On January 6, 2021 an armed shooter could have done to Congress

[Hortensis]

20. Also the Capitol Police? While the senate chamber was being cleared,

for instance, police with pistols were there guarding every door (there were a lot of doors) and an officer with an automatic weapon was in the center prepared to take out any crowd so unfortunate as to break in.

In the house chamber, the officer who shot and killed Ashli Babbitt on the spot had his gun on her for some while before she was finally able to barely start to climb in.

Even if someone could have gotten an automatic weapon that far, it wouldn't have been quite the same.