Thread: What could cause this?

At 300m you do start to see minor wind effect the groups. Have a look at 'Applied Ballistics' and deliberately dial in a 10m/sec crosswind.
You'll see a couple of clicks to account for. 0.2milrads at 100m is 2cm, so that is 6cm drift at 300m (plus or minus one for height depending on direction).

Anything minor adds up too but if it were me i would leave the scope as is but really make sure that crosshair doesn't move independently to the target when you support the rifle / hands off / no cheek weld etc and bob your head slightly when looking through the scope. I do this every time (when you know you can risk taking the time). Bad parallax will definitely amplify any changes coming from your body position (that the terrain might be causing). Especially if your elbows aren't level....