Here's how the polarizing M16 or AR-15 rifle went from a symbol of America's lost war in Vietnam to being owned by about 16 million Americans in 70 years

• The M16 or AR-15 rifle is one of the US's most divisive symbols
• To gun enthusiasts, it is an effective, lightweight weapon. To anti-gun advocates, it's a symbol of mass shootings.
• Experts estimate approximately 16 million adults in the US now own at least one of these rifles.

On Monday, a mass shooter killed three students and three adults at a school in Nashville. The shooter used two AR-15 rifles in the attack.