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.
450's get used by people building .223 ammo all day every day and are preferred for lighting off ball powder in that case. The thicker cup on them is also liked by guys like me shooting .473" bolt face chamberings with small primer brass like .308win/6.5CM/6CM/6XC etc.... With thinner cupped primers like the CCI 400's we get a lot of pierced primers unless you go to small pin and bushed bolt face. With 450's or F205MAR or CCI #41's, never get pierced primers. I use them in 6XC specifically to avoid pierced primers.
It's hard to beat CCI 450 MAG PRIMERS.
They are my go to primer for all my SRP case needs. From the ARs to long guns.
Very consistent on weight and Velocity.
Also, I have never pierced one in my AR-15's. You just can't beat them for reliability. I have tested and compared just a bout all of the name brands over time and like I said, you just can't beat them. OPINIONS vary on this subject.and this is what I found that works for me.
Merry Christmas Everyone..