Have you tried asking Sierra who makes the brass they use in their loaded ammo?
I just recently talked to Sierra and asked who makes the brass they use in their loaded ammo, ironically enough it was regarding some of their 204 Ruger ammo particularly the Prairie Enemy 36gr BK stuff, and got the smartass answer of "what does it say on the rim". When I pushed further and told the Richard Noggin it says Sierra but I wasn't aware Sierra makes brass he told me they don't divulge who makes the brass they use in their loaded ammo. Guess I wasn't aware the brass Sierra uses in their loaded ammo was some kind of industry secret. Maybe you'll have better luck but they guy I talked to was a D-bag.