People, ammunition selection is important. For the 7.62x39, several distinctions have to be made. First, among military ammunition types, there are two variants we must separate; the original M43 Russian ball cartridge and the M67 Yugo ball round. The original M43 was known to be a good penetrator of light foliage and intermediate barriers, but possessed poor terminal effects, as noted by Fackler and others. This is because it penetrated for 8 inches or more before tumbling. The later M67 Yugo ball round was introduced with an air pocket in the nose that effected much faster tumbling characteristics, often with 4 or 5 inches of penetration. Also as noted by Fackler, the Yugo round is much more terminally effective than the M43 round, and sacrifices little for penetration in most barrier types. I've seen the effects of the Yugo round on small deer from a Yugo SKS and it is devastating.
I am not sure about the newer Military Classic line, but the old 122 gr Wolf FMJ in the black boxes was built and performed very similar to the Yugo M67 ball round.
Some of the JHP for the 7.62x39 were known to be very inconsistent. They would either perform like FMJs or fragment violently, and you never knew what you'd get. One exception was the Uly 8m3 round. This was a consistent and devastating terminal performer, providing fragmentation, expansion, and penetration identical for home defense.
Then there are the newer rounds, the most notable being the Hornady SST round. These can bring the potential out of the round while offering better accuracy in most rifles than the Russian ammunition. There are also some JSPs out there with good reputation for lethality on medium game, though I've never bought these being quite as effective as the .30-30.
The 7.62x39 is, IMO, the finest intermediate powered assault rifle round ever, bar none, period. There are some, like the Grendel and the SPC, that might perform better at range, but never reached the popularity or availability of the 7.62x39, nor its economy. The 5.45 and 5.56 have flatter trajectories, less recoil, and allow more ammunition to be carried, but sacrifice performance for this convenience. The 5.56 in particular has always been known as a marginally adequate performer.
I find it interesting that people give the smaller caliber rounds a longer effective range. While they might have flatter trajectories, and may be easier to hit with at longer ranges, I wouldn't give them longer effective ranges. Sighted in at 200 yards, the 7.62x39 125 gr load leaving the muzzle at around 2300 fps, will drop a foot and a half at 300 yards. At that range, it still has the mass, velocity, energy, and momentum of the .357 Mag at the muzzle. That means aim COM out to 300 yards, with the lethality to take someone out of the fight. At contact distances, the ball round will go through a cinder block or a foot of living, standing fir, and still be lethal.
For comparison the 5.56 is a poor penetrator of intermediate barriers at any range and is looking at around 2100 fps from a bullet of half the mass at 300 yards. That is right around the velocity so-called hydro-static shock ceases to damage the elastic tissues of the human body, and the .22 caliber, 62 gr bullet just does not have much going for it without the velocity.
Drop is consistent because gravity is consistent. So you can give any of them an optic with a BDC, like the ACOG, and increase hit probability to much farther than average infantry firefight distances. It's much more difficult to make the smaller bullet perform like a bigger bullet once it gets to its target. If I can't have an M1A chambered for a real 7.62mm, give me an AK. I'll hold my sector all day long. Bring it.
