Criminologist also blames American myth of "armed lawbringer" and patchwork approach nationally, with data on mental health or previous convictions often unavailable for background checks.
Are guns really just an American problem? After all, Norwegians, with 1.4 million weapons and gun clubs galore love popping off a few rounds at the weekend, and in Finland it is the same, with plenty of weapons in the system for hunters and sports enthusiasts.
Guns are available in Europe, but there are few mass shootings, so what exactly is the deadly difference?
"Countries like Switzerland, Finland and Norway, the guns that are usually purchased are shotguns and rifles, they're purchased for sports shooting or target shooting or hunting, whereas now the majority of firearms purchased in America are for self-defence, they're often handguns of high magazine capacity or they are these new assault rifles which are being marketed for self-defence purposes, so we are seeing a real ratcheting-up of the firepower available to citizens in a relatively unregulated way and that is fundamentally different to the types of guns and the types of ownership that we see in most European societies," says Peter Squires, Professor of Criminology and Public Policy at the University of Brighton.
Switzerland lets its reservist soldiers take their weapons home in the interests of rapid mobilisation, but once again, mass shootings are all but unheard of.
"Switzerland, for example, is one of those countries I would call a 'highly civilised gun culture'. It generally involves gun ownership by people who are doing military service and they get their guns through that military service. They are trained in safe gun handling, and eventually when they leave their military service they can keep a firearm at home, but it must be locked away and the ammunition must be stored separately," concludes Squires.