Did what I should have done last night, and broke down the Glock to clean and lube it, and went beyond my usual field-strip, toothbrush the slide assembly and snake the bore, drop a little lube and call it done. (Given that the usual advice seems to be to break down the slide assembly for cleaning every 3K rounds or so, I don’t think I was being that neglectful).
Took apart the slide assembly. The firing pin was pretty clean (I mean, it left some dirt behind when I wiped it down, but nothing bigger than some dust), and most of the rest of the slide assembly was the same. The extractor, though, had collected a bunch of gritty-looking nastiness, and the firing pin safety had a small collection of its own. Didn’t appear to prevent function (I never had any failures to extract), but I cleaned it all up anyway. Found an empty rattling around the safe in the process, (from a previous range session, I didn’t have any open containers with me at the firing line this time, and there was no brass in my pocket either), and compared the striker impact to the photos of the duds, and the empty was definitely a center hit and showed a faint rectangular outline around the main impact point, so whoever it was that had “light off-center strikes” in the pool wins a no-prize. It’ll probably be a couple of weeks before I can make it out to the range again to test-fire (and make sure I got the extractor in right).
I’d say “why only those three rounds,” but the answer is probably somewhere around “tolerance stacking.”