In my experience, when I use straight wheelweights I have trouble with the mould not filling out completely. If I add a touch of tin in some form, it makes the mould fill out easier and I get significantly fewer rejects. Adding more than 2% tin, however, is a waste of tin (which is more expensive than the lead). I usually add solder to bring up my tin content. A lot of the new plumbing solder has a very high tin content and it is easy to use, since most of the time it comes in wire form on a roll. Just unroll the wire and split it up to whatever amount you want to add per pot of wheelweights. Even a 0.5% of tin added will surprise you as to how much easier the bullets cast. I usually find it best to calculate how much you want to add per pot, make a pot full with tin, cast it into ingots, then make another pot full and cast it into ingots too. Keep making ingots of your bullet material until you run out of tin. Be sure to mark your ingots in some way so you don't confuse them with ingots that don't have tin added. If you try to just make your wheelweight and tin mix one pot at a time and then cast it into bullets, it will be hard to keep your tin content constant. You will find yourself adding wheelweights to some of the alloy that already has tin in it, then you won't know how much solder to add to keep your mix constant. An inconsistent mix will give you inconsistent bullets, and will waste tin.
I used to try to mark my ingots of wheelweight/tin alloy with a magic marker, but I found the stuff would fade or get wiped off. Now I make my straight wheelweight ingots using a standard Lyman ingot mould, but make my alloy ingots using a different ingot mold. I found an old cast iron mould used to bake cornbread into small round shapes (at least that is what I think it is supposed to be used for.) I use it to make my ingots of alloy. With their different shape, they can't be confused with my straight wheelweight ingots.
