CCI changed their magnum rifle primer formulation in 1989 specifically for the old chemistry spherical propellants. These are St. Marks Western Cannon series powders of the type developed during WWII to use nitrocellulose recovered from over-aged artillery propellant. The oldest still in production is WC846 (BL-(C)2 in canister grade) which was originally developed to load 303 British ammunition for the Allies in WWII and later adopted for M80 7.62 ball ammunition. These propellants get their progressive burning characteristics from use of a deterrent that penetrates the grains, leaving a high concentration at the surface (why they are hard to light) and a diminishing concentration toward the center. This means the outside burns much more slowly than the inside, requiring a higher start pressure to sustain burning, with the burn rate speeding up as it burns inward in order to increase the rate of gas generation (progressivity) despite the fact the grain surface area is diminishing as it burns toward the center. The CCI change added some metal particles into the priming mix to increase the heat of sparks landing on the deterrent's surface to better burn into it.
These powders include:
H110 / 296 (WC296)
HS-6 / 540 (WC540)
HS-7 / 571 (WC571)
748 (WC748)
H414 / 760 (WC760)
H335 (WC844)
BL-C(2) (WC846)
H380 (WC852)
US869 (WC869)
In my experience, Accurate 2520 also benefits from better ignition and other slower sphericals likely will as well. Ramshot claims its more modern spherical deterrent chemistry ignites more easily and doesn't need the higher start pressure a magnum primer provides.
So, why doesn't the use of a magnum primer always prove superior in every application of those powders and why is it sometimes necessary with stick powders? Generally, this is about the cartridge case size. A magnum primer's main purpose is to provide extra pressurizing gas to a cartridge case for better ignition and sustained burn. But if you have a very small volume case, such as most pistol cartridges do, even a standard primer provides higher pressure than it does in one of the old long revolver cartridge cases. Indeed, in the small volume behind the bullet in a pistol case, a magnum primer often creates enough pressure to start unseating the bullet before powder burning is well under way, thereby expanding the burning space and causing an irregular hesitation in the buildup of pressure. So it can thereby actually increase velocity variation and lower velocity by having the peak occur in a larger burning space expanded by the bullet movement.
At the other extreme is a larger case, like a 30-06, that is loaded with an easy-to-light powder like IMR4895, but that has been loaded to a low loading density of 80% or so for a "Garand load". This leaves a lot of extra empty room in the case, and that can lead to a standard primer having a hard time providing adequate starting pressure for the powder. In this instance, the extra pressure from the magnum primer can make ignition more regular. But here you will find that if people use a range of loads that go from lower to higher loading density, the magnum primer can be found to do better with the lighter loads for the reason described, but that a standard primer does better with the fuller loads that leave less empty space in the case.
So there tends to be an optimum primer pressure for each powder that is affected by both case capacity and loading density. Perhaps they ought to make primers in pressurizing capacities that vary from levels 1 to 10, so everyone can find the best balance for their load. But then again, maybe not. Just imagine how much more load development testing that would involve.
