The first most used gun all over the world is the 5.56 mm caliber M16. More formally United States Rifle. M16 and its family M16/A1/A2/A3/A2 has been the primary weapon of the United States infantry. From 1960s it has been the primary US rifle. It has also been used by 15 NATO countries. It was first used for army in 1964 and is the most produced rifle in its caliber. It is being used by US soldiers in Iraq, Afghanistan, and other countries in which US has deployed its army.
The second most used gun all over the world is the AK-47 otherwise known as the Kalashnikov was made in 1947. It is a gas operated assault rifle used mainly by Russian soldiers in the cold war. It is given its name Kalashnikov after its creator Mikhail Kalashnikov. It had the upper hand over most auto-loading guns in the World War II because it was capable of selective fire, it had moderate power and had comparable range. Nowadays it is mostly used by terrorists all over the world.
