputation.