Over on the Epic Games forums, the community manager “raczilla” had the following to say:

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. 

This isn’t only being applied to Epic Games, mind you, it’s something that Microsoft will be enforcing all companies to comply with as an effort to keep things a bit safer and more family friendly.  No provocative poses for your Avatars, drug references, weapons, etc.  If you do have guns equipped now, however…like said above, you’ll be ok to keep them.