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Any standard rule of thumb on powder reductions when switching from standard (CCI200) primers to magnum (CCI250) primers? Most of my loading is for a 257 Roberts (IMR 4350 and H4831SC) and a 308 Win (Varget and RL15). Thanks

I've used them interchangeably in various calibers without problems.

Jim

No need for Magnum primers in those calibers unless you use a ball powder.

I have 800-900 mag primers, and less than 100 standard. Just wanted information in case I ran out of standards before I could find more.

For those cartridges, you can use them interchangeably. There may or may not be a difference in accuracy; perhaps even for the better. Also, in cartridges of that size, there is no need for magnum primers even with ball powders.

Not what Winchester,Speer and Ramshot recommends

I've run about four instrumented tests on pressure and muzzle velocity using Federal and CCI standard and magnum primers in cartridges 30-06 and smaller. With proper control of chamber and cartridge temperature, I have never found a real MV or pressure difference between any of those four primers.

That is not to say there is no case where primer makes a difference. I'm sure that a very large primer is beneficial in the 50 BMG, for example.

All that said, I do think that many of the reported differences in those primers are artifacts from lack of control of temperature, failure to apply proper statistical tests, failure to randomize the experiment, etc.