Tin/Lead alloys do age soften, they stablise after a while but that does not mean they do not age soften from just cast.

Lead-tin (Pb-Sn). Which metals do we add to lead to make better bullet metal and why? The first and most obvious need here is to make the alloy harder, but there are other factors that play into this answer as well. Historically, tin was used because it was readily available in pure form, mixed easily with molten lead and contributed desirable properties to both the molten and solidified alloy (castability and hardness, respectively). Tin also increases the hardness of the alloy but does not interfere with the malleability of lead (a key point that we‘ll return to). Tin lowers the viscosity and surface tension of the molten alloy, allowing it to fill out the mould more effectively, resulting in a higher quality bullet. Tin is limited in its ability to harden lead, achieving a maximum hardness of about 16 BHN at 40% tin.

These binary lead-tin alloys undergo slight to moderate age softening upon storage (1-2 BHN units), with the harder alloys undergoing more of a change than the softer alloys. The hardness of a binary lead-tin alloy generally stabilizes after about 2-3 weeks.

Heat treating binary lead-tin alloys does not provide any change in hardness. At typical lead pot temperatures, lead and tin are infinitely miscible with one another, at the eutectic temperature (361 F) tin is still soluble to the tune of 19%, but at room temperature tin is still soluble in lead at the 2% level, meaning that as the bullet cools down there is significant precipitation of a tin-rich solid solution in the form of granules and needles in a matrix of lead-rich solid solution.