What major manufacturing oversight led to widespread failures and the subsequent recall of early M16 rifles during the Vietnam War?

The Vietnam War era saw one of the most controversial military product deployments in US history. The M16 rifle, introduced as a technological marvel, quickly became infamous among troops for its reliability issues in combat conditions. This poll explores a critical aspect of this military product recall crisis that changed how the US military approaches weapons testing and procurement.

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Changing the rifle's specified gunpowder type without updating cleaning protocols or providing cleaning kits
Using aluminum instead of steel for the receiver to reduce weight
Reducing the barrel thickness to cut production costs
Switching from wood to synthetic materials for the stock and handguard