Some are aluminum, which is no good for casting. I also found that some wine or other liquor bottles used to have the corks sealed with a tin cap, but most are aluminum these days. You can usually tell by the weight. The tin ones are also much softer, so you can dent them with your thumbnail or tear the metal easier. You can also use a match or lighter on a corner to see how easily it melts. Tin melts really fast and drips, whereas the aluminum seems to kind of crinkle and dry up.
I think a better source for tin is leadless solder purchased wholesale from a plumbing supply store. I bought several dozen 1 lb rolls a few years ago for a fraction of what they'll cost in a local hardware store. This solder is 95% tin and 5% antimony. I can easily duplicate Lyman #2 or 1-20 hardness with wheel weights or plumber's lead, as checked with an RCBS hardness tester. You'll have to experiment, as each batch of wheel weight alloy will be a little different. Just segregate batches of bullets by weight and hardness.
