The '98 Mauser is plenty strong enough to hold the pressures of the 300 WM.
If something "locked up" it wasn't because of a weak action, something else is the culpert, hard to say what, dirty chamber, faulty loads, or what ever...........I can't see it from here but it's not because the action is too week.
As a side note, find another gunsmith.
CPT USAR Ret
USAMU Sniper School Oct '78
Distinguished Rifle Badge 1071