.25. Cal std or slug liner?

There is no such thing as a “Heavy” liner anymore. The new slug liners are laser-engraved with the word “Slug”.

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The standard superior liner should work really well on all pellet weights, including the "heavy" 34gr FX's. It's only when you start getting into the heavier weight slugs that you'd want to step up to the slug liner, although it still may shoot the lighter slugs better than the standard liner as well.

Standard superior liner if you'll shoot mostly pellets, plus some slugs. Slug liner if you plan on the opposite. Personally I run a 600mm superior as I shoot pellets most of the time.