The 8x51mm will have a much smaller shoulder than the .308. All of the. 307 family, the .243Win., 6mm Rem, 7/08Rem. .308Win. and .358 Win. have a .454" dia. shoulder, but also that 20 degree sloping shoulder. A .454" shoulder is the same diameter that P.O.Ackley chose for his "Improved" ctgs.