Leitner-Wise offers an AR-style rifle in .50 cal. That's .499. I got to shoot one of the semi-auto versions, and it's a monster. The muzzle flash alone eliminates the need for any additional lighting in a room
I heard a 'rumor' that it was designed in response to the SEALs need for an engine-block-busting round on a platform that didn't need wheels or 2 people to operate. Got out of the business before I heard whether or not they had an order put in or not, though. Not a trump card to full auto, but only because they don't make it in full auto that I know of.
The cool part of it for civilians is that the company offers(-ed?) a conversion kit for the standard AR-10. All it is is a different barrel assembly and bolt. Never cared much for owning overtly military-ish stuff but I think it's a sweet idea.