Which of these is the mildest small rifle primer-

Remington 7.5
CCI 400
Win WSR

In my testing in .223 the CCI 400 would be the mildest, but the results are load dependent.
In other words depending on different powder, powder charge, bullet, brass, rifle, you will see shifting results.
FWIW in 5 of the 6 tests I did, the CCI 450 provided a lower velocity than the CCI 400.

Were you surprised at the lower velocity of the 450's?

A bit, seeing as how the CCI 450 are magnum primers.

This may be common but I was not aware of it. Thanks.

You never know what you are going to come up with in doing these tests. That's why it's usually worth the effort to do so. A year or so ago, when shooting groups (and chronographing) using three primers, CCI-400, CCI-450, and Remington 7 1/2s, in two Colt ARs, I found the CCI-400 provided generally better accuracy than the other two primers, though the differences were small. I realize the CCI-400 is not recommended for AR loads.Sometimes, velocities of standard and magnum primers are virtually identical. Switch powders and the velocity difference may be significant.

From all my research and talking to primer people the only difference between a SRP magnum and non magnum is cup thickness. LRP big difference.

cci

Good grief, try them all and see which one works best for you!