For gun homicide rate, among all the countries in the world, the US rates 28th, behind such stalwarts as Honduras, El Salvador and Jamaica. Some of these countries ban the private ownership of firearms, though not all do.
Sounds like the majority, if not all of the shooting was conducted using two 5.56mm semi-automatic rifles equipped with bump stocks (that use the recoil of the rifle to enable rapid trigger pulls). The combination of rapid fire and jostled trigger hand makes these weapons highly inaccurate. In fact, from a range of 400 yards (the range of the shooter to the concert venue) you'd be lucky to hit the broad side of a barn. The shooter was just spraying the field, and if you got hit you were unlucky.
The shooter was a 64 year-old with no criminal background. Given that shooting was likely conducted solely with two weapons, I'm not sure what regulations people might suggest that would prevent something similar from happening in the future? National firearms backgrounds check and annual license? Nope. Limit gun ownership to 5 or fewer weapons? Nope. Or 3... or 2? Nope. I don't have an answer. Modifying your rifle for automatic fire is already illegal, though that doesn't stop people from doing it.
In the meantime, the vast majority of firearm deaths are due to handguns.
Year to date in Chicago alone, there are almost 500 gun homicides so far this year... (2,401 shot and wounded just in that one city) a problem that dwarfs the issue of mass shootings by a factor of about 100.