I had a similar experience with a H&H .500 3 1/4". I was sizing .500 3 1/4" Basic brass with an RCBS 7/8"x14 die. It appeared that the die wasn't thick enough at its base and that the web area of the case was spreading the die - thus, not being sized correctly. I bought a set of 1"x14 dies and presto! - problem solved.