I read it two different places I believe it is one response that has been worded like absolute. do not use federal primers because of the slam fire issues and possibilities..
It's good to be cautious loading for the M14. Yes, the Fed 210M's were used with the possibility of slam fires. They were supposed to be more sensitive (Lee used to insert a caution in their instructions about NOT using 210M's in their tray fed priming tool because of the possibility of "sympathetic detonations", where if a primer goes off while seating it, the primers in the tray could also go off from the shock.
I and others used them knowing the warnings (I've never seen anything formally stated btw). One question I asked myself is "how did Federal load their highly regarded GMM with their primers safely?" I did reduce my risk of a slam fire in the other places where I could. I cut the primer pockets so that primers seated proud were not a possibility. I also sized my cases to below chamber headspace to try to minimize the risk of an OOB detonation (little different but related problem).
