Microsoft has established a new policy that will go live on the 1st January, 2012, dictating that no weapons will be sold for Avatars anymore.
This was revealed in a post by Epic Games, warning, ”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.”
So, if you want your Avatar to wield a weapon, better buy them now before the policy goes into effect.