Good post, Mike. For sure the styphnate initiator is an explosive. However, there is very little of it and it doesn't contribute enough gas to have a big impact on pressure. The propellant main charge is also small relative to the powder charge, so it is not a big contributor to chamber pressure. Thinking about this, I can imagine that location of the stray primer within the powder might also be an issue. Say we have the stray sitting directly over, or very near, the intended primer, then it might ignite from the first primer firing instead of cooking-off. This would, in effect, give the powder a double dose of ignition and might account for a higher chamber pressure.
Mike, the load data is interesting. "Hotter" probably doesn't exactly describe the way the primer ignites differing powders. Have you looked for a pattern between primers and powder burn rate?
8-b, I'd use a strain gauge glued to a stout test barrel. No big loss if you bulged the chamber.
I load with a P&W 800B. The primer feed tray is directly above the incoming hulls. I have, on occasion, had primers misfeed into the guide rail and drop into hulls. Far as I know, I've always caught this event, but now think I'll rearrange some things.
Another good discussion - thanks all.
