While multiple witnesses claimed to have seen the Secret Service Agent with a Machine Gun or Submachine Gun, it is later identified as an AR 15. The experts incorrectly referred to it as an Assault Rifle.
Assault Rifles are a specific type of rifle capable of selection between single, automatic, or burst fire. The AR 15 is Semiautomatic which is one bullet fired for each pull of the trigger.
Trevor Hayes, who plays Agent George Hickey, is seen writing his statement with his left hand. He is later seen carrying the AR-15 with his right hand.