Black powder cartridges
Assuming the gun is a centre-fire and not a pinfire, Gamebore still make black powder loads in 12-bore.
I tested some at my local shooting ground a few years ago and found they had a very gentle performance, as befits cartridges made for guns which are much older than their owners.
If your gunsmith does not stock them, he should be able to get them for you.
Snag is they are quite expensive, with an on-line price of £530 per 1,000.
A cheaper alternative may be to contact a specialist supplier, such as Peter Dyson and Sons, of Hanley, Holmfirth, Yorkshire, Tel: 01484661062 or www.peterdyson.co.uk
Dysons advertise handloaded black powder cartridges in both centre-fire and pinfire.
If you decide to try your gun at a shooting ground, do ask first.
Some black powder cartridges can be quite noisy (although, from memory, not the Gamebore ones), and some sensitive souls don’t like the volumes of sulphur-smelling grey smoke they produce.