If your goal is to eliminate or mitigate mass-shootings then restricting guns appears to be a viable solution. Australia has all but eliminated these crimes after cracking down on guns; Canada has similarly tiny rates of mass-shootings despite similar rates of gun ownership. It makes sense intuitively that reducing access to guns that facilitate efficient mass-shootings would cut down on them.
That being said, the goal should be to ultimately lower the overall violent crime rate and gun restriction is not the way to do that. Socio-economic reform is probably a better route and would avoid the ridiculous politicking the US encounters over gun laws.
Most mass shootings are premeditated months in advance and are done using guns which were acquired in preparation (not sudden bouts of rage). Gun control is simply a form of buyer obfuscation, not gun sales reduction. If a mass shooter must resort to waiting a few months for background checks to go through, it is likely that they will do so. The majority of mass shootings are done by people with no previous history of violent behavior. Mass shootings aren't a gun problem as the rate of gun ownership doesn't correlate to mass shooting incidents in any first world country.
We should see for every three mass shootings that the US has: Switzerland, Cyprus, Sweden, Norway, France, Canada, Austria, Germany, and many other countries with approximately
30 guns per 100 residents should see at least one mass shooting a piece (since the US has roughly three times the gun ownership rate of those countries). However, we don't see that occurring. Despite many European countries having 30 guns for every 100 people, they simply don't go on mass shooting sprees as often. The data doesn't support the correlation claims you are making.
Most of Europe has relatively lax firearm possession laws. It's also much easier to get a suppressor (something of which I'm jealous). I believe that the lack of mass shootings is due to cultural homogeneity over gun possession rate, but I don't have any data personally backing that up. Cultures that are less ethnically diverse in general have less crime. This isn't limited to one race, either, which suggests that culture clashing is something that is undesirable across the board.