In my experience wearing out 16 or so .308 Win. and 30 caliber magnum barrels in competition, they all shot great starting with the first round fired. Never worked up a load for each, either, as long as the same bullet was used.
US Navy Distinguished Marksman Badge 153
Former US Navy & Palma Rifle Team Member
NRA High Power Master & Long Range High Master
NRA Smallbore Prone Master