It takes a couple of weeks for the cast bullet to reach an appreciable degree of increased hardness from the fresh cast. Then, the only hardness difference is surface hardness. When you size it, you are right back to as cast hardness if the sizing die actually resizes the bullet.
It is like "Case Hardening" that is only suface hard.
Years ago, when I was deadly curious about this subject, I took on the project of casting, recording hardnesses of many bullets and then setting them up for a period of time. I had enough bullets from the cast to use one at different times to test. My hardness tester is a Saeco type and it punches a depression in the surface of the bullet, thereby rendering it useless unless your turn it over and use the base.
I tested one or two every day out to 3 weeks. Until a couple of weeks passed, there was not an exceptionally noticible change. However, by about 3 weeks, it would reach an equivalent of 2 numbers of Brinell hardness difference (harder). That is one degree on a scale of 1 to 10 on the Saeco Tester.
I would have that much difference in different batches of wheel weight material, so I discontinued worrying about the aging of bullets.
It was fun, though, but not worth my time.
Harold Clark
I don't recall the exact times and testing results, but I will look for my notes on that subject if you are terribly interested.
