User blog comment:SociallyAwkwardEpic/Asked and Answered: Rockstar Editor for Consoles, The Lab for Consoles, Mods and More!/@comment-9101338-20150507215459/@comment-25368842-20150507220051

See here. Looks like a Pistol .50 with an "engraved" skin. So could just be a new Ammunation Mod for any weapon.