Benjamin 312 Seal Polishing Bushing Tool
thingiverse
Use a 3/8" inch aluminum rod with some 400 - 600 grit sandpaper superglued to the end of the rod. Then use this bushing to center the rod in the backend of the Benjamin 312 barrel. The rod should be about 8" long roughly. Then, by hand, rotate the rod so that it spins and polishes the flat face of the shoulders where the air seals are making contact. There's a YouTube video on it by NorthWestAirgun.
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