What I've used is mostly tin, 95% as I recall,, a bit of antimony, about 5%, and the best had silver in undisclosed quantities. This is what I recommend. But don't hesitate to experiment with whatever you find. I learned of the above by experimenting and will describe that experiment below.
I purchased a one pound roll of lead free solder at an estate sale which had copper and several other ingredients and is for filling large gaps in plumbing. In other words, the molded metal is thicker in consistencey than without the copper etc. I don't know that it would be any better for improving castability , or even as good, as the tin, antimony, silver alloys. The reason is that copper and many other metals cannot be alloyed into tin lead alloys, but is only mixed in, and will segregate out when held in the molten state in your melting pot for extended periods of time, which is what happens when bearing babbit is used in bullet casting alloys.
In short, those alloys won't hurt, but may not help as much as just tin, antimony and silver, which all assist in the flow charistics and the silver espeically puts a shine on the bullets, even with very low quantities used. An example, which stuns me every time I remember it, is. -- I was casting pure lead push through slugs and the scrap alloy I was using was producing a discolored, slightly blued surface. I jabbed a two inch long length of the above solder, (1/8 inch diameter wire) into the melt and immediately made another cast, without stiring the solder in. That set of bullets was shiny! The solder had alloyed itselt into the entire 20 pounds of lead before I could cast the next set of bullets and I had the mold in my left hand ready to cast when I put the solder in! I simply laid the roll of solder down and lifted the pour handle immediately with my right hand intending to stir while the set of slugs were cooling! --- There is a wealth of information about alloying in that little experiment! One doesn't ever need to stir the pot to mix the alloy up, as some imply to be essential. Except with mixes, such as babbit and probably the speical solders you name, which must be ladle cast or stirred every couple of casts if using a bottom pour pot to keep the alloy uniform.
