>>527800140
It would still lack penetration because it's slow and heavy, and if you want more energy and velocity you'd be using a 20mm antimateriel rifle firing 20mm cartridges filled with much more propellant instead of a 12 gauge shotgun.
Nowadays slugs in shotguns are only used in hunting, while shotguns in modern warfare are used for breaching doors and last ditch defense against small drones using buckshot.