You are NOT correct. You have got the cart before the horse.
The issue of the type of powder mattering for the use of magnum primers occurred AFTER the magnum primer concept was developed. Specifically, it occurred in 1989 when CCI changed their magnum priming mix formulation to add metal particles for hotter sparks to better ignite spherical powders with heavy deterrent coatings. But their magnum primer line existed long before they figured out they needed to make that change to improve spherical powder ignition. Not the other way around. I know this is so both because I still have some magnum primers from the '70s and early '80s, before that change was made. The reason we know when that change was made is because we have it from the horse's mouth in
this article by Allan Jones, who worked on primer mixes for CCI when it was done.
The whole reason magnum primers were developed originally and before the powder-type ignition issue was addressed is that magnum cases tend to have larger-than-average internal volumes. Smokeless powder that is confined requires a minimum amount of heat and pressure to sustain burning without squibbing out until the heat and pressure from its own combustion gets high enough to provide that. The empty space between grains plus any empty space left over when the charge is less than 100% load density is greater when the case is large than in smaller volume cases. Thus it requires a larger primer gas quantity to bring the pressure up to that reliable burn-sustaining level in those larger case. Magnum primers are made with a larger priming mix charge weight to provide that larger gas volume.
Note that the volume above is the total empty space volume and not the volume of the powder charge unless the load density is 100%. 15 grains of H110 takes up 80% of the total volume available in a 30 Carbine case. 16.7 grains of H110 takes up only 65 percent of the volume of a 357 case. The total space between the grains of the larger charge plus the empty case volume leftover is greater in the typical 357 Magnum load.
There are always exceptions in a given case because of the large range of bullet lengths and seating depths affecting the powder space. This is why you see some loads with a magnum primer that don't really need one, but you will also run into loads without a magnum primer that would probably benefit from one. For example, the 30-06 case is large enough that a lot of common Garand loads don't fill the case well. I've seen several examples of people cutting down group sizes with those loads by going to a magnum primer, even with easy-to-light single-base stick powders. The practical thing for the handloader to do is to try both where both are an option and see what works best.
