Actually it's even crazier than originally thought
now if you read the bill text...
you see them banning not only .50 caliber firearms but any 50 cal bullets not owned prior to the enactment of the ban. So eventually 'd get the the point where you can still legally own the gun but have no legal ammo for it. Note the way it's written it's not 50BMG specific. 50AE could be illegal, 500S&W too, 50 BeoWulf, 50GI, etc
Of course banning 50 cal bullets makes perfect sense because at least 2/3rds of the 500 people murdered in Chicago with guns were killed with 50 cal bullets right?