If you are shooting smokeless then harder bullets with Antimony should work, but watch the ratio of Tin to Antimony as funny things can happen over time re hardness, the Lyman cast bullet book is a good read on this.

If you are planning on using BP or equivalent then stick to lead and tin (around 20:1 ratios by weight).

Look on flea bay , had luck there in the past for tin or known high tin alloys. Buying commercially pure tin can be a bit £££

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