It’s probably just a .50 Beowulf upper and the reporter is too G-d D@mn stupid to know the difference.
Lol, reporters think anything that shoots more than once an assault rifle. They still routinely report that police fire their service revolvers.
Glad this guy got snagged before he got going. Bad boy rifle and armored up too, whew, that could have been hellish.
I know they made a .50AE (Desert Eagle) caliber upper for the AR-15 some years ago along with some modified magazines.