that sort of rules out a couple of things...scope slipping forwards/backwards will do the verticle string thing.
projectile holes are round??? so its not a case of keyholing stuffed barrel.....
there are a couple of good local gunsmiths who could check it over for you.
20cm is shocking....maybe get someone else to try a group of 5 and see if it miraculusly shrinks group to 2"
one other thing...are you holding forestock or letting it move on rest????
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