I think we need to look at all options.  My gut tells me this is more of a cultural/metal health issue.  If some evil person wants to kill a bunch of people, s/he can use guns, vehicles, fertilizer, etc.

But, for instance, if you reduce the supply of guns/ammo, prices for those things will go up - both on legal and illegal purchases.  That might keep some guns out of the hands of criminals who are often not financially well-off.  But, this might also increase the occurrence of gun thefts.  So, I don't know.  The rate of violent crime in America has been dropping for years.  But, the occurrence of high-profile mass -shootings is rising.  Regardless, there are no easy answers to this problem.