I think the general thrust of your point makes sense. There have always been a lot of firearms available in America, and that remains the case. But gun sales have upticked of late, so it's not quite a matter of it being a constant, I would say.The number of firearms available in America has been pretty much a constant , but this country never had mass shootings to the frequency that it has gotten to now , which points out and establishes that there is another factor involved aside from the weapons themselves that needs to be discovered in order to address the issue of mass shootings properly and hopefully effectively.
Worth considering, there were substantial changes to research on gun violence, and the impacts of gun ownership on violence (the Dickey Amendment). These things aren't single 'slam-dunk' causative effects, but I think there are things that can be looked at from a legal and access point of view that somewhat inform change over time. Equally I would agree that there are cultural and health-related issues that are at play...I just don't understand why acknowledging that should then preclude action on either a legislative or research front.