How does groups form? Is it the gun, the shooter, wind pushing them slightly to the left?
If you think your gun is capable of printing 1/2MOA groups and you, as a shooter can be as consistent as 1/2MOA, your groups can be as big as 1MOA.
When you add wind, even 1 m/s breeze which you hardly can even feel, it will throw your shots significally off at hundred if not compensated with point of aim (or scope adjustments).
When stretching distances further, shooters accuracy often stays linear, but external ballistics does not, so things get even worse and 1MOA accuracy at 100 meters can be 2MOA at 300 meters.