The short answer is none.
Martin Heemeyer had one in his self made tank, but never fired it. Albert Petrosky had one in his truck, but didn't kill that cop with it. There are a few others, but nothing really all that notable and no deaths that I am aware of.
Groups like the VPC like to include .50's in a list of everything seized from various criminal actions as if their hyperventilating over their very presence meant a damn thing. It's all just part of them trying to get yet another firearm banned.
Groups like VPC and Brady are in violation of USC Title 18, sect 241. In most cases, their victim disarmament laws they constantly push should be considered legally equivalent to "accessory to murder".