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Background In January of 2019, Washington State (WA) passed Initiative 1639 making it illegal for persons <21 years-old to buy assault weapons (AWs). This study aimed to evaluate the effects of WA-1639 on firearm-related incidents involving AWs by those <21 years-old in WA, hypothesizing a decrease in incidents after WA-1639.Methods Retrospective (2016-2021) data on firearm violence (FV) events were gathered from the Gun Violence Archive. The rate of FV was weighted per 100,000 people. Total monthly incidents, injuries, and deaths were compared pre-law (January 2016-December 2018) vs post-law (January 2019-December 2021) implementation. Mann-Whitney U tests and Poisson’s regression were used for analysis.Results From 4091 FV incidents (2210 (54.02%) pre-law vs 1881 (45.98%) post-law), 50 involved AWs pre- (2.3%) and 15 (.8%) post-law. Of these, 11 were committed by subjects <21 years-old pre-law and only one occurred post-law. Total incidents of FV (z = −3.80, P< .001), AW incidents (z = −4.28, P< .001), and AW incidents involving someone <21 years-old (z = −3.01, P< .01) decreased post-law. Additionally, regression analysis demonstrated the incident rate ratio (IRR) of all FV (1.23, 95% CI [1.10-1.38], P< .001), all AW FV incidents (3.42, 95% CI [1.70-6.89], P= .001), and AW incidents by subjects <21 years-old (11.53, 95% CI [1.52-87.26], P= .02) were greater pre-law vs post-law.Discussion Following implementation of WA-1639, there was a significant decrease in FV incidents and those involving AWs by individuals <21 years-old. This suggests targeted firearm legislation may help curtail FV. Further studies evaluating FV after legislation implementation in other states is needed to confirm these findings.