A small number of police departments are showing interest in a new type of video camera that can be mounted directly on officers’ guns, saying it may offer a better view of officer-involved shootings than body cameras. Some law enforcement officials and civil rights groups are skeptical. Among the cons, they point out, is that gun cameras start recording only after weapons are removed from holsters and won’t capture what led to officers drawing their guns, or other interactions with the public. They also say they should be used only as a complement to body cameras. Besides the better view,...