Usually leather, like dixa said.
Test the seal by putting a single sheet of toilet tissue over the joint, and firing a pellet out of it. If the tissue does not move, that is not where the power went.
Some careful work picking at it with a heavy sewing needle, can sometimes revive the seal a bit. Work around the outside, poking the needle in, and prying it up a bit. Carry on with the same, and you can work the seal out of it's seat. A spacer or washer fitted under the seal can allow you to get more use out of it, too.
Last time I needed seals, I made them. I used a brass tube from the hobby shop, sharpened on the inside edge, to cut holes through some thick leather. I used a quite oversize hole punch to cut the seals-to-be loose. I clamped a couple or five of these on a mandrel and turned the outside diameter to size on a lathe.
The new seals take a bit of effort to install. I used a bit of nylon rod and a hammer.
Leak test the seal before you remove it.
Measure the inside and outside diameters of the seal ( a millimeter ruler is fine) to see what size you need, if you are going to order one. There is some flexibility, but one size rarely fits all, and about no one stocks parts for old airguns based on make and model.
Cheers
Trev
