This may enter the realm of a bad question but I have read here that belted magnums are headspaced at the neck rather than at the web, If this is true it would explain why a friend just sent me a Wby mag shell with it’s neck separated off of the shell. This is not to gain reloading information but to understand how the cartridge lines up in the chamber? Please delete if bad! Thaks Vic
Belted magnums head space on the belt. Firing .300 Winchester in .300 Weatherby will stretch the heck out of the shell, I see it all of the time being done.
As for your friend,either the wrong ammo is being used. his dies are set too far down and he is shoving the shoulder back and work hardening the brass and when it is fired it is blowing the shoulder forward and failing, or for some reason his chamber/throat is way too large.
Now that being said, if your chamber is sloppy,or your using some once fired brass from a diff rifle, you can resize to the shoulder of the case. It is confusing to be sure.
If the chamber is tight, use the belt as the head spacing.
On first firing the headspace is controlled by the belt (the belt being nothing but a very thick rim).
Chambers for belted cartridges tend to be quite sloppy
The chamber cut for the belt runs from .220-.228 (SAAMI), while the belt on the cartridge itself is in the .213-.219 range.
One thing to watch for (if you collect cartridge dimensions like I do) is that people who reload these often set up their dies to HS on the fire-formed shoulder, rather than bringing them back to original dimensions.