To follow up, and clarify. First, very important, 7.62 NATO (7.62x51) while dimensionally identical to .308 Win is not the same ammo. If I remember right, the max pressure on .308 Win by SAAMI standards is 60,000 PSI, and 7.62 NATO is rated at a max pressure of 50,000 PSI. I don't have a reloading manual in front of me (deployed right now), so I may be a little off, but the .308 Win does have about a 10k PSI higher max pressure than the 7.62 NATO. I believe surplusrifle.com or .org did an article on this. Again, deployed so I'm limited to what websites I can get to, and sometimes the web filter blocks stuff for no apparent reason
. The whole point though is just to be careful with .308 Win in older 7.62 NATO rifles, i.e. the Spanish '96 Mausers that were converted for the Gaurdia Civil, and like weapons.
On the subject of the 7.62x39 it's very similar, ballistics wise, to a 30-30. Trajectory, and velocity are nearly identical on bullets of the same weight. Think of the 7.62x39 as a Russian version version of the 30-30 that was designed to run in semi autos. Those same weight bullets though are on the heavy end for 7.62x39 and light end for 30-30. The 30-30 is typically loaded with a bit heaver rounds as you can see. Hunting loads in 7.62x39 are less common but still available. Both cartridges are good medium range 30 cal rounds with low recoil, and the versatility to be used for defense, hunting, and even plinking. The SKS lends itself to hunting duties a bit easier than an AK, and still works quite well in the defensive role. Hopefully that puts the 7.62x39 into a little clearer perspective.
Edit: Just to add a bit to the 7.62 confusion, the 30-06 is 7.62x56mm