I’m not saying you’re wrong, because I haven’t tested it(or even shot 210’s) but this does not jive with commonly held held beliefs regarding Federal primers. According to MANY reports, the 210 and 210M are the same compound on the same paper. The SUPPOSED only difference is the actual employee making the primers. The primers made by experienced and skilled employees get labeled 210M. Apparently doing the job sloppily can yield acceptable, but less than ideal results, and taking care to do the job properly and/or developing some specific skill, can yield much more consistent results.
That’s the general consensus from those “in the know”. It’s entirely possible for that to be correct and YOUR batches of 210’s to yield a different velocity than 210M’s. It’s also possible that those “in the know” are wrong and/or full of crap.
That said, I only worry about changing primers on a load causing a safety issue if I know I’m changing from a very hard cup to a very soft cup. Usually pressure differences are minimal, but how much pressure. Axis can take is worth noting. My first pierced primers resulted from a load developed with zero pressure signs using CCI primers(one of the hardest) and substituting Winchesters(one of the softest).
