You can still enter the armory through the front door by a glitch.
1.) Buy lots of big health packs and drop a big mountain of them in front of the door.
2.) Step on them and rapidly jump toward the door. the game will mess up and let you bypass the door and fall under the floor. After that it is quite possible to jump your way back up to the armory first room.
Note: If you don't go through the door after trying for a while. Keep buying health pack to make the mountain bigger.
I believe there are lots of videos on Youtube on the subject. Be sure to check them out.