Get someone who is a good / known shooter to shoot some groups.
I don’t like to admit it , but I’m the biggest variable when it comes to accuracy with my Rifles.
I’m not trying to be a Smart Arse , but make sure that you have eliminated yourself from the equation.
I’ve had similar problems in the past and nearly pulled my hair out trying to solve the problem , mine was a damaged muzzle crown that wasn’t obvious, but was the cause of much drama.
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