As we know, the Marvel: Avengers Alliance game is kind of bugged. One of the more annoying bugs concerns the golden weapons acquired for mastering a chapter to 5 stars. It's annoying because it generates a lot of questions. Not that questions are bad, but the same question, over and over, gets mightly old.
Never fear, the answer is here.
First, in order to obtain the chapter mastery reward, you must have five stars in ALL SIX missions in the chapter. 30 stars total. If you have 5 stars in the non-premium missions, you must also five star the premium.
Next, if you have all 30 stars, and you don't get a message telling you that you have recieved the golden weapon, don't panic. Check your inventory by trying to change weapons. Sometimes, it will get in your inventory without giving you the message.
If it's not there, refresh the game. Then check your inventory. Frequently, if you get your five star mastery out of order (meaning mostly, X.6 gets it's fifth star last), it will be in your inventory at this time.
If it is still not there, go back and check your missions. One time, I had a mission roll back 4000 points under the 5th star. Annoying. I did the mission again, then refreshed again, and Voila, there was my golden weapon. I have heard stories about it taking a whole game day to produce the mission progress rollback (or recalculation, or whatever you want to call it), so it might be worth trying again the next day.
Finally, at this point, you should file a ticket with support. That can be done here: http://playdom.zendesk.com/requests/new
You will have to register and get your Facebook ID, but the ticket submission form has a button that will allow you to do so.