Bangladesh, Feb. 9 -- For more than a decade, Mexicos war against powerful drug cartels has been defined by imbalance. On one side stand state and municipal police forces – often underfunded, poorly equipped, and overstretched. On the other are criminal organizations that increasingly resemble irregular military units, armed with weapons capable of overwhelming even federal security forces. Few developments have widened this gap as dramatically as the proliferation of .50-caliber ammunition in cartel hands.

Originally engineered for the US military, the .50-caliber round is not simply another type of ammunition. It is a battlefield instrument, designed to disable vehicles, penetrate fortified positions, and neutralize threats at ex...