During the Vietnam War, which critical issue led to an emergency recall and modification of early M16 rifles, causing numerous American casualties due to weapon malfunctions?

The development and deployment of military equipment has sometimes been marked by notable product recalls and modifications. One of the most famous examples occurred during the Vietnam War, when American troops faced life-threatening malfunctions with a newly issued service rifle. This poll tests your knowledge of this significant product recall in military history that ultimately led to substantial weapon redesigns and changes in military procurement procedures.

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Change in ammunition powder without proper weapon testing, causing excessive fouling and jamming
Insufficient waterproofing causing corrosion in jungle environments
Manufacturing defects in the rifle's plastic furniture causing breakage in combat
Excessively light construction leading to accuracy problems at long range