Most factory loads are near maxed out and it can be unwise trying to improve on some of them unless you know that the factory loads are tamed......such as the 7MM Mauser and other cartridges.
Many modern rounds such as the 7mmstw,and 300ultramag are loaded very mild by remington.Even 300wby rounds loaded by remington are loaded much milder than the norma loadings.
By the way I have measured many belts on factory belted magnum loads and several of them expanded by more than .0005" after one firing.I have seen 300wby factory loads that expanded .002" at the belt after one firing in a factory mark V rifle.