I am quite new to all this but recently purchased an old HW35 .22 from around 1978. I'm enjoying the open sights but would like to try out a scope, probably a basic 4 x 32 or similar?

The grooves measure 12mm at the smallest point and this ties up with Hiller's book - I understand most newer rifles have smaller grooves of 9 - 11mm. Is it difficult to find something to fit the older HW35?

Another point on open sights, I have mainly used it indoors at a distance of around 8 metres (longest I can set up when the house is empty) and the best group of 5 was 1/2 inch. The rare occasion I tried in the garden (v. difficult due to ******* neighbours) with open sights I could hardly make out the central target at 25 yards to line up let alone hit the bull... How do people get on with open sights at longer ranges?