The following is a portion of an email exchange with CCI:
Small rifle primers
#41......................................Mil. Spec. primer, thick cup, magnum primer charge, angle of anvil change.
BR4.....................................thick cup, standard anvil and standard priming mix held to a tighter tolerance.
Justin M./Technical Service Rep.
2299 Snake River Ave.
Lewiston, ID 83501
Alliant/Blazer/CCI/Speer
(800)379-1732
The CCI BR4 primer is a standard and not magnum primer mix. The CCI 450 and CCI #41 both use a magnum primer mix. In addition to a thicker cup the CCI #41 also uses a different angle on the anvil which is how the CCI #41 acheives less sensitivity. The best source of CCI primer information is either a quick phone call to CCI or shoot them an email.
As to any advantage to using a magnum primer with Reloader 15? Reloader 15 is an extruded type powder so really a magnum primer in 223 Remington loading isn't necessary. While the CCI BR4 primers should work well I have also shot groups just as tight with CCI 400 standard small rifle primers. I suggest you see which primer delivers the best results / accuracy in your specific rifle and run with it. The Remington 7 1/2 is another primer I like in 223 Remington.
Also for what it may or may not be worth the below images were a result of using CCI 400, CCI 450, CCI BR4 and CCI #41 primers. The load was 26.1 grains of H335 powder below a Sierra 53 grain HP Match #1400 bullet. The cases were all loaded with each charge weighed. The target was at 100 yards using a Remington 700 bolt gun.
Worth noting is the velocity chart in the second image and while the CCI #41 afforded the best group that may not be true as I was close to my sky screen edge when I shot the target so was more concerned with just getting each 10 shot group over the chronograph than trying for group size. Anyway, this is why you really want to see what works best in your rifle with any given powder.
Ron