longbow, its kind of expensive but is incredibly smooth on the inside, the balls just glide through it. it makes the gun much more efficient, and less ball breaks because its so smooth that balls that are too big for the barrel glide through it, when with another barrel they would tear apart because they are bigger than the ID (internal diameter) of the barrel.
Or for like 35 american you could get a J&J ceramic
Or for like 35 american you could get a J&J ceramic