Good question. First of all, we need to understand that reloading manuals are not carved in stone. Think of a reloading manual as a report. In essence, a reloading manual says, “We tried this particular component combination, and these are the results we obtained.” When you duplicate the load shown in a manual, you’re using a different rifle (even if it is the same make, model, and caliber), a different lot of powder (even if it is the same brand and type), a different lot of cases (even if they are from the same manufacturer, etc.), a different lot of, well… you get the idea. There are an amazing number of variables that affect any load combination. With the difference in the manuals, you’re just seeing first hand examples. Again, start low, and work your load up.
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