The 5.45x39 has a lot to offer.
It's flat shooting, has a very high velocity, and very minimal recoil, especially when used with the AK-74 muzzle brake.
The Russians took a long look at the .223/5.56x45 US M16 in Vietnam, and liked what they saw.
They especially liked the idea of a more controllable rifle in full-auto, and the fact that the soldier could carry MUCH more ammo if it was lighter.
In Afghanistan the Russians were having problems with the standard 7.62x39.
It wasn't very good at longer ranges, had heavier recoil, and the amount of ammo the soldier could carry was limited.
Worst of all, they were facing the Afghans who are small, skinny, scrawny men.
The 7.62x39 does tumble after hitting, but it doesn't start to tumble until after about 8 inches of penetration, and only has time to tumble once in the average size person entering point first, tumbling over and exiting butt first.
The Afghans were so thin, the 7.62x39 didn't begin to tumble before it exited, and unless a bone or vital organ was hit, they could survive the strike, even though they had minimal to no medical care available.
The 7.62x39 was basically "paper punching" them and they were surviving.
So, the Russians essentially found a way to "cheat" the Hague Conventions which regulate military ammunition.
The Hague Accords ban the use of hollow point, exposed lead, or other type of bullets that are designed to expand and cause maiming of humans.
Because of the Hague Accords, military bullets are full metal jacketed and specifically built NOT to expand.
The Russians heard about the early Vietnam experiment in which several thousand of the then AR-15 full-auto rifles and one million rounds of Remington .223 ammo was sent for field testing.
The early AR-15 and it's Remington ammo quickly developed a reputation for reliability, and for the "killing power" of the ammo.
This was due to the bullets tendency to tumble after striking.
The Army more or less ruined the reliability of the rifle and the killing power of the ammunition in the famous "development" program for the M16.
The Russians took the idea of a small caliber bullet that was very stable and accurate in flight, but that destabilized once it hit, and tumbled, causing greater wounding.
They developed the 5.45x39 round which was specifically made to tumble wildly on impact.
The bullet has a mild steel outer jacket with a copper plated coating to prevent rust and to lubricate the bullet during feeding.
Inside the outer jacket is a slightly harder steel core, which is NOT an armor piercing core, even though it's reported to be able to penetrate both sides of a US steel helmet at 300 meters and author David Fortier was able to put a 5.45x39 through both sides of a US Kevlar helmet at 300 yards.
This mild steel core is flat on the ends and slightly tapered on both ends.
On top of the steel core and surrounding it on the sides, is a soft lead cap.
In the tip of the bullet is a hollow air space.
This produces a bullet that's weighted toward the rear, and makes it very stable in flight, but which causes it to destabilize once it hits.
When the bullet hits, the soft lead cap and the steel core slide forward into the air space, and this causes the bullet to destabilize, and it tumbles wildly.
Where the 7.62x39 begins to tumble around 8 inches, the 5.45x39 begins to tumble around 2 1/2 inches, and tumbles so wildly, it can tumble inside an arm or leg.
The 5.45x39 typically can tumble at least twice in the average size person.
While the bullet tumbles, it's also pitching and yawing, describing an almost corkscrew-like erratic course through the body.
One quirk of the 5.45x39 is a near 90 degree veer off course near the end of it's travel.
This erratic path, along with the weird veering off at an odd angle makes finding and treating the wound path almost impossible, especially with primitive medical means.
With the 7.62x39, an Afghan medic could hope to probe the path to find and extract the bullet.
With the 5.45x39 probing was useless.
Due to the enhanced wounding and maiming abilities of the bullet, the hard to impress Afghans nicknamed the bullet "The Poison Bullet" or "The Devil's Bullet".
With the 5.45x39, the "paper punching" of the bullet stopped, and Afghan soldiers started dying from the un-treatable and maiming wounds.
Gangrene became a major killer among the Afghans.
All this is born out by experiences of American hunters using the 5.45x39.
One writer is hunting the small Georgia deer with the round, and others are reporting shooting coyotes with it.
They all say that the 5.45x39 seems to perform as well as American commercial soft point, expanding bullet sporting ammunition.
The writer reporting on shooting a Georgia deer showed photos of major internal damage, including clear evidence of the tumbling bullet.
He reported that the bullet had veered off at an odd angle when it had slowed, had not exited, and he had been unable to find it so far.
While the 5.45x39 is not as accurate or powerful as the US .223/5.56x45, it does appear that the Russians have succeeded in fielding a bullet that out performs the US bullet.
The Russian military including Spetznaz prefer the 5.45x39 and consider it the best rifle and round in the world.
They are reported to be using the 7.62x39 in house to house fighting in Chechnya, due to the increased penetration of the heavier bullet.
In every other case, they use the 5.45x39 by choice.
Here's a sectioned 52 grain 5.45x39 7N6 PS bullet.
Note the mild steel outer jacket, the slightly harder tapered and flat ended steel core, the lead cap on top and around the core, and the hollow air space.
Maybe it's because of all those dead komrades they saw.