STEP ONE:
THE MOST IMMEDIATELY EFFECTIVE WAY TO REDUCE MASS-MURDER DEATHS
Focus on Magazines/ drums, not gun design.
Semi-automatic hunting rifles, pistols and assault carbines all have similar RPM (bullets/ deaths per minute) if fed from high-capacity magazine; all are semi-automatic, auto-loading weapons. 30-shell shotgun magazines now available!
Simply Outlaw Possession of any magazine/ clip (or other device) capable of feeding more than 5 (maybe 10 bullets. (similar to silencer/ suppressor laws)
Absolutely No Exemption for existing 10+ mags. Possession is prima-facia evidence; 1+yr Federal Prison, and $50,000 fine.
Result: 40% to 75% fewer bullets fired per incident.
No single act will save as many lives.
No 2nd Amend. issue since magazines are not 'arms' or even, ammunition.