Guns have never been in short supply in America. What has been in short supply is loving but strict upbringing by both parents (one man and one woman).
When did mass shootings start? In 1966, when Charles Whitman, an abused child of an abusive father killed and wounded four dozen persons, mostly by gunfire, before being killed by police.
Whitman — in method and madness — was a harbinger of what now seems to be an epidemic.
The cure for the epidemic isn’t “gun control” or “red flag” laws, both of which are demonstrably failed policies. It is a moral rebirth of the nation, which is unlikely to happen as long as leftists and “establishment” types are in charge.
For a lengthier treatment of this subject, complete with (slightly out of date) statistics, see this.