I owned a couple of SKS's.
Neither shot better than about 8" at 100m.
Ammo was chinese surplus (steel case) and both rifles were Chinese. One was a D-mag, one was regular fixed mag.
Testing at longer range showed that they were incapable of consistently printing on a 4 foot screen at 400 yards.
That said, the ammo is cheap like borsht, so are the guns. Cheap center-fire, fun to shoot. Reliable, not very accurate.
I have heard of better accuracy standards from Russian guns (Tula arsenal rings a bell for some reason.)
As for the SVT-40, well, the one I have played with was unreliable, and it's gas system very finnicky. It wasn't mine, but I helped tear it down to try and get it working again. It was fired in a series of 200m rifle matches (SR Matches 1-4) and the score was abysmal compared to Enfields being fired alongside it.
The SVT does use the 7.62x54R (rimmed) round, so it's a bit more spendy on the ammo side.
That's my experience....hope it helps!
NS