So far as I know, retarded people haven't really been an issue with guns. The main issues seem to be people with a propensity for violence due to mental illness, and people presumed to have a propensity for violence because of their convictions for crimes.
In at least one recent case, that of Dylann Roof, the shooter was unambiguously ineligible to purchase a firearm because of existing law, but was nevertheless greenlighted to buy because of the inability to communicate between federal and state authorities.
I certainly don't think all the legislation that would be constitutionally permissible is on the books, but there are a few things that meet with near universal approval, such as background checks so that violent criminals and psychotics can't just buy a gun with no problem, and there are laws that would cover these cases. They just aren't being enforced.
That would seem to be the pragmatic place to start, at least if people were actually serious about doing something possible, instead of just demanding all guns be done away with, or that everyone has the right to their own roof-mounted machinegun on their SUV.