Or... seastone bullets(even if they're probably pretty expensive, I guess pirates should be able to get their hands on some, even if they had to make them themselves by converting seastone cuffs into bullets or something)- I guess it's not, as the marines ships were coated with seastone in the amazon lily arc or something if I remember correctly, so it cannot be that rare. Epic Plot Hole? - Or both. If I was an infamous gunner in One Piece, I'd always have some Kryptonite with me, knowing that the most powerful are usually devil fruit users. Especially Logia fruits seem to literally convert people into demigods.