So the reason I showed that was to illustrate some points about ammunition. The 7.62X51 and 7.62X55 ammo are full-sized battle-rifle cartridges, and travel in a relatively straight line compared to some other types of military ammunition. It is easier to get those bullets to hit where they are needed without a great deal of compensation.
On the other hand, the short assault rifle 7.62X39 cartridge has a "rainbow" trajectory than requires more compenstation for bullet drop because the projectile is travelling with less energy as it is a less powerful cartridge. This is not illustrated in the above scans because bullet drop is not much at 100 yards. However at 200 yards it starts to show, and it is downhill from there with the 7.62X39. So it is a good cartridge for 100, maybe 200 yards, but you need lots of practice to use it at 300 yards and beyond.
The question is - at what range do you engage the enemy? Full-power 7.62X51 is powerful at long distances (700m) and travels with less bullet drop, but this energy is wasted if you are only fighting at 200 m. The round are longer and considerably heavier than 7.62X39.
Compared to the NATO 5.56, the 7.62X39 is said to have better penetration on solid objects like wood. Yet with its higher velocity, NATO 5.56 penetrates body armour better. So in modern warfare, you want something that can penetrate body armour and 5.56 is better in that regard. However, many complain about the lack of "killing power" of 5.56, as well as its weak velocity at longer distances.
Most "ideal" cartridges fall in between the 5.56 and the 7.62X39. I think this is a great idea. Personally, I find that 7.62X39 is just a little too "stubby." The trajectory isn't straight enough, it doesn't have quite enough velocity even though it is a heavy bullet that is good at going through cover. On the other hand, I find 5.56 too puny, like a really fast .22 that is kind of pointless or wasted in a 7-9 pound rifle. But something in 6-7mm would have only a little more recoil than 5.56 (which isn't very much to start with), and yet would have more killing power, more effectiveness at longer distances, and better penetration of cover. It would be like a smaller, faster, and more flat-shooting 7.62X39. That is why the 6mm and 6.5mm seem like good, nice cartridges to me. I have fired 6.5mm Swedish which is the same principle applied to the full-sized battle rifle calibers and it appeared to be an accurate round that had enough power without too much weight and recoil.