room made of stone, watch-tower
碞 centers on a stone-built structure used for observation or shelter: a watch-tower or a room made of stone.
碞 combines 石 (stone) with 品, which may have contributed a sound element. The exact historical development is uncertain.
Three mouths 品 sit atop a stone 石, like a stone room with multiple openings for watching: a watch-tower made of stone.
For ガン, picture a stone watch-tower shaped like a giant can: can -> ガン, and the stone room is as solid as a can.