Cartidge comparisons, and a joy to shoot.
There was no reduction in power when the soviets went to 5,45x39 cartridge in the AK74. It's a 7,62x39 (AK fodder) necked down to accept the smaller projectile. The total energy is about the same, the result is that the lighter projectile is flung faster. Like 5.56 NATO, the projectile tends to tumble on impact with tissue, and has about the same energy.
BTW: The 7,62x39 made it debut in a semi-auto battle rifle called the Simonov, or SKS, in '45. The SKS cost more to manufacture than the AK. They looked better in parades, however.
The 7.62 NATO is a completely different size cartridge, and there's no point in comparing it to these lower energy cartridges. The Soviet equivalent is 7,62x54, and is used in heaver rifles and for longer range shooting, and has the same kind of overall effectiveness as 7.62 NATO.
In the news recently: The US army is going to answer complaints about desert dust and the M16's, with a shoot off against some other rifles to consider for use in the middle east. Too bad they don;t consider the Kalishnakov or one of its varients. The Israelies make a nice one called Galil, chambered in 5.56 NATO.
I have an AK that they sell here in the States, it's been neutered for no full-auto. It's a lot of fun at the range.