I have a savage 12 fv 223 with the factory 9 twist. For the first 500 or so rounds the gun was shooting great (average groups around 3/4 moa at 100 y w/ 55 gr varmint extreme and varmint nightmares from midsouth). Then a few weeks ago I went to the range and on 4 straight groups the 1st shot would keyhole and then the next 4 would group normally. I thought the barrel was dirty so I cleaned it with wipe out until there was no color coming out of the barrel. The next time out I only shot about 20, 75 gr A-Max and they were fine ( in fact I shot my smallest group ever out of the rifle at 200 y). Today I went back to the range and again the the first round would keyhole and then the other 4 would pass through the target correctly (again with 55 gr varmint extremes). I suspect a dirty barrel again but am not sure why the gun would be fouling so bad now. Prior to this the gun could go 100 or more rounds before accuracy would fall off. Could this destabilization occur for other reasons?

I have checked the crown and that appears to be okay.

Any help is greatly appreciated.