Honestly I think the rise in mass shooting is more coupled with a glorification of the act through the media than a rise in gun ownership. Gun laws haven't lessened recently, yet the number of shootings has skyrocketed, so personally I feel that although it is an important issue it isn't the core of the problem. We don't treat our mentally ill with any competency in this country, and the media infames the shooters. In my mind that is the core of the issue.
The media does seem to have a great effect. Want to die, kill someone, and be famous? Mass shooting. Media swarms and your face is plastered on everyone's TV screen all over the country. As it is, all the news stations hone in on everything about the shooter and give minor condolences to the victims. If we focused on the victims of the shooting and downplayed the shooter as much as possible or even ignored him altogether, I'd be willing to bet we'd see a sharp decline in mass shootings because the incentive of being made famous as quickly and easily as possible is taken away.
Gun laws are only as strong as the weakest link. Houser had domestic violence history, had a history of destructive crime, had been involuntarily committed, had been denied a conceal carry permit, and he still legally purchased a gun at an Alabama pawn shop which he used to kill two people in a different state. What is increasing is the ability of people like Houser and Roof and those before them to find like-minded individuals on stormfront, 4chan, reddit, or wherever else who post violent reactionary rhetoric that encourages them. Certain media influences are getting increasingly ridiculous, and ultra-conservative political groupies stopped pretending to be reasonable a while back. These cultural influences are what these folks are stewing in. Better medical care would surely help, if these people weren't convinced to turn it down or opt out of it.