Hi Bonanza,

I've never done any extensive primer testing, though I've read of some. It is widely held that "weak" primers produce superior accuracy. Thus, magnum primers should be used only when standard primers are insufficient to properly ignite the powder charge. This would occur when very slow powders are used, and when very large amounts of powder are used. Frequently (but not always) one implies the other.

It is my experience that # 4350 is at the cut-off point. In the 30-06 standard primers work best. Many reloading manuals recommend magnum primers in the belted magnums when loaded with # 4350. I use them myself in the 300 Win Mag and the 375 H&H, but I am not convinced that they are needed, and may even be inferior. Perhaps this summer I'll load two batches of otherwise identical ammo the test the hypothesis.

Very cold weather is another indicator for magnum primers. Powder becomes harder to ignite, and pressures are reduced when air temperature is greatly reduced. Loads for very cold weather should be developed in cold weather. Likewise, loads for very hot weather should be developed in hot weather, as the opposite is also true. Pressure is increased when it is hot outside. Primer choice could easily depend on temperature.

Large cases alone do not signify a need for magnum primers. Ammo for my 458 Ruger #1 is loaded with # 4198 or # 4895. Neither benefits from magnum primers.