Firearm availability is correlated with increased risk of suicide but its link with suicidal ideation remains unclear. Previous studies are limited by retrospective reports and prospective designs with lengthy gaps between assessments that are ill-suited for measuring fluctuations in suicidal ideation. This study used ecological momentary assessment (EMA) to repeatedly assess suicidal ideation in a sample of 138 U.S. adults (81 handgun owners, 57 non-owners). Participants received six EMA prompts per day for 28 consecutive days. Results revealed no group differences in the frequency, maximum amplitude, or variability of suicidal ideation across male and female handgun owners and non-owners. Stability of suicidal ideation significantly differed across groups, however (F(1,132) = 4.5, p = 0.036); male handgun owners had the strongest stability and male non-owners had the weakest stability. Stability of suicidal ideation was significantly lower when participants reported a firearm was nearby as compared to when no firearm was nearby (F(4,17732) = 5.6, p < 0.001). Results suggest firearm availability increases reactivity to the environment, slows recovery from acutely elevated risk states, and may increase vulnerability to sudden shifts to higher risk states characterized by increased probability of suicidal behavior. Although these effects were observed in both handgun owners and non-owners, they disproportionately impact handgun owners because they report being near firearms more often.