Two Questions about the new Dlc
How can we go back to the armory after you beat it in the second playthrough
Man common their is plenty of post and YouTube Video about those subject, many won't answer your question cause it's been a Month the same question is asked evreryday...
You can go back the same way you went in, you use the fast travel station. The DLC, like many others isn't technically "attatched" to the mainland so you need to teleport there via the fast travel network. I dislike this simply because the developers only put 1 fast travel pole in the area(this is the same for Zombie Island) so you always start off at the beginning of the area.
There are 2 missions that marcus offers that let you go back into the armory.
And then, if sb had already done the two entries given my Marcus, and later on figured out the Armory "unlimited timer" glitch. and now wants to go back, to give that glitch a try, he beats Knoxx"'s ass and when he goes up to the door, he finds the gate closed, except going online, is there any other clue?
If the gate's closed it mean that he went in the wrong room in single player doing the Glitch, if he were online it means that he or somebody else went into the wrong room still... So he has to find someone online who's doing that quest for the first time ;)

Yes and yes.