Surplus military ammunition (right) used to be available for under $100 a case; now it's over $250 a case, but still cheaper than Lapua factory ammunition (left).
If we look at the M193 and M855 loadings of 5.56 NATO, they have necks of approximately four to five inches, maximum damage cavities four to six inches long and maximum penetration of 12 to 14 inches.
The 7.62x39's terminal ballistics using standard military ammunition are poor due to low muzzle velocity, the impact velocity is low, which prevents fragmentation. The bullet construction has no features that aid bullet expansion or fragmentation. In short, standard military loadings in 7.62x39 have terminal ballistics similar to small handgun rounds with non-expanding bullets.
However, the 7.62x39 does have a redeeming quality: it penetrates intermediate barriers - such as thin metal or a chest-rig of loaded magazines - better than 5.56 mm. Just as the War On Terror was getting underway, the U.S. Army Special Operations Command (USASOC) created the Special Purpose Rifle - Variant (SPR-V) program to fill the need for a modular assault rifle capable of shooting 5.56 in addition to 7.62x39 mm. This project was eventually cancelled, but Special Forces soldiers had come to realize 7.62x39 mm provided increased lethality over 5.56, especially when penetrating barriers. The "Enhanced Rifle Cartridge" (ERC) project followed, which created the 6.8 SPC cartridge; however, that's another story.
At close range where the AK is most effective, accuracy may not be critical, but other than on a training range like this one, terminal effects are critical.
This "underbore" configuration leads to several intrinsic characteristics. First, ballistics are limited by powder volume. This means that enough moderate or slow powder cannot fit in the case to produce proper pressure and high velocity. Faster-burning powder must be used to produce rifle pressures; however, due to the burn rate, the velocity of medium and heavy projectiles will be limited. Second, lightweight projectiles that can be fired at high velocity will have ballistic coefficient (BC) values too low to retain a velocity advantage over much distance. For example, if a lightweight bullet could be shot 300 fps faster than M43, but its BC was half the M43's FMJ, the 300 fps muzzle advantage would be lost by even 100 yards. Thus, it's pointless to try to gain a lot of muzzle velocity at the expense of BC with 7.62x39. With modern powders, the military-standard 2330 fps may be improved by 100 to 130 fps. Third, 7.62x39 will retain performance as the barrel length is reduced.