Re: 215 vs. 215M
Eric,
I think you will be ok. I have used the 210M's for load developement, and then ran the regular 210's for my 308 for a long time, due to not being able to find more of the "m"s. I can not tell much if any difference. Most seem to agree on this, but some have said that there are lots of the non "m" primers with wider standard deviations and extreme spreads. This probably amounts to a bunch of nothing as most of our shots are not far enough out to hurt you that much from verticle dispersion.
Good luck!