The 15XT has a built in seal.
It is the Cylinder.
The purpose of a cylinder seal is to write signatures in early times.
Brake wheel cylinder or axle sealBrake wheel cylinder or axle seal
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a seal (i.e seal) between the cylinder and the head on a car
I suggest you search the web for "Air gun repair" and locate someone near you to repair the rifle. See the link below.
walrus seal
The name for an eared seal is Otary
Apply a finish to the Aluminum alloy cylinder. Recommend an annodize treatment. Another alternative is to install a bronze ring under the seal, if possible.
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A Ringed Seal
In ancient near eastern times, people used cylinder seals as a type of signiture for documents. The cylinder seal was made of clay or rock and had reliefs carved into them.