Where did some Muslim shixtbird get the ammo? It is not beyond the realm of possibility that he was stocked up, but I'd call it highly unlikely. What government agency supplied him?
The whole AR pistol thing is weird too. Nothing against AR pistols, but few gun people are going to use one. A good conventional high-cap pistol would have been more effective or better yet, a carbine.
They always seem to use the particular combination of features that the Dems (and deep state) are Jonesing to ban at that particular point in time.
