WHoever told you that is full of it.
The "real" ak-47 is nearly identical to the AKM exept for having a milled reciever body and parts machined to fit into it. Can't use a milled stock on a stamped and vice-versa, but handguards interchange as do magazines and muzzle attachments (usually, if they are in standard 14x1LH).
What you usually see is called an "AKM", using a stamped sheet metal reciever, and they are in 7.62x39mm, as were the AK-47's.
Military use is generally resricted to 7.62x39 or 5.45x39mm, with .308 and .223 versions earmarked for export.
The only .308 ak's I've EVER heard of were the Yugoslavian Export Models, M76 "sniper" models converted from 8mm to .308 (har har), the short lived .308 Israeli Galil, some valmet sporters and the sporting Saigas imported by EAA/RAA.
Those are NOT common, except for the Saigas, and magazines will be crazy difficult to find, especially if you've got an M76 or a Valmet.
Youc an scope any AK, this is usually accomplished by using a side rail on the reciever to mount a bracket upon which a scope, if military, is integral, or, if sporting type, a weaver rail is attached to.
You can also use the Top Cover of the gun to mount a scope, and there are all sorts of rails and ring setups for this from .22 rings to weaver, and beyond.
Accuracy depends on the individual gun, but due to the number of flexing parts in the long stroke design it's hampered a bit.
Still, I've seen some of the 5.45x39mm (newer russian round) AK's shoot on par or better than a stock AR-15.
In 7.62x39mm?
My romanian ak in 7.62x39mm works just fine, gives acceptable (5-6 inch) groups at 100 yards iron sights.
My AMD-65 is a crap shoot beyond 75 yards, but then again it's not the toast of the town with a 12 inch rifled section of the barrel either.
They're fun to shoot in ANY round...
Though..
If I HAD to pick my caliber, it'd be 7.62x39mm, based on ammo cost and availability alone.
There are many .223 ak's out there, but magazines can be a problem to get working right, especially if you have a Norinco one...the romanian .223's take East German ak mags fine, so that's a bit easier.
.308 AK's? Forget finding new hi-cap mags. They're nonexistant. You'll have to convert your saiga somehow to take m-14 mags. Valmets and the like are too costly for me to even begin contemplate owning much less shooting.
5.45x39mm ak's are a BLAST to shoot, magazines are ALL over the place...
But ammo is impossible to find in many areas and must be ordered online. It can get very cheap, as cheap as 7.62x39mm at times, but the supply is always in flux, at least from what I've seen.
They are more than acceptable for hunting, especially in .308 or 7.62x39mm-and can easily be fitted with a lower capacity magazine for such jurisdictions that mandate it. I am lucky enough to live in one that does not.
the 7.62x39mm is the ballistic twin of the .30-30 rifle round, and the .308 speaks for itself.
The .223 can be used as well, state allowing, but shot placement is important.
I've heard of guys taking game with 5.45mm guns, but not sure of the legality in many areas, the bullet certainly has enough stopping power to do so as long as reasonable (.223 style) ranges are adhered to.