Very common problem with late NH WSM guns. I've got a 7mm WSM and a 300 WSM, both very early rifles without the problem.
I've also checked end of production WSM rifles in the same calibers with egg shaped chambers.The 7mm would chamber one type of commercial ammo fine, another type with some force required on the bolt, and another brand of ammo would not chamber under any circumstances. Best bet is a rebarrel.
