Some things that you may have gleaned from this discussion:
1) Use of magnum primers is OK, but work up from a starting load. Do not substitute components afterwards. Use of magnum primers may result in a large increase in pressure. If your load is already near max pressure, and you substitute in a magnum primer the results could be catastrophic.

2) Ball (spherical) powders can be harder to ignite, so magnum primers are often used for that application. Reloading manuals will often indicate this.

Note that all military small arms ammo made by Lake City AAP uses magnum primers. When it absolutely, positively has to go bang – even when it’s 50 below zero.