ABSTRACT Many firearm deaths, including suicides and homicide deaths from intrahousehold violence, occur during acute risk periods. As such, some suicide and violence prevention strategies focus on reducing firearm access at heightened risk periods, including preemptively and temporarily removing firearms by applying Extreme Risk Protection Order (ERPO) laws, and deferring firearm access using mandatory waiting period laws. Though both types of policies have some, albeit mixed, empirical support, the prior ecologic studies do not allow for a detailed understanding of whose deaths are being prevented (e.g., the gun owner, cohabitants, or other community members) and when potentially preventable deaths are occurring. Filling these critical gaps requires longitudinal, individual-level data. The ultimate objective of this work is to rigorously quantify the effects of ERPO and waiting period laws on mortality risk among handgun owners and their cohabitants. Specifically, using the Longitudinal Study of Handgun Ownership and Transfers (LongSHOT), an already- assembled multi-decade cohort of over 25 million adults in California (2004-2021), the proposed work aims to estimate the effects of California's ERPO law on risk of suicide among handgun owners (Aim 1), and on risk of homicide among cohabitants of handgun owners (Aim 2). Further, the proposed work responds to a common but unexamined critique of waiting periods – the possibility that first-time gun owners will be unable to protect themselves or family members from imminent threat during the waiting period – by estimating the risk of homicide during California's mandatory 10-day waiting period among new handgun purchasers and their cohabitants (Aim 3). By contributing this empirical basis for understanding the effects of ERPO and waiting period policies, this work will directly support the purpose of RFA-CE-23-006 (“to rigorously evaluate the effectiveness of innovative and promising strategies to keep individuals, families, schools, and communities safe from firearm-related injuries, deaths, and crime”) using existing data sources under Funding Option A.