According to a post on the Epic community forums, 'gun-like' items are set to disappear from the Xbox LIVE Marketplace as a result of a new policy instated by Microsoft.
If you already own the Lancer and/or the Hammerburst, you will be able to keep those items. Otherwise, you only have until the end of the month to get them, as they will be pulled on January 1st. There is no full list of items that will be pulled, but if it's gun-like and you want it, better get it while you still have the time.
Gears of War 3 gun avatar props to be pulled. 
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