Keep the 9mm handgun out for just another
minute and go back into the main room, then enter the restroom on the
opposite side of the room and headshot both guards before they can blink an
eye.
then climb up into the vent. Go to the end and jump down, then
grab the K3G4 from the box, switch to it, then access the computer to open
the gates and get a checkpoint.
Kill them both (dont bother to headshot, because theyre the only
ones in the Senate Building except one other without flak jackets) and leave
the room.