My kids cleaned one SKS that had a good sized piece of "chat",chert or flint in the gas tube.Very sharp edges.We just guessed that it got stuck in the cosmoline when it was being packed away and accidentally got there.
I've seen adults just clean them enough to get them to function but the thorough cleaning can really pay off.
Another foreign object found in an SKS: a very wiry black hair in an Albanian specimen...apparently armorers aren't required to wear hairnets.:)