I shoot a 30-378, and a 338 RUM. they are both at thier best a little dirty, the 338 likes at least 1 fouler, the 30-378 starts to shoot after about 3. they both start to loosen up at around 20 shots.
really tho, every gun has its sweet spots, often a product of the owner, weather it started with a rough barrel, weather it was broken in, hand lapped, etc..
my 338 hasnt seen alot of rounds, but the 30-378 has some where in the neighborhood of 2000 rounds in the same barrel, i've cooked bullets from 4000+ to what i am shooting now, 3450, 180 gr. it could use a new barrel, but still holds in the 1-1/4" range. dont over heat,clean and use a good protectant when stored, depending on what you use it for, it could last a life time.