What was the primary issue that led to widespread failures and the urgent recall/modification of early M16 rifles during the Vietnam War?

In military history, product failures can have catastrophic consequences on the battlefield. One of the most notorious examples was the early deployment of the M16 rifle during the Vietnam War. Despite being touted as revolutionary, initial versions suffered from serious reliability issues that endangered American troops. This poll tests your knowledge about this famous military product recall and the changes that saved the reputation of what would become America's longest-serving service rifle.

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The rifles were issued without cleaning kits and marketed as 'self-cleaning' while using powder that caused severe fouling
The lightweight aluminum construction couldn't withstand jungle conditions and frequently cracked under normal use
The plastic components melted during extended firefights due to barrel overheating
The smaller caliber bullets (.223) lacked stopping power and frequently jammed when fired rapidly