Steel Soldiers now has a few new forums, read more about it at: New Munitions Forums!
Microsoft MSN, Live, Hotmail, Outlook email users may not be receiving emails. We are working to resolve this issue. Please add support@steelsoldiers.com to your trusted contacts.
Here's another source for a sealing washer:
https://www.mcmaster.com/93781A035/
I'm not sure if that will work as well, based on the picture. Do you know if the rubber is thicker than the metal?