Best gun in the game (barring the Gauss rifle, but you'll be lucky to get one shot with it in the whole game, the ammo is so rare) is the Vintorez (Vintar in vanilla).
Super accurate, almost completely silent (Just a barely audible "whuff"), and a scope. I sold mine for a Dragunov, thinking the world's most famous sniper rifle would be better, but it sucks. The scope is way off and you have to aim several inches below the target to hit, sometimes so low you can't see them.
I got a vintar and played around with it, and then bought a dragunov from the colonel in the duty base. The vintorez, while "accurate", technically, is extremely hard to aim with any precision beyond fairly close "long" range (whatever range it is where aiming just above their head hits them square in the temple), and the bullet moves so slow it's a nightmare to lead a moving target *just* enough to land a headshot. And the silent part doesn't seem to do a damn thing, when I fire it everything within a mile knows just where I am, and starts shooting at me, whether it can see me, or even has a gun.
The dragunov on the other hand, has amazing accuracy at range, at least out to 100 meters, probably further (if there's even a place where you have a clean line of sight that far...). Put the crosshairs between their eyes at a hundred meters, pull the trigger, and they drop. The only range you need to aim lower is within 30 meters or so, and then it's more "aim at their jaw to hit them between the eyes", just like with all the assault rifles. The only downside is its ammo costs a mint, at 1800 ru per 10 rounds. (Ironically, nothing seems to notice when I fire it, even if I hit their buddy square between the eyes five feet away from them...)
The SVUmk2 isn't as good, and I'd rate it below the vintorez. Both are also incredibly ugly compared to the dragunov (both the in-game models, and the actual guns), though that's secondary to utility, of course. The SVU isn't so bad, but the vintorez really just looks like shit.
Of course, this is unmodded, so they may very well be different for you.
I still don't know what the hell 3 is talking about. No hostiles show up on your radar unless you can see them, and even enemies armed with something as inaccurate as an ak74 (so anything above the lowest level grunts) hit you to devastating effect at range. Ones with nato guns even more so, and at longer range. The ak74 is the first gun you get that can reliably hit anything at range, the TRs 301 is probably the first that can hit anything at range with enough accuracy to get a headshot with any reliability... Maybe the rapid, though that's still a bit below the nato guns as far as accuracy is concerned... Again, this is what I've seen in an unmodded game, maybe what he's got makes all guns really accurate, and jams a few spikes into the brain of the already lobotomized AI...