I have never, EVER SEEN an SKS shoot inside 5" at 100 yards! They are normally "C__p"
If you have one that shoots good, you better keep it, and frame it! It is indeed rare!
My 7.62X39 Ruger M77 Hawkeye shoots both .310 & .308 just fine. I stick with the .308diameter though because of huge bullet selection, and the accuracy I normally get.