Your manual likely point out that the use of magnum primers instead of large rifle primers changes the load. I believe they advise you to consider the magnum primer to be an additional gr of powder in terms of its effect on the load. In your case, that would suggest you could expect a diminished performance of the load when substituting the large rifle primer. while it may follow that you are reducing the load by the equivalent of 1 gr there may be other factors at work. As you know the intent of the mag primer is to ensure a good and even powder burn in the magnums high capacity case. this is likely info that you already know but I think some caution has to be exercised as too reduced a load in a magnum case can have serious consequences as well. I wonder if a call to customer service rep from one of the companies who supplies loading manuals would provide some expert advice? I would be interested in hearing their reply.