I bought a K-31 about six months ago. First time I shot at the range I had the strange sensation that I had a scope mounted on the rifle... had about same group as you.
Did your rifle have the previous owner’s name under the butt plate?
I took the butt plate off to look for that a few weeks ago...I had heard about that practice, but it was not there. I bought mine at the Tucson gunshow, so maybe the previous owner here in the states took it out. Didn’t think to ask at the time I bought it, I may have not know about it at the time.