Re: 215's or GM215M's ?
My best information comes from a respected shooter and LR rifle builder, Kirby.
If you are patient w/the search function here, you will find a post when he reported that the 215s are a bit "hotter" than the GMs. That seems to be important with slow powders over 100 grains and higher.
From that post, if I remember correctly, he uses 215s exclusively.
Learning that, I switched from the GMs to the 215s and haven't noticed much difference. However, the variables that I'm having trouble with make a larger difference than the difference in primers.