I'm getting into some old and obscure guns, parts of which are likely to unavailable. If a gun has a synthetic seal and replacements are not available, can one be fabricated? Do you find an existing seal that is over-sized and turn it down? Or can you replace it with an old-school leather seal. It's quite often that owners of old guns have to make their own leather seals. (Please - no steering away from the idea. This is just a what-if.) Since a leather seal does not fit as tightly, would it tend to generate more piston slam or would the leather buffer it enough?
Again, not asking if this would be the best way, etc. Just feasible.