AK-47 (most used rifle)

The AK-47 is an ancient gun, with the date of its inception enshrined in the name. The 47 stands for 1947. The K is the initial of Mikhail Kalashnikov, the gun’s designer. The A stands for Avtomat, meaning automatic.
The AK-47 is also an incredibly popular gun. Wikipedia puts it this way: “It was one of the first true assault rifles and, due to its durability, low production cost and ease of use, remains the most widely used assault rifle in the world – so much so that more AK-type rifles have been produced than all other assault rifles combined.”
Here is a beautiful cross-section animation that shows what is going on inside an AK-47 as it fires:

http://www.youtube.com/watch?v=lQe864rGLyk&feature=player_embedded

See How Machine Guns Work for more details on different mechanisms.
Which is better, an AK-47 or an M-16? If accuracy is the criteria, then the M-16 is the winner:

http://www.youtube.com/watch?v=G6BpI3xD6h0&feature=player_embedded

But there are other criteria as well, including cost, resistance to jamming, etc. The AK-47 is rugged and reliable even in bad conditions. Also, the design makes field stripping and lubing an AK-47 easy, which is another important feature on the battlefield:

http://www.youtube.com/watch?v=TigS4SFst-E&feature=player_embedded

Don’t have access to an AK-47? Given the accuracy problems, perhaps you should consider the Winchester model 1894 as an alternative. This video shows you why:

http://www.youtube.com/watch?v=Vgr3kTU68uw&feature=player_embedded