Outside neck turning is a more precise way of achieving uniform neck thickness. Forster makes the one I use. It can be used by hand with the Forster case trimmer set-up, or can be used with a drill press for mass production. However, the minimal case neck reduction required in forming .400 Whelen from cylindrical brass should not result in nearly as much increase in case neck thickening as sizing down to .30, .338. .358 or even .375. I would try sizing a case or two and measuring the neck thickness with a tubing micrometer before investing in neck turning equipment.