I let the gasket for the CO2 cartridge on my Crosman 357 pellet gun dry out to the point that it would not hold pressure when a new cartridge was punctured. I was able to remove the old gasket and take some measurements and this printed model worked like a charm. Can probably be scaled to any size needed for other pellet gun gaskets, but I don't have any other experience.
I printed it in Overture TPU that hadn't been dried recently with 100% infill and 0.05mm layer heights. I had some stringing in the interior and the surface didn't look good (I think the extruder tension wasn't quite right but it's a quick print and I don't do flexibles that often). I was able to trim up what I needed and everything worked great, holding CO2 for at least several days.
Original, dried out gasket shown.
The author hasn't provided the model origin yet.