High primer, FTF .357 mag. It went off on the second try. After the first FTF i kept muzzle pointed down range for at least 15 seconds. Pulled to examine...best i could come up with was a high primer. S&W model 27s dont really have a reputation for light strikes
the gun was ruled out as the culprit.
Over crimped plated bullets. Had about 10 keyholed rounds of the batch of 100. This was in 10mm with 180 gr. Berrys plated hybrid hollow points. Fixed by purchasing 3000 pieces of HAP 180 from Hornady with a real jacket and called it a day. I much prefer hard cast bullets or jacketed.
Ive also pushed plated bullets too fast in 357 mag. causing extreme horrible accuracy (thought id just use the same recipe and improvise on bullet weight for weight which is a Reloading day #1 NO-NO)
Plated bullets look very pretty, but they are very soft pure lead cores (like 8 on Brinell) with a very very thin copper alloy plating, no thicker than a piece of paper. Sometimes plating is very brittle from the heating/cooling or electronic plating process, and literally crumbles off the core like broken glass when it goes down the barrel. Plated bullets and Cylinder gaps on revolvers are a crapshoot whether you get small stingers to the face, even on a light load and a perfectly tight gun. Plated is the worst bullet to deal with IMHO and the benefits they provide are outweighed by the potential for issues.(Yes, some of the problems ive had with them were my fault)
Loaded a primer in sideways once, lucky it didnt go off. It must have flipped sideways, it was the last one of the cycle of 100 on my Dillon 550B. Ive looked at the primer position ever since, just a habit now, among so many other small habits you pick up and do involuntarily over years and years of small mistakes that you DO NOT want to replicate.