A friend has a .223 that he was ready to sell because it would not group at 100yrds.
He is very good with rifles and tried everything.
One day just for grins he started blasting away at long distance targets,200yrds and farther, it was a tack driver.
For what ever reason it seemed liked the bullets were not stabilized at 100yrds. No matter what weight of bullet or load he tried.

Good luck!