Bullet Fireworks

Bullets bounce everywhere. The bad guys shoot a lengthy burst of sub machinegun fire as the hero runs through an industrial plant. The bullets impacting on steel railings, for example, give off bright flashes of light. This scene is a dramatic event for almost anyone watching the hero in a time of great peril. What can you say about the physics here?

Answer

Normal bullets are copper-clad lead and do not spark upon impact with steel or any other surface. Just go to a grinder to check out the properties of copper versus other metals in the production of visible sparks. The grinding of steel produces sparks everywhere, seen even in sunlight. The many small, hot particles of steel are actually burning. Now grind a piece of copper tubing. No sparks. You might see an occasional spark due to contamination on the grinding wheel or in the copper. The copper bits ground off do react with oxygen, but they do not get very warm. Please do not grind lead because toxic particles will be released into the air and, besides, lead is known not to produce sparks. So the conclusion is that practically no bullets produce a brilliant flash of light on impact.

In support of the film depictions is the fact that the military does have machine gun bullets containing white phosphorus so that the point of impact can be seen by the gunner. These bullets are used also to ignite fuel tanks and other possible containers of explosives by producing sparks to ignite the vapors. But phosphorous bullets are very rare outside the military