With ammunition costs still less than 5.56, the AK remains a popular training platform, as shown in this photo from a Tactical Response carbine class.
The popularity of the 7.62x39 in the United States and the SKS and AK-47-based sporting rifles is based primarily on the historic rock-bottom prices of surplus 7.62x39 ammunition. Within the last decade, cases of 7.62x39 could be had for less than $100; however, today that price has risen by about 250 percent. Although it is popular for simply blasting and plinking, the 7.62x39 has ballistic value.
Two military loadings dominate 7.6x39 ammunition, both use FMJ projectiles. The original M43 loading used a 123-grain boat-tail FMJ bullet with a large steel core. Later, the Yugoslavians developed a new bullet with a flatter base and no steel insert. These changes were made to enhance terminal effects in tissue. Chinese military ammunition in 7.62x39 has a steel core with no lead, and is currently not available to import into the U.S. due to language of the AP ammunition ban. Standard mil-spec 7.62x39 ammunition has a muzzle velocity of 2329 fps with the 123-grain projectile.
The AK design is tolerant: factory, surplus, and a variety of handloaded ammunition runs fine through this Krebs Custom rifle.
Overall, the bullet pokes in the medium the "neck length" distance, then it expands, fragments, and yaws to create usually a football or fan-shaped large internal wound, and then the largest fragments continue penetrating to the maximum penetration depth. Ideally, the initial upset depth is one inch or less; up to three inches is acceptable. The length of the maximum damage cavity should be as long as possible in the first 12 inches of penetration, and the cavity height and width should be as large as possible to ensure maximum damage. Finally, the total depth of the penetration should be between 12 and 18 inches.