「Filled with metal」or,「filled metal」: a hollow shaft for delivery of a BLASTING DISCHARGE as it applies to GUN BARRELS (「filled with metal」as lead shot, and later, a bullet). Recall that the first guns were front-end loaded musket-type BLUNDERBUSS weapons with a ramrod used by a「person taking stance with downward thrusting motions」into the hole with a black gunpowder charge. Once again, an indisputable warning not to engage in excessively vigorous reciprocal rod motions just prior to the expected moment of firing, not forgetting the soft spot of the wrapped powder charge that might permit penetration with resulting disastrous consequences for the person loading the barrel, if the charge exploded while ramming the rod with too much force, and who if not killed from that type of trauma, might carry around some disparaging nicknames during the remainder of his natural life.
「Emphasis on rod with friendly accommodating anal coitus.」…or「person thrusting downward with rancid copulation」undoubtedly describes a BLUNDERBUSS, and a BLASTING DISCHARGE. A BLUNDERBUSS is a weapon that scatters pieces of shot at short range, or, 'an unsubtle person' also known colloquially as a buttfucker.
By applying one's imagination only slightly, the origin of the English expression 'get the lead out of your pants/ass/shaft/pencil' can be easily derived, especially when the etymology of lead is known to be 'plummet, heavy bluish-gray drips falling straight down' as from「person downward thrusting spurts of rancid copulation」discharging bullets that kill.
Some scholars have suggested that「metal filled」euphemistically referred to a HEAD HOLE FILLED, the hollow part of an axe, mattock, or adze head where the handle is inserted and then the remaining gaps were (eventually) filled with molten lead so that the head will not fly off. The original elements are a bit graphically cruel, as「metal」is a「person taking stance with downward thrusting motions of penis」and perhaps an「inverted child at birth」with the infant's head fontanelle positioned quite obviously as just about to pass through what resembles the mother's「squatting legs.」The axe handle here is the「thrusting downward penis during copulation that fastens and attaches itself to」the axe head,…or, the infant's head as the case may be, an indisputable warning not to engage in rambunctious vaginal coitus just prior to the expected delivery date, and not forgetting the soft spot on the baby's head might permit penetration with resulting disastrous consequences for the surviving child, who, if not killed from that type of trauma, still might carry around some disparaging nicknames during life (such as…fuckhead?).

External source definitions …excluding politically incorrect concepts and other meanings deemed offensive today; may include extended definitions, euphemisms, and mnemonics)
Chinese: ancient firearm; gun
Japanese EDRDG: gun, arms
Unihan extended: ancient weapon, blunderbuss