I believe you’re thinking of Gary Kleck, not Lott. Besides, Lott is a trickster who will claim the U.S. is not the leader in the world’s mass shootings, not by counting the number of mass shootings (because that would easily place us at first) but by counting the death toll from mass shootings against countries that had experienced unusual extremely deadly terrorist attacks (Norway in 2011 and France in 2015 for example.)
Also, Kleck and Lott are just about the only two gun violence researchers that actually suggest guns make people safer.