To get the death machine with unlimited ammo and to keep (along side your 2 other weapons) you must kill a zombie and get the perk. After equiping it, get hit by a zombie and get put onto final stand, after getting revived by a team mate you should have the death machine along side your 2 other weapons. you can swithch weapons any time without losing the mini gun but make sure you dont pick up any other perks for example: carpenter, insta-kill, nuke, x2 points, death machine, fire sale etc... please enjoy this glitch whilst it lasts  because it has already been patched on x box.