Hey, Sean, what you're looking for is the breech seal. And yes, it's a different size from the Shadow. They ARE available from Gamo, but they'll try to tell you that the rifle has to be taken apart to replace it, which violates the warranty, so they don't sell them as a service part. You'll have to call and convince someone that you know better, that seal CAN be replaced with the rotary still in place. The other option would be to check with CDT, I think he planned on getting some awhile back, he may have some in stock. If not, I'm sure he knows the part number. Good luck!
Dave