I always use the standard 215 over the 215M primers simply because in my testing several years ago, the standard 215 primers were noticably hotter and I found with powder charges over 100 grains, the consistancy of the standard 215s seems more consistant.
Also, in cold weather, which I mean as below freezing and especially below 10 degrees F, the hotter primers just seemed much more consistant.
Another area I prefer them for is when using ultra slow burning ball powders. Again, the hotter the primer the better for consistant ignition.
I would not say that these are not the best for every application but for the very large capacity chamberings, or when shooting in very cold temps or when using heavy doses of ball powders, I find the standard 215 superior to the match version.
Again, these tests were done several years ago. In the specific test I was using a 200 gr SMK in a 300 RUM. IT took nearly 2 grains more H-1000 to achieve the same velocity with the 215M primer as it did with the standard 215.
My velocity spreads were also tighter with the 215.
This has held true when I started testing my first three Allen Magnums so I have just stuck with the standard 215. The added savings in cost are just a side bonus.
For smaller capacity magnums or conventional rounds, I would say you would not see this advantage, at least not to a measureable amount.
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