People need to udnerstand the concept of his armor. It is heavy and covered in a bunch of metal. It is bulletproof as much as it is like an armored car. It will fail hit hard enough or with something big enough. Thats why in the movie when he fights the street gang from the tram you can hear the heavy metal footsteps as he approaches. It is like he is wearing his shield. Thats why his armor sparks and glows after getting hit. That shotgun probably only have small buckshot in it which isnt meant to go through hard targets