22 RF Barrel Issues

Hello Folks

I have a 22 match coming up so have been out practicing - radical eh!!

Anyhoo I'm shooting a rifle I haven't used much (a Winchester M52 replica). Eley Club (for short range) and Eley Match for further out, It was/has been shooting pretty consistent groups between 0.5 and 0.65 at 50 yards

However the dammed thing has me beat - its developed some very nasty habits.

I was shooting some 100M tests the other day and the groups started to open up dramatically. Oh dang I thought, must need a clean (although its only about 200 rounds since it was last cleaned).

Cleaned it, with de-carbon cleaner. Nothing else changed, didn't adjust anything.

First group after clean was 1.2", second group was 0.54. Put it aside and went to work. Got home and commenced practice, bugger me, first group 1.8" (one very wild shot), the next group 0.52 Gave up in disgust.

Now I know some barrels will take a while to settle after a clean but what could I have done to "convert" a nicely performing rifle into a cold bore menace!!