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The standard 9 1/2 is a pretty mild primer, but it should be adequate for most loads, especially loads using extruded powder.
Magnum primers are called for when any one of these things is true:
The extruded powder charge is more than 60 grains, or
The spherical/ball powder charge is more than 40 grains, or
The temperature is lower than freezing.
If in doubt, use the magnums, but do NOT switch TO magnums if the load is already near maximum. Back the load off by 5% of the charge weight first.
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