Ok...maybe I'm an idiot. I've seen on some diagrams that the breech seal is on the barrel block. On the Magnum, there is no seal. The seal is on the piston side. That's still a breech seal, right? And removing that would still be fairly straightforward, yeah? Just coax it out and lube up the new one and carefully place it in, yeah? I got correspondence back from Gamo USA; they said they can't ship the seal out to me. They instead directed me to Gamo Spain. Geez.
Shim it! Pull the factory breech seal out, line the groove with dental floss, and pop the breech seal back in on top of it.I have been shimming breech seals like this for 5 years or longer. I bekieve I learned how to do it on this forum. It works like a charm.