The first AK rifle and the 7.62x39 mm cartridge were adopted by the USSR in 1949. But where did it come from? Why did the Russians switch from the 7.62 to the 5.45, and why did Kalashnikov consider it... a big mistake! And, last, but not the least – what new caliber was recently created in Russia, to replace the old 5.45x39 cartridge?