Leather has been used as a sealing material in all sorts of applications ever since the beginnings of engineering. Leather is a remarkable material. It has slight elastic qualities, is hard wearing, flexible and is also easily worked into required shapes. It was these properties of leather that permitted the gunsmiths of the continent to make the very first pressurised airguns as far back as the late 1500s, and there is a bellows airgun from 1580 in the Livrustkammaren museum in Stockholm.
Replacing the breech seal on a vintage rifle like this Cadet only requires a few basic tools - but the job is far easier if you have a means to hold it stable
Leather provided the means for gunsmiths to create a seal that would be tight enough to make an airgun work effectively - and also to fill the bellows to fire the forges that would allow them to heat the metal to be worked at high temperatures.