Bulged barrel, muzzle damage. Had a few where I've uncovered that now, one (.243) took a lot of head scratching to work out what was up. The bulge wasn't visually apparent but once we worked out what was up the issue was obvious by placing a straight edge on the outside of the barrel. And it was far enough back that the barrel wasn't able to be docked as well, very sucky.
In the shooting from it, two shots touching then the next few anywhere within about a 500mm circle or worse.
The next possibility would be throat or headspace issues, but I wouldn't think that it would show in such a sudden extreme group change...
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