I could train someone with no prior knowledge of firearms, who was fairly intelligent, responsible, etc. to use a shotgun effectively for home defense in about 15 minutes on a range, and maybe an hour or so on use of force/escalation of force at a table. That's both the most morally defensible firearm ownership and the most likely positive use for a firearm, at the lowest overall cost (risk, financial cost, training time, etc.).
Concealed carry of a handgun, use outside the home, precision rifle, etc. would be a lot harder, but I'd be quite happy if most people just learned minimal home defense with a shotgun and nothing more.
