Gerard-

I also dread the day that lead can no longer be used as rifle projectile.

Still, the rifles I most enjoy owning and running are the very early cartridge rifles from the transitional period (mid-19th century) such as my 20 and 16 bore pinfire double rifles.

Lead and copper are not the only bullet material choices.
I haven't looked at tin as a material - most of the thought I've (most times just idle thoughts in the background noise of my mind...) had have been around bullet design (such as your driving-band bullet design) more than material.

It appears as if the design formula you're following is one that seems to work - and is something that owner/shooters of vintage Nitro doubles have embraced.

The concern I most often hear from the crowd (with regard to pre-nitro rifles with fluid steel and damascus steel barrels) on projectile material is that the copper (often referenced around jacketed bullets) can prematurely wear the barrels out.
Would this be the same issue with tin in a driving-band design?
At bore-rifle bullet sizes, would the tin bullet be tough enough to do the job..?

Things to chew on for those of us who love our old pre-nitro BPE and pre-BPE double rifles...

Cheers
Tinker