Since assault weapons are not a major contributor to US gun homicide
[interesting admission right off the bat] and the existing stock of guns
is large, an assault weapon ban is unlikely to have an impact on gun
violence. If coupled with a gun buyback and no exemptions [meaning no
"grandfather clause"](tranlation: 'Confiscation') then it could be effective.
An exemption for previously owned magazines would nearly eliminate any
impact. The program would need to be coupled with an extensive buyback(confiscation)
of existing large capacity magazines. With an exemption the impact of
the restrictions would only be felt when the magazines degrade or when
they no longer are compatible with guns in circulation. This would take
decades to realize.
Finally, "universal background checks" will not help (and will indeed exacerbate the problems of straw purchasing and theft) unless guns are registered. California's rulers love gun registration, because when they later ban the registered firearms, without a "grandfather clause," they know just where to go to confiscate them (earning the state "extra credit" from the Brady Campaign).