Most people using an AR buy cheapo 115 grain FMJ to shoot from their $600 AR. Cheap ammo from a cheap AR will group at 5-6 inches shooting with a bipod from a bench with a minute between shots with no wind at that range.
Throw in wind, stress, sub-optimal support, and rapid fire and it’s a very difficult shot.
Which is good and bad. Good in that assassinations should fail. Bad in that it means this asshole was shooting up a crowd.