Just for the record when the original Prometheus pellets with the steel head and clear skirt ended, ZINC was used instead, not tin.
Prometheus did make a run of super quality match pellets in tin for the US military to be used in recoilless target rifles, and considered using tin for pellets more widely, but the idea was abandoned once the price of tin went sky high.

As such all the non-toxic pellets were made from zinc, but lead was also widely used for more traditional sporting pellets, including those pellets made for Daystate, Logun and FX.
One of the best materials available for making airgun ammo was gold, but that too was considered too dear, though its use would have ensured that all shot pellet remains would have been recovered, so would have been very 'green'!