Transverse barrel movement was measured during the firing of a Ruger Precision Rifle chambered in 6.5 Creedmoor. The use of laser vibrometers enabled high-speed measurement at good resolution during the projectile in-bore transient. Lateral vibrations have been broken into constituents based on source using a combination of experiments and finite element modeling. Individual contributors to overall vibration discussed include firing pin impact, primer ignition, and the combined load of combustion gas pressure and projectile–bore interaction. Good correlation was obtained between barrel motion in the vertical plane and model predictions during the in-bore period. It was concluded that action of the firing pin and primer impulse contribute significantly to the overall dynamic response of the barrel.