Yes. A gun would have that effect.
The observation I will make however is that Libby activated the video recording at time before any weapon, if there was one, was produced since we don't see one in the stills. Something other than a weapon with respect to BG (or even BG himself) caused Libby to start recording. As GH points out BG was about 80 feet from the girls and the girls were already off the bridge at the South end - definitely Libby. It would take at least 30 seconds and likely 45 seconds to a minute for BG to cover the distance across those bridge ties to the girls. That would be quite a head start if the girls ran. So the key question to me is: What is it that kept the girls from making that run for it? My own opinion is that there were one or more persons (who are involved) near or behind the girls when this all started in motion.