According to the study done by William Landes at the University of Chicago and John Lott at Yale from 1977 to 1995 the only public policy that has ever been shown to reduce the death rate from multiple victim public shooting are concealed-carry laws.
No other policies had any effect of the frequency or carnage from such crimes including: waiting periods, background checks or the death penalty.
Therefore all citizens of the United States in good standing should have this right guaranteed to them by the US Constitution.