[shibumi]

The gait was also what I based my conclusion on.

That, and the fact that a REALLY aggressive dog would not have given the officer that much reaction time. (My little GSD can reach any point in the yard in under 2 seconds.)

[untrained skeptic]

I think the video has been slowed down. Notice that even though the bullet seems to slow the dog some it slides and rolls nearly ten feet past the officer. The dog isn't jumping up when it was hit, so to go that far after being hit by the bullet it would appear to have had to have been running fast.

The gait looks bouncy, but that seems to be from a slow frame rate. The security camera likely had a slow frame rate even before being slowed.

The TV station probably slowed down the video so it was easier for people to see what happened, but it has the effect of making it appear like the dog was coming in more slowly and that the officer had more time to react.