In the first episode The Neutral Zone, in the aftermath of the robbery, an AK-47 can be seen lying on the floor in the basement of Rally and May's house.
First built in the Soviet Union in 1947, the Avtomat Kalashnikova, Model 1947 was developed by Mikhail Kalishnikov for use in the Red Army. Using a 7.62x39mm round and weighing 3.47 kg (7.7 lbs) empty, the weapon was the standard Soviet firearm from 1948 to 1975, when the AK-74 was adapted. Later, this weapon became the standard 'bad boy' gun when it was sold by the Soviet Union to revolutionaries and insurgency groups all over the world, as apposed to the U.S. Colt M-16 (later the AR-15).
Unlike most firearms, which were built to tight tolerances, the AK-47 had very loose tolerances. This allowed the gun to suffer a significant amount of abuse, yet still operated quite well. During the U.S.'s involvement in the Vietnam War, some U.S. troops abandoned the use of the standard-issue M-16 (which the early versions suffered from jamming and an inadequate 15-round magazine) for the AK-47.