According to Epic Games Community Manager Will Kinsler, Microsoft will be removing all gun-like Avatar items from the Xbox Live Marketplace on January 1, 2012:

Heads up! Starting on Jan. 1, 2012, the Lancer and Hammerburst avatar items will no longer be available on Xbox LIVE Marketplace. If you’ve purchased the items prior to Jan. 1, you will be able to keep them. A new policy goes into effect for all gun-like avatar items on the Marketplace, so get them while they’re hot.

Any gun items purchased before the new policy goes into effect will remain on your account, unaffected. You can search the Xbox Live Marketplace for these soon-to-be collector’s items here. The four Gears of War Avatar guns are:

The policy is a curious one, especially consider the Xbox 360’s three best-selling titles are all hyper-violent shooters: Halo, Gears of War, and Call of Duty. GamesBeat has reached out to Microsoft for an official statement.