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.
According to Senator Rand Paul “about 100 rounds” were fired in 10 minutes. Resulting in 5 wounded. The shooter’s weapon has been identified as an SKS, 7.62x39mm. 100 rounds of 7.62 weighs slightly more than 3.5 lbs. How many pounds and pre-stuffed mags do you think he carried? The local PD was there at about the 10:30 minute mark from what I can tell. There is no possible scenario where this guy could of “killed” them all, with his lousy hit rate and the local cops closing in, as the hysterical OP alludes to. The math doesn’t work. The last thing I am is naïve, just very pragmatic.