Get hold of Tony at Tromix. He'll walk you through other mfg's problems. Sounds like an extractor. The 458 is a little wonky when it comes to extractors. I have a Radical upper and ended up getting a new extractor from Tony and it runs fine now. If you are looking for anything else big bore, get hold of Zach at Atheris Rifle co. I got his Beowulf upper last year and it works fine (once you run it a bit) and am waiting on his bushmaster upper now. Complete 450 upper cost me less than $400 on sale a couple weeks ago, and I know it will work.