Headspace at the shoulder. Having an exact or real close headspace measurement taken at the bolt face between belt & shoulder is unlikely. Ignore the belt, a useless feature put on certain cartridges & retained only because some shooters think the belt is needed with cartridges having a shoulder.
A .458WM should have a belt. There is no such thing as a .47-70 rimless.
"I took the rifle to a gunsmith today who checked the head spacing with go- no go gauges and said the spacing was good." The chamber might be within specs. Repeated F/L sizing driving the shoulder back at each sizing will result in a case separation just forward of the web.
F/L size at the shoulder & forget about the belt. Getting a 6.5mm, 7mm, or .30 PRC is an option (no belt).
The 1976 Wimbledon match having a score of 299-21X was won using a .300WM head spaced at the shoulder. A shoulder based headspace would have more precise control vs belt length that might vary a few thou inch from cartridge to cartridge.
Anneal the brass to reduce spring back & head space on shoulder. I use a .007 feeler gauge between bottom of F/L die & top of shell holder that assures consistent shoulder-based headspace.
Browning ain't (is not) going to say the chamber is screwed up. There is an option of buying "long" brass.
