D&L Airguns. Probably. Google will find their site.
If it can be removed, measure the inside and outside diameters, as well as the length, as well as you can, and have that info on hand. If they cannot provide a part number replacement, they may be able to provide something that can be made to work.
If the seal is leather, a new one can be made from some very hard leather cut with some sharpened tubing punches.
Get hold of the thickest and hardest leather you can find, and think in terms of sharpened up cartridge cases and the like to make punches from. A small hammer and a block of wood, to drive the leather washer into place. You want the washer to be both oversize on the outside dimension, and undersized on the inside, so it will squeeze out to be longer than it is thick when installed, if that makes any sense. Make a couple different sizes, try them and see what works. BTDT.
Lay a sheet of toilet paper over the breech area and see if it buggers off when the shot is fired, as a leakage test. Shouldn't move.
Cheers
Trev
