Epilogue - I should start out by saying that a couple decades ago the trigger on this gun began functioning like a single trigger, as in one trigger, one shot. I took it apart and found the most minute piece of dust in the selector mechanism. That was the precedent for me (and others) to suspect gunk.
I took the stock off a second time the other night, but did not try anything more heroic in terms of disassembly. Over white paper I blew out the interior of the receiver and indeed a chunk of dark stuff fell from in front of the right hammer, but it was soft and I doubt it ever caused any problems. There was nothing else to be done so it all went back together.
Oh, I checked the firing pins and found that they protrude about 0.052" from the breech face. That's healthy.
In a subsequent test fire session, I again got a misfire. Like before, the shell had a tiny nick in the primer, not enough to detonate. I measured the distance from the head of the shell to the top of the primer cup and got a 0.031" gap - pretty deep. I measured a bunch of other unfired shells and was getting a value of 0.008"+/-. These were old, old Winchester primers. My takeaway is that about 1 out of 20 of those primers had the cup set sufficiently deep to prevent reliable ignition. Who would have thought?
