My Ruger BH 357/9MM will pierce Magtec SP primers about 50% of the time. Light loads don't fix the problem. The Magtecs still fail with 9MM using test loads with Lyman 358242 (120 gr) over Promo 3.0. The problem with Magtec SP primers is less pronounced with 357/38 Spl but still the occasional failure. Evidence is that the primer is black and there is powder residue on the hammer. No sign of flatening or cupping.
Good news is that yesterday with over 100 rds each using CCI and Win SP primers using my usual mid range loads in both 9MM and 357 I didn't see a single primer issue. The fired CCI and Win primers look as normal as can be. Lesson learned.
I've gone thru over 10K of Magtec SP primers in different 9MM autos and my Ruger SS and never noticed a problem.