I add 5% tin to my bismuth shot that I make. Not that I make enough to keep any manufacturer up at nights. I read several years ago that pure Bismuth tended to be somewhat brittle and had the potential to shatter but when mixed with another metal that property went away. So just like they add antominy to make lead harder you can add another metal to bismuth to make it less shatter prone. I make absolutely no claims to be a metalurgist or expert in any way but that has worked for me. For all I know there are ten better metals to add to Bismuth.
