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My Superior Slug Liner 1:16 did the same thing. I thoroughly cleaned it with Iosso Triple Action Oil Solution and Iosso Bore Cleaner. Now it stays clean longer and shoots H&N Heavy 40gr and Javelin 40gr slugs sub MOA.
I have read from big shooters that cleaning a slug liner after every 50-60 shots is normal if you wanna see a good performance. I know if I wait until a 100 count my scores are ruined.
I tried scoring target papers @ 100 meters with slugs, I didn't like the results and I am back to pellets. In an average day at my range about 200 shots still gives acceptable results. Always clean and buffer my liners for next incoming range day.