I would be curious if it was a sizing issue vs a hardness issue. Different alloys will shrink more or less depending on mixture/purity. While I haven’t cast bullets or balls, I did cast several thousand pounds of fishing weights over a couple year span as a small side gig weight producer. I always noticed my pure lead casted much better than dirty mixed lead. Came out of the molds much easier. I always assumed the purest lead maybe shrink a bit more than the unknown alloys/contaminated stuff.
Melting points change with lead too, dirty mixed stuff needed to be poured much hotter than the good stuff.
Actually the opposite is true..
Pure lead and tin has a higher melting point than the alloy! As you mix the two metals together, the melting point drops from 327C for pure lead to 183C for a 61.9:38.1 lead - tin alloy. This is called the Eutectic alloy! (see I did not sleep through all my metallurgy lectures!). This is how you can make really low melting point alloys such as Cerrosafe (..which uses cadmium). Google "lead tin phase diagram" for more information. A famous demonstration in metallurgy is to show an alloy that is liquid at room temperature, even though the parent metals are solid at the same temperatures!
For a given temperature, an alloy, provided it is all melted, will produce a sharper casting than a pure metal as the melting point is lower and the liquid is more fluid. Pure metal and Eutectic alloys go directly from liquid to solid with no "mush" phase. Some alloys go through phase changes as they cool, which can also affect moulding performance.
I remember at my first metallurgy lecture, the lecturer came in with a basket of frozen metal billets and placed one in front of each student. He said nothing more about them and introduced the lecture series. The billets were really cold, with frozen ice condensation on them and flowing vapours. As can be predicted, the students (including me) played with them, and started to pick them up as they warmed. One student suddenly cried out in pain as the billet they were holding became too hot to hold. This was a demonstration of phase changing.. the lecturer before the lecture had heated the billets up in a furnace above a point where the alloy changed phase, taking in energy in the process. He had then dunked the billets in liquid nitrogen, freezing the metal in the phase changed state. As the metal heated up, it went back through the phase change and released the energy it had taken in as heat.. Oh how we laughed!
You do however need to make sure the metal is clean by fluxing it and skimming off the dross. What you will find is that alloys either side of the eutectic will go through a "mushy" phase as they melt, and you need to make sure the temperature is high enough to melt the mush before pouring. Some contaminants such as zinc will produce a very mushy melt, which is really difficult to mould using pouring methods. These alloys are used for white metal casting, but are usually pressure cast at higher temperatures.
Alloys are always harder than pure metals because of crystal interference, which can be increased by subsequent heat treatments, although this is seldom done in soft metal alloys.
For more advanced geeks.. see here:
https://chem.libretexts.org/Bookshe...ria/Liquid-Solid_Phase_Diagrams:_Tin_and_Lead
