Anyone who has dealt with depression can tell you how this works. The suicidal urge doesn't last very long, but guns are waaaaaaay too convenient. Guns are far from the only way to kill yourself, but their widespread availability and the immediacy of the action are unique among suicide methods.
The theory sounds right, but it doesn't look like the data supports it.
We have a waiting period here in the USA when you purchase a firearm from a store. Is the waiting period not long enough in your opinion?
There had been a Federal waiting period, but it was ruled unconstitutional in 1997. Some states have waiting periods. See https://en.wikipedia.org/wiki/National_Instant_Criminal_Back...
This isn't so much about "I want to kill myself; I should go buy a gun" but rather about "I want to kill myself; my dad keeps a gun in his sock drawer". Talk about responsible gun ownership all you want; but if you own a gun for personal defense (as the majority of gun owners do), it's probably not locked up most of the time.
In addition I have this philosophy that if you want to interfere with peoples freedom you are responsible for making their life (as judged by that person) worth living - not just preventing them from killing themselves.
Regardless, it would be interesting to know if there was data on the effect on suicide when you get increased gun control. The data for suicide that I have seen seems to imply that the availability of guns would increase the suicide rate for men, which is also the biggest demographic for suicide victims.
It should also be noted that the data set they used only include mass killing with guns, and not mass stabbings. Those also happens, including here in Sweden a year ago at a school. But if you don't like that data then bring more and we can discuss.
https://www.cdc.gov/nchs/pressroom/sosmap/Firearm.htm
Two things pops out. First that the variance is massive within the united states. The second part is that 63 percent were suicides. It is very doubtful that a single correlating factor accounts for the variance.
This is the problem when discussing political issues like gun control. Gun related crimes has shown correlation with gun ownership laws. Gun accidents is naturally correlated to gun ownership. Mass public shootings however do not when adjusted for the population, but it draws so much political attention so it always get put into the discussion regardless if it match the data.
Gun accidents is also a rather weak-ish argument for gun control. Accidental deaths and injured caused by guns is statistically unusual, and no where near the top for the population in general or kids specifically. As famously noted in the book freakonomics, swimming pools comes in a big second place directly after Motor vehicle accidents in accidental deaths. The best argument in my view being made for gun control based on accidents is a moral one, in that even some accidents is bad enough when the owners don't have any reason to own a gun.
Which leaves us with the parent comment and gun related crimes. Assault and robberies. Those are where gun control seems to have the strongest connection with the data, and where it would do the most good. People on the lower end of the social economical ladder getting shot by other people on the lower end of the social economical ladder. It is not flashy, its not a winning political platform, but its where the data points to. I would not call it irrationality, but rather human fallibility in focusing on the rare and extreme and not see the common.