222 Rem Lost Accuracy at 500 Rounds?

[Webster]

So I have a custom 222 Rem for the first 500 rounds was shooting lights out with a Nosler 40gr BT. I would regularly get .200" groups at 100 yards with very little effort if the wind wasn't too horrible. I would need to extend the seated bullet by about .010" about every 200 rounds. I even had a factory hornady 50gr load that shot .200". I loaded up a couple dozen rounds to check seating depth again and I am getting .75" groups with mostly horizontal regardless of seating depth. The factory load is now also at .75" No changes to anything gun or ammo related, checked action screws, scope etc.., and velocities still good with single digit Std dev. What do you guys think is going on? Going to do a good carbon scrub with JB bore paste. Any other suggestions?

0.010" every 200 rounds sounds like a lot. How ofter do you measure touch. I think it's harder to measure touch accuratley as the beginning of rifling loses it's sharp machined edges. The target will tell you what works. I have 4200 shots on my 6BR varmint hunting rifle. I don't see any obvious change in groups. Never changed seating depth. The rifle normally shoots about .350" groups. Averaging .200" would win a lot of serious bench rest aggregates. Good luck sorting it out. Let us know if you find out what's going on.