What the hey…

As an Englishman and an Americanophile, I do see the obsession with guns is a nationwide issue for the USA. Over here, no private citizen should own an assault rifle or a handgun without a special licence-that leaves most weapons in the hands of criminals! Those in rural areas may own a hunting rifle or shotgun, but the security checks are thorough and quite onerous, including a police visit to your home to ensure that the weapon is safely secured when not in use. We still have shootings but nothing compared to the USA.
I do not understand why anyone in a civilised country would want/need an assault rifle.
On the second point regarding those with a mental impediment, doesn't the Constitution say that you cannot incarcerate a person who has free will and is not a felon?