My preferred policy:
Declaring a place "gun free" without putting in effective means of enforcing that does more harm than good.
All gun buyers should be subject to a background check.
The overall length of short barreled rifles under the NFA should be measured with folding stocks folded, since someone trying to conceal such a gun would fold the stock.
High capacity magazines holding 10 or more rounds should be regulated as Title II NFA weapons. Current owners of such magazines who don't want to comply with those regulations should be allowed sell them to government at a fair price, exchange them for lower capacity magazines, or convert them or have them converted to lower capacity magazines.
Looking at mass shootings since 1982, it seems that killers with military-size magazines holding 20-35 rounds were significantly more lethal than those with magazines holding 10 or less.This is unsurprising, since this is the size favored for the primary weapons carried by most soldiers on the modern battlefield. In the Mother Jones database of mass shootings, shooters with guns holding ten rounds or less killed about 7.1 victims per shooter. Those with 20-35 round magazines killed 10.3 victims each.
Magazines holding over 10 but less than 20 rounds are currently popular choices for modern semiautomatic pistols and offer increased firepower for defensive use. New production for civilian use was illegal under the Federal Assault Weapon Ban (AWB) of 1994. For mass shooters, they increased lethality only modestly, to 8.3 victims each. Oversized magazines holding more than 35 rounds were not significantly more lethal than those holding ten rounds or less, probably because of an increased tendency to jam. Such large magazines are rarely used for military assault rifles or machine pistols, probably for the same reason.
In spite of the enormous stock of pre-1994 high capacity magazines grandfathered under the Assault Weapons Ban, the ban seems to have eventually had a significant effect on criminal use of such magazines. These gains were reversed after the act lapsed.
The ban seems to have been somewhat effective at keeping the most lethal 20-35 round magazines out of the hands of mass shooters. Only three shootings that I know of used that size magazine while the act was in force from 1994-2004, the Caltrans maintenance yard shooting of 1997, Westside Middle School, and Columbine, compared to five since it lapsed. Six deaths per mass shooter was the norm during the ban, and nine per shooter since.
It has been argued that magazine capacity has little impact on criminal lethality, since a shooter could simply carry more magazines and reload more often. And yet at least three mass shooters were subdued while trying to reload: the Long Island Rail Road shooter of 1993. the Thurston High School shooter of 1998, and the shooter of Gabrielle Giffords and others in 2011, in spite of the wide availability of high capacity magazines that allowed the shooter to fire many shots without reloading during most of the period since 1982.
It has been argued that mass shootings are only a small subset of gun homicides. This is true, but policies that reduce the body count in mass shootings will also reduce the body count in homicides by criminals.
There is some evidence that the AWB had a positive affect on the broader impact of shooting casualties. Shooting deaths were rising before 1994, and fell during the 1994-2004 period. They were roughly flat after the act lapsed in 2004, but there was a significant countervailing factor. Gunshot wounds increased significantly after 2004. The only reason the death count did not rise correspondingly was that better emergency room treatment saved more victims.
No further federal regulations of assault weapons, as defined in the Federal Assault Weapons Ban of 1994, are desirable. Regulations should concentrate on the objective considerations of magazine capacity and concealability.
States and municipalities should not be prevented from setting reasonable safety training standards for gun owners.