The reason that happened isn't hard to pinpoint at all. It's because, from the very start of the decade, high-profile serial killers were making the news on the regular, and at least part of the reason for that was that they so often proved to be extremely hard to catch. In fact, the first killer to terrorize the '70s was never caught at all. He called himself the Zodiac. Speaking of guns, his weapon of choice was a .44 Magnum.
You know who this looks like? White dudes.
He was just the first in a long line of notorious killers to make headlines throughout that decade. Ted Bundy, John Wayne Gacy, the Hillside Strangler (which was actually a pair of cousins) ... if there was ever an ideal time to be a serial murderer, it was definitely the '70s. Just that so many of these stories surfaced at the same general time probably would have led to a spike in gun sales on its own, but it didn't help that some of the biggest names of all used guns to commit their crimes. Along with the aforementioned Zodiac Killer, the infamous "Son of Sam," who practically held NYC hostage with fear for more than a year, also relied exclusively on firearms.
And the advice of his neighbor's (apparently very naughty) dog.
It wasn't just those headline-grabbing crimes that made citizens of the '70s such avid consumers of weapons. All over the country, crime of all sorts became more common over the course of the decade. It's hard to say why for sure, except no it's not. As I've mentioned before, starting around the early '70s, de-institutionalization became all the rage. The number of available hospitals and beds for people suffering from mental illness in this country dropped dramatically. Huge numbers of mental patients were suddenly on the streets. What kind of impact did this have on crime numbers? Let's go to the charts!
This is a chart. As you can see, it displays things.