The 7.62x39mm was developed as a medium-power round. In other words, it bridges the gap between the 9mm (or 7.62x25) submachine guns of the WWII era and the full-powered battle rifle rounds like the 7.62x54r. As such it was designed for the combat ranges most often seen in combat - 300 meters maximum. Beyond that individual soldiers are harder to see and are best engaged with crew-served weapons. Therefore the 7.62x54r was considered to have excess power for the application at the cost of bigger and heavier rifles, increased recoil, heavier and more expensive ammo etc. Yet the subguns were pretty much limited to under 100 meters and the light pistol bullets (FMJ) had only limited stopping power.
So while the x39's 122-grain bullet will shoot to 1000 yards, it has some major ballistic disadvantages over the larger-cased, heavy bullet x54r. The velocity advantage from the big case is significant in flattening trajectory and minimizing wind drift. In addition the heavier, longer, and more streamlined bullet retains its downrange velocity (and energy) better than the stubby and less streamlined lighter bullet. So that is why the 7.62x54r round has a major advantage when it comes to making easier hits at long range.
Having said that, you need to realize that basically all the 7.62x54r and 7.62x39 ammo is not match grade in accuracy, and in fact is not noted for its guilt-edged accuracy at all. Couple this with the so-so bores of most surplus Mosins (shot with corrosive ammo) and coarse iron sights, and you can see why engaging long range targets is a real hit or miss thing.
So while it may be fun to lob bullets from these two rifles out there at (large) long range targets (and I would qualify anything at over 400 yards to be long range), they really don't offer the level of accuracy that most long range shooters demand.
Therefore, I would enjoy any SKS or Mosin Nagant for what it is and have fun lobbing shells way out there, I wouldn't get to tightly wound about serious long range stuff with either of them.
For that purpose the easiest way to go is with a modern scoped sporter bolt action shooting a modern US caliber, whether a "deer rifle" for casual long range shooting, or one of the heavy barrel tactical/sniper rigs that the serious people use.
For more info on long range trajectory stuff go to the SCOPE, OPTICS & BALLISTIC INFO & FAQs sticky post at the top of this forum and then scroll down to the SIGHT IN AND TRAJECTORY RELATIONSHIPS part of the thread.
BTW - as an owner of a 1896 Swedish Mauser 6.5x55mm rifle, all I can say is that while it is very accurate for a surplus military rifle, basically any of my modern scoped deer rifles from Ruger, Winchester, and Remington can exceed its hit capability at long range in any one of the usual common calibers - .243, 25-06, .270 Win, .270 WSM and 30-06.