I had the same problem wth my .454, I was sizing bullets to Freedom Arms specs, but still I had problems with WFN bullets to chamber. Not all die manufacturers account for alloy springback, and that is what happened to me. When accurately measuring bullets a couple of days after sizing, they exceeded the FA specs for that particular cartridge. Now with a proper die and my bullets land right at .452, which is the throat diameter of my gun, and they chamber just fine. Try to measure your bullets with a micrometer some time after sizing, they might actually be larger than you think. Solutions could be to size .0005 to .001 under throat diameter, or to use an alloy with less springback.