Teton Ted: Glad you came to this board with that question. Stuff like this gets passed around by folks who " heard it from someone", and end up doing all kinds of silly things.
Cast Round Balls are made of as pure Lead as we can get. They are as soft as we can make them so that they upset in the barrel when the gun is fired, pushing the cloth patch around them into the grooves of the barrel to provide a gas seal. " Aging", " Tempering", or any other treatment of soft lead changes absolutely nothing.
Having said that, when casting ALLOY LEAD bullets, with antimony and tin added, usually for pistol and revolver BULLETS, ( not round ball for BP revolvers), the alloys do harden just a little by sitting for a few days. Attempts to harden bullets further have been done by heating them in ovens, and then cooling them quickly by dousing them into cold water, etc. but no real improvement in hardness occurs over just letting them sit for a couple of days. The degrees of added hardness is in the range of 1-2 degrees of Brinell Hardness, and is not enough change to spend much time doing, or worrying about. The higher the Antimony content, the more Hardening is involved, because Antimony hardens lead, but can also make lead more brittle, so it shatters rather than flows on impact.
People have played around with CAST BULLETS for years, trying to get different kinds of performance. Mostly it fails. The hard alloys, that have lots of antimony and tin, are just very difficult to run through Lubresizers, wearing out machines, and men who are trying to size them on manually operated machines.
MY shoulder got the sorest, I swear, the night I helped my father size some very hard cast alloy bullets he wanted to test. Dad Volunteered "Me" to do this after his shoulder began to hurt. The bullets were so hard, they did not expand well to take the rifling in his revolver, and accuracy suffered. He was trying to see if it was possible to cast a bullet hard enough to pierce metal car doors reliably. He did make up some semiwadcutters of hard alloy, but put a steel BB down in the mold before pouring each bullet. With that " steel " nose, the bullets did knofe through car doors from about any angle, but that is another story.
Don't " Age " cast round balls. Its very counter-productive to what you are wanting a round ball to do when shot out of a muzzleloader, both inside the barrel, and when it impacts a target(game).
