I had a similar experience about 15 years ago when, following the oft-advised practice of using a magnum primer for spherical powder, I substituted a CCI 250 primer for whatever the manual called for in my .257 load. Never got to the book max, as a primer fell out a couple grains under that. No indication whatever that anything was wrong until I opened the FN bolt and saw the case. No more such silliness for me.
