Epic Games has declared that the Hammerburst and Lancer Avatar props are going to be removed on January 1, 2012, as part of a new anti-gun policy. Microsoft will be barring any Avatar proper that resembles a firearm. Gotta protect those kids while they're playing Halo and Gears!
If you bought these items already, you will be able to keep them, but there will be no more "gun-like" items on sale in the Avatar Marketplace.
Microsoft has a strange list of things that Avatars cannot have or do, pretending there's some iconic image to uphold like Mickey Mouse. Unfortunately, Microsoft does not realize that Avatars aren't childhood heroes on par with Disney characters. They're pointless little monstrosities, and having guns was about the only interesting thing going for them.