Mexicalli police said the device, mounted in the bed of a pick-up truck, used a long plastic pipe as a gun barrel and was powered by compressed exhaust gases from an old car engine.
The cannon could fire cylinders packed with drugs that weighed up to 13 kg, the police said.
It had a range of about 400 metres – enough for its operators to park near the Mexican side of the border and shoot the packages across into U.S. territory, where they would be picked up by the smugglers’ accomplices.
The device was found after US border officers told their Mexican counterparts that they had confiscated a number of drug packages that appeared to have been fired over the border.
They discovered the cannon atop a truck parked near the border, but the people with the truck managed to flee the scene.