What common calibres are in the UK?

[Steve]

It's nice to think of cartridge guns, but let's not forget about muzzle loaders. A muzzle loader made with modern techniques can be effective, ammunition is easily made using old tech.

If you look around, suitable components are everywhere, for example the steel hydraulic pipes clipped to JCB arms or tractors, tough chrome-moly steel bar in a socket set or a car drive shaft etc. etc..

A crude muzzle loader will put meat on the table if an air rifle isn't up to the job.