You can see Allen pointing the long gun at the agent in the lower left as he runs into the scene, but it doesn't look like he fired it. I see no visible evidence Allen fired the shotgun at the agent as he ran through the scene, there's no visible gun blast from the muzzle, and no recoil is evident. Shotguns typically create a lot of visible blast from the muzzle. I just don't see any.
The agent in the lower left fires his pistol at Allen as Allen points his long gun at him. You can see the puff of smoke as the agent's pistol discharges the first time. Then the agent fires again two more times as Allens runs past. The three times the agent in the lower left fired his pistol at Allen there are other officers directly in his line of fire.
The officer along the back wall, standing with a couple of other officers, drops to the ground as if hit immediately after the first shot is fired by the agent in the lower left. After the officer along the back wall drops to the ground, he crawls and is helped to his feet, then led into a side room by another officer. Is this the officer who was struck in the vest?
They're saying Allen fired his shotgun. There's even a circle on the video superimposed over the muzzle of Allen's shotgun, but I don't see any visible sign the gun was fired. The other circles, superimposed over the agent's pistol when the agent fires it at Allen, clearly show muzzle blast.
I hope there's additional video from different angles/locations.