STEP ONE:
FAST AND EFFECTIVE WAY TO REDUCE MASS-MURDER DEATHS - without haggling over gun design.. or the 2nd Amendment.
Focus on Magazines, Not gun design, a much more complicated issue.
Semi-automatic hunting rifles, pistols and assault carbines have similar RPM (rounds per minute) firing capacity if fed from high-capacity magazine; all are semi-automatic, auto-loading weapons.*
Simply Outlaw Possession of ANY magazine/ clip (or other device) capable of holding more than 5 (maybe 10) bullets; (similar to silencer/ suppressor law)
Result: 40% to 75% fewer bullets fired per incident:
Deal with gun-design ban separately.
(Only effective Ban would be on all semi-automatic weapons.. as Australia did in 1996; no mass killings since and 1/5th per capita murder rate of U.S.