In my .260 AI I am using Peterson SRP brass. A little more capacity than the creed, but not a ton. When I first did load development with rl26 and 147 eld-m's, the best primers were br-4's, e.s. was somewhere in the teens. Then I switched to Berger 156's with same primers, powder and brass, and velocity was all over the place, like 75+ spread. Did a primer test with the br-4's, federal magnum primers, Remington standard, and the cci 450's. The 450's brought e.s. down to the low teens consistently. With all those different primers, velocity average was all within 50 fps or so average of each other. Anymore, I do a primer test as part of my load development unless a load just really comes together without changing anything.