Pumpblast. Tin, up to a one in ten mixture will harden lead up to a point. That's 10 percent of a very expensive ingredient. It has been prover, that 2 percent tin will enhace the castability of lead, and that any more is a waste of material. Wheel weights already have anywhere from 1/2 a percent to 2 percent tin in the mix. I alway add a 1 pound ingot of linotype to 10 pounds of wheel weights. This will give enougfh tin to aid casting. In fact, I may even reduce the amount of linotype to half that, as some experimenting has shown that to be sufficient. You could take your 50/50 bar solder, and cut it into forths, and one of those forths would probably be more than enough.
Yes, tin doe harden lead slightly, but antimony hardens it more. Arsenic, if your wheel weights has it in the mix, will make them even harder. In my experience, non-arsenic wheel weight average about 12 BHN on the hardness sale. I have a batch of weights I got in the mid-1970's that have arsenic included in the mix, and they read 15 to 16 on the BHN scale. Heat treat these bullets and the non-arsenic metal runs from 21 to 26 BHN, and the arsenic mix will go from 29 to 32 BHN. Let me tell you, you cannot scratch the 21 to 26 BHN metel with a fingernail. You will think it is something other than a lead alloy.
I have a bunch of 1 in 10 lead ingots I bought years ago. Bullets from that stuff only go 10 to 12 BHN and is, in my opinion a waste of tin.
Wheel weights alone do a fine job in most handgun loads, and surprisingly well in tghe 30-30 Winchester. I like the abit harder in cartridges like the .44 magnum and .357 mangnum. For rifle .308 Win. and more powerful, I heat treat.
I hope this answers youe questions.