i recently installed the g18c barrel on the KJ g23, it fits and cycles with no mods only thing is the slide was set back a bit because the hop up wouldnt not move back far enough but that was only about 5mm but i did some modification on the barrel and now it sits properly
haven't fired it with rounds yet because id like a longer inner barrel first
i will be working on the slide next, yo see if i can get it to work but it looks like i would still need the stock 18c bbu and fire control although i dont know if the bump in the frame is mandatory for that to work, i also noticed that the KJW bbu will not fit i think it needs the inside of the slide needs to be filed down