Which innovative product design feature of the M16 rifle was initially criticized by military leadership but later became standard in modern military rifles?

The M16 rifle has been a cornerstone of American military equipment since the Vietnam War, but its development and adoption was not without controversy. This poll explores the product design history of one of the most influential military firearms ever developed, which changed modern combat tactics and equipment design philosophies. Test your knowledge about this iconic weapon's journey from concept to becoming standard issue equipment for U.S. forces.

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Its lightweight composite materials and plastic components
The smaller 5.56mm caliber ammunition (compared to traditional 7.62mm)
The direct impingement gas system that eliminated the need for a gas piston
The modular design allowing for easy field stripping and maintenance