horse's bit
銜 centers on the metal bit placed in a horse's mouth for control. The meaning is specific to this equestrian tool.
銜 combines 金 (metal) and 行 (to go, move), suggesting a metal device used while moving—a horse's bit. The exact historical development is uncertain, but the component combination fits the meaning.
Picture a metal (金) bit that goes (行) into a horse's mouth, controlling its movement. The outer 行 wraps around the inner 金 like the bit sits within the horse's mouth.
For カン, imagine a can (カン) of metal bits used for horses. The metal can holds the bits, and the sound 'can' directly cues カン.