I had seen somewhere on the Internet where some bullets coating would react with powder - I forget the details.
I've had powder go off due to poor storage conditions - exposed to a hot wall repeatedly over many years, but it typically corroded through the case at the mouth and/or the primer. Your's looks to be corroding at the base of the projectile where powder meets coating.
Yes, the possibilities include breakdown products from a deteriorating propellant (which typically include nitric acid) collecting there near the base of the bullet because of the direction the rounds face in storage, and attacking preferentially in the crevice.
It doesn't look like an external cause.
That makes sense.
I'm not sure about the coating on those bullets. I don't see MidwayUSA still listing them, not sure Speer still makes them.
I didn't smell any acidic aroma, in fact the powder I pulled from a corroded case still smells like it should. There was none of that brown dust you see and smell when powder goes bad. For sure you can detect the nitric acid smell when that happens. This is different, just corrosion where you see it, the rest of the powder doesn't look affected.
The fired round that sounded less than full power probably had more powder deterioration than the one I pulled.
DF.
