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No better than 8". For most rifles, and most conditions, group size increases linearly with distance, provided the wind is perfectly calm...which it usually isn't.
If you have a very accurate rifle, you will see groups get a little vertically egg-shaped at 500-1000 yards, due to variation in muzzle velocity.
The wind is the big factor in spreading groups horizontally. That effect is cumulative as the bullet travels, so it increases more rapidly than just a simple linear curve.
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