Britain is a country with stronger gun control laws than the United States, and lower murder rates. But Mexico, Russia and Brazil are also countries with stronger gun control laws than the United States -- and their murder rates are much higher than ours. Israel and Switzerland have even higher rates of gun ownership than the United States, and much lower murder rates than ours.
Even the British example does not stand up very well under scrutiny. The murder rate in New York has been several times that in London for more than two centuries -- and, for most of that time, neither place had strong gun control laws. New York had strong gun control laws years before London did, but New York still had several times the murder rate of London.
It was in the later decades of the 20th century that the British government clamped down with severe gun control laws, disarming virtually the entire law-abiding citizenry. Gun crimes, including murder, rose as the public was disarmed.
Meanwhile, murder rates in the United States declined during the same years when murder rates in Britain were rising, which were also years when Americans were buying millions more guns per year.
The real problem, both in discussions of mass shootings and in discussions of gun control, is that too many people are too committed to a vision to allow mere facts to interfere with their beliefs, and the sense of superiority that those beliefs give them.
Any discussion of facts is futile when directed at such people. All anyone can do is warn others about the propaganda.