As bad as the shooter was he would most likely ran out of ammo before killing a significant number of people.
Your post is either incredibly naive or incredibly optimistic. The targets (Republicans) were pinned down in a dugout while the shooter was moving toward this same dugout.
Without a threat from the Capitol Police he would have walked up and been able to shoot them at point blank range.
Even as clearly bad at shooting as he was, (your point I believe) he likely had sufficient ammo to handle this final event.
Moral for the trapped congressmen, — do not hide in a place with no escape route because you do not know how many rounds the shooter has.