Doesnt work any longer!!! Buy tickets to the museum online and pick them up in the main ticket office building in the Alexander's Garden outside of the Kremlin. It's the only place you can do it now whether you've prepaid online or not.
This article describes you how to buy tickets for the Armory Chamber museum and get inside without leaving the Kremlin's territory. It assumes you're already inside the walls looking straight at the Armory building.
The entrance on the left (or the closest to you, if you've just entered the territory from the second gates), possibly with a line and black fences is the main entrance to the Armory. Museum sells you tickets only at several limited intervals throughout the day: starting with 45 min before each session there starts. Sessions are: 10:00 AM; 12:00 PM; 2:30 PM; 4:30 PM. First, check the time. If it's within that 45 min window and it's not 1:00-2:30 PM, because that's when they have a break in the office inside the building, then try to go strength through the main entrance, if there's no gigantic line. At the entrance try your best at explaining the lady that you want to buy a ticket inside and point right behind her, cause it's literally where the window is, you'll see it, just few steps forward. If, and that sadly can be true, the lady will tell you that the only way to buy tickets is outside, even though the unmistakably ticket window with all the prices and information is literally 3 meters behind her, send her a scathing smile, wish a good day, go out and get to the plan B.
You've noticed that there're two entrances (one on the right or further on the left, if look from the entrance perspective). Another's being the one to the Diamond Fund, other small but expensive collection in the same building, which you may also want to visit. But since it's all connected inside you can reach the same ticket office that you just saw but from within now. The whole trick is, the Diamond Fund usually doesn't let people in at any random time. Strictly paced excursions, people usually need to buy tickets in advance. But if you speak some other language, they let you in just like that, cause all their excursions are in Russian. So, show your best foreigner at the entrance and keep repeating Diamond Fund until they let you in. Take steps down, dismiss the Fund's ticket office and with knowing what you're doing look take another steps down, like you're going to store your bags and cameras, cause that's what you actually need to do to get to the Fund. By the way on the right from that steps and office is the actual Armory entrance that you'll take a little later.
After those steps down follow the long corridor to the same ticket window you saw minutes earlier. And as if you never heard the woman 3 meters next to it telling you anything about it, ask them for the Armory tickets. She cannot reject you in any circumstances other than they ran out of tickets (500 per seance) or there's to little time till the end of the current session. If there're any additional questions, which shouldn't be the case, just decisively repeat again what you want, like you won't accept any other answer or put some Russian word at the end, like "Pozhalusta" - Please, to make her feel important, then you'll guaranteed have it:)Fee is 700 RUB/ 200 RUB (students) per person including free audio guides. Pick it on your way in, leaving any document or 2000 RUB as a collateral. Don't need to store anything, but the clothes for the armory, unlike Diamond Fund that will ask to leave even umbrellas. Better store them now, by the way, along with you cameras and phones, even if they don't have cameras in them, if you plan to stop at the Fund later. Tickets there are 500 RUB each with no discounts. And as said, can get there at any moment until 5:00-5:30 PM. Because the Kremlin works till 6:00 PM.
Now go through the same entrance to the armory that you've passed before, take a long set of steps up, dial 100 on your audio guide in the main hall and enjoy the exhibition.