Fragments associated with explosive volcanic eruptions range from microns to meters in diameter, with the largest ones following ballistic trajectories from the eruptive vent. Recent field observations suggest that bombs ejected during Strombolian eruptions may collide while airborne. We developed a Discrete Event Simulator to study numerically the impact of such collisions on hazard assessment. We show that the area where bombs can land might be significantly increased when collisions occur. As a consequence, if collisions are dominant, the deposition distance cannot be used to estimate important eruption parameters, such as exit speed.