I have a Custom 358 STA barrel from MGM. First, I would never buy another barrel from them again and there customer service is very poor. Second, the chamber looks like it was cut with a butter knife. I sent the barrel back to the them after I noticed the brass was getting all marked up and you could feel the marks in the brass with your fingers. They told me the warranty was voided because I had a KDF installed. When I asked them what did having a KDF installed on the other end of the barrel have to do with a poor cut chamber on the other end. His reply was that the person that made my barrel no longer works there because of issues. I asked him to replace the barrel and he said he couldn't. He said he could clean everything up but I would need a custom set of dies afterward because it would take a lot to do that and of course they would not pay for the dies. He suggested that I just shoot it like it was and the brass shouldn't crack around one of the crazy rings around it that you could easily feel with your fingers.
I haven't really shot it much since I got it back but I did have a local smith polish the chamber to the best he could without causing any issues. The other problem I have with this setup is it shears the forearm screws(8x40) off with in 7 shots on the bench. The 358 STA is a power house but I need a cure for the screws shearing off. I really like the cartridge.
Take this info however you like, its just my experience with MGM. Thanks