I also think it was a squib. Watched the video at least 5 times and in all that looking didn't see any flash coming out of the end of the barrel. What was visible was the flash from the B/C junction and the barrel coming free of the frame.

One other thing about this video may be a result of the frame rate causing some "strobing" but I didn't see ANY movement of the cylinder when he pulls the hammer back. So another possibility is a cylinder that was "clocked" just enough to cause the bullet to strike the forcing cone far enough off center to cause the same effect as shooting a squib.