Experiments located at pulsed spallation neutron sources have the potential to make ground-breaking observations of exotic particles including axions, dark photons and heavy neutral leptons. These experiments make use of proton beams in the 800 MeV to few GeV range that impinge on targets embedded in beam dumps to produce pions, muons and photons, as well as spallation neutrons, which are the facilities' primary goal. Very rarely, new particles may also be directly produced or produced through mixing, and can be observed in nearby detectors. Reducing the background at these high-rate facilities is crucial to obtain sensitivity to new physics. Precision timing for the particle arrival with respect to the beam spill can be an effective method for rejecting backgrounds. In this paper, we discuss the use of the “gamma flash”—the arrival of the first gammas from interactions in the target at the detector—to determine the start of the beam spill to nanosecond precision. We present application of the gamma flash technique to new physics searches at the Coherent CAPTAIN-Mills (CCM) experiment, conducted at the Lujan Center beam dump at Los Alamos National Laboratory. The methodology we present can be applied in present and future short-beam-pulse experiments, including those proposed to be located at the PIP II Rapid Cycling Synchrotron at Fermilab National Laboratory and the European Spallation Source facility with the short-pulse upgrade.