Whenever you change a given component in your load recipe, especially if it varies from a specific book recipe, you always back off 10% and work back up. Even if you go from a magnum primer to a standard primer, you follow this logic. Primer/powder/load compression/crimp/bullet weight/bearing surface...there are too many variables to consistently predict what effect changing any of them is going to have on the resulting pressures.can small magnum rifle primers be substituted for small rilfe primers in the .223rem. poppa