Not to steal your thread, but here's what happened to me once sort of along the lines you're asking about:
I got a large quantity, 15 pounds or so of 20-something year old IMR 4198 powder for $2. each, all in sealed, clean, unopened 1lb. cans. They sat around a few more years, and I stumbeled across a load in a old 'Cartridges of the World' book that called for 40 grains of it in 7.62NATO, or .308.
So I loaded up a bunch...... what the heck, it looked OK, and smelled ALMOST as strong as new powder.......
About the 5th or 6th one I fired blew up my Springfield M1A. The bottom of the bolt face blew out, the gases went into the magazine and it blew out/opened up like a flower. A big chunk of the stock let go under the bolt and really 'whacked' me on the right forearm arm ( I shoot lefthanded).
I sort of figured out later:
The load wasn't exceded... But the old powder had dried out, even in the metal cans over time.
(New powder is still a little moist with the Acetone they make it with when packaged & shipped from the factory. This factor is figured in with the listed charge weights.)
But;
Once dried out.... I think each little speck of powder has about the same energy as it did when new, but without the little bit of moisture, they are each a little lighter too,,,, so, you also include more of them for a specific charge weight. Hence------ a overloaded case.
In later years, I weighed some of the old 4198, and some from a new can, then put them both in a .308 case. The old stuff fit without being a compressed charge, but it did come up higher in the case than powder from the new can did. That sort of confirmed that there were more little specks of it to make 40 grains, than using the new stuff to make the same weight.