Is there any other boss that drops the Infinity gun?
I remember some mini boss in a DLC drops one, or has a chance too. Doc Mercy just isn't dropping it for me.
Any legendary has a 1/20 (5%) drop rate from the boss it's assigned to, the infinity pistol is assigned to Doc Mercy. Any legendary has a 1/1000 (.1%) to be found in a chest, random drop, or in a skag/stalker/whatever pile.