These laws prohibit the purchase and possession (or in the case of Washington, the sale and manufacture) of assault weapons, high-powered semi-automatic firearms. While state definitions vary, these guns typically fire rounds with up to four times the muzzle velocity of a standard handgun round—inflicting greater damage on the human body. A full discussion on Assault Weapons is here.
From 2009 to 2022, nine out of the 10 mass shooting incidents with the most casualties involved the use of at least one assault weapon. A 2021 study found that the federal prohibition on assault weapons and high-capacity magazines was associated with a significant decrease in public mass shootings and related casualties, and prevented at least 11 public mass shootings during the 10 years it was in effect. The researchers also estimated that had the law remained in effect from 2005 through 2019, it would have prevented 30 mass shootings that killed 339 people and wounded 1,139 more.
Source: Cal. Penal Code § 30510; Cal. Penal Code § 30515; Conn. Gen. Stat. § 53-202a; Del. Code Ann. tit. 11, §§ 1465, 1466; Haw. Rev. Stat. § 134-1; Md. Code Ann., Crim. Law § 4-301; Mass. Gen. Laws ch. 140, § 121; N.J. Stat. Ann. § 13:54-1.2; N.Y. Penal Law § 265.00; Washington House Bill 1240 (2023) (to be codified)
9/10: Las Vegas, NV, 471 people shot, 60 fatally; Orlando, FL, 102 people shot, 49 fatally; Aurora, CO, 70 people shot, 12 fatally; Highland Park, IL, 53 shot, 7 fatally; El Paso, TX, 46 people shot, 23 fatally; Fort Hood, TX, 45 people shot, 13 fatally; Sutherland Springs, TX, 45 people shot, 25 fatally; Uvalde, TX, 38 people shot, 21 fatally; San Bernardino, CA, 36 people shot, 14 fatally; and Parkland, FL, 34 people shot, 17 fatally. Everytown for Gun Safety Support Fund, “Mass Shootings in America,” accessed January 5, 2023m https://everytownresearch.org/maps/mass-shootings-in-america/.
The mass shooting at Fort Hood is the only incident among these 10 that did not involve an assault weapon. The shooter used a handgun equipped with a high-capacity magazine. Investigators recovered multiple 30-round and 20-round high-capacity ammunition magazines at the scene. “Investigators Detail Ammo Found at Fort Hood Shooting Scene,” Dallas Morning News, October 21, 2010, https://bit.ly/3BmF1kg.
2021 study: Lori Post et al., “Impact of Firearm Surveillance on Gun Control Policy: Regression Discontinuity Analysis,” JMIR Public Health and Surveillance 7, no. 4 (2021): e26042, https://doi.org/10.2196/26042. Study defined mass shooting as four or more victims killed with a firearm at a public location. Familicides and felony killings were excluded.