Risk and protective factors for firearm assault (FA) have been established, but little is known about factor preceding transitions in FA behavior. We modeled covariate effects on individuals’ transitions in FA behavior (Yes/No) using inhomogeneous, continuous-time, Markov Chains. 3287 assessments were made across five initial biannual follow-ups, and two additional biannual follow-ups (an average of 2.2 years later) from a follow-on study; 2687 pairs of transitions were observed (2414 No-FA → No-FA; 89 No-FA → FA; 121 FA → No-FA; 63 FA → FA). Non-firearm peer violence (HR = 2.31, 95% CI [1.28,4.21]), firearm victimization (HR = 2.57, 95% CI [1.31,5.04]), and marijuana ASSIST sum (HR = 1.27, 95% CI [1.05,1.54]) all preceded transitions into FA, but not transitions out of FA. Delinquent peer associations both hastened transitions into FA (HR = 1.19, 95% CI [1.00,1.40]) and slowed transitions out of FA (HR = 0.84, 95% CI:[0.72,1.00]), with analogous findings regarding attitudes favoring retaliation. Efforts to prevent FA initiation should focus on those currently reporting firearm violence victimization, and on factors indicating an escalating delinquency trajectory (e.g. non-firearm violence, substance use), while programs focusing on peer influences and social norms may be effective at preventing FA regardless of current FA status.