hi,
i just noticed that replacing one rune to another cost same amount of gems as put rune on clear gun, also rune which was on gun disappeared (maybe it should, i dont know that).
https://l2wiki.com/classic/Special_Abilities if this information is correct, then replacing rune should cost 56 b gems, but for me it cost 112.
(i removed iss rune from other gun and try to replace it on valhalla instead of feoh rune).