I found this result on my 338/378 weatherby and on my 378. May have something to do with the Weatherby neck. Haven't bothered to try the collet die on my 338 Lapua IM, went straight to Redding and bushings. However I use only Lapua brass which is some of the best on the market. It comes pretty damn straight and uniform on neck thickness.

It is better to be judged by 12 than to be carried by 6.