Interesting! My son is having problems with his Browning 338WM chambering. I hadn't come across this problem before, and had me going. In reading all the imput here, I feel that's possible the problem with his chamber and dies. In my rifles I hadn't had any of those problems. About 25yrs ago I switch to neck sizing my belted mag, without any trouble. I was loosing case in 3 firing. I was and still do have a 308 NM rifle. It's shorter to the shoulder and longer neck than the 300WM. I happen by accident in trying neck sizing for that cartridge, but only could get a 300WM neck sizing die. I didn't want to wait for 6 to 9 months to get a special order die.
Being the difference in the two cases, the 300WM neck sizing die would only size about 3/4 of the neck. Leaving the balance unsized. It work just fine, and stop the case separation at the belt. My case life was extened to about 12 firing before I would loose the primer pockets. So now my cases are fired formed up and including about a1/4 to 1/3 of the neck. The balance of the neck was sized. It reduced my groups some, but I was close to 1/2" @ 100yds. I was and do shoot 165gr Nosler BT and AB in that rifle @ 3300fps. I am 5+grs over the manaul hot load. That why I began to loose primer pockets. I learned a lot during that time of going to the range at that time. My sons rifle is in Montana and I am in Mexico-North (aka Calif) presently so I can't measure his cases. So I order 3 die this morning. One sons is about 180m mile away from us and my brother reloads too for belted mag rifle. So I got a die for the 3 of us being we are in different places. The other is I don't want to wait until I get to measure the cases of my son. I can see the dies are not cheap either. Thanks for the imput. It's only money!
