A friend & I scored a couple thousand Winchester SPP that were at least 20-25 years old. Cannot attest to the price, but these SPP’s were of the nickel plated kind.
Upon inspection 90% of the primers had corrosion on the surface, some primers had more, some less. The corrosion didn’t appear as typical rust but a kinda yellow-brownish film. They must have been stored in a humid environment. However, they all worked but you could really notice the corrosion.
Powder will not really degrade as long as the container is the new plastic bottle type & new air cannot enter. Older paper containers with tin lids & bottoms will clump & become different to meter. Should you encounter/possess any of this my suggestion is to wait for the Queen Victoria’s birthday celebration. A few bundles of this powder in tissues strategically placed in the Burning SchoolHouse make a fine display.