I'm with Art. Wood will often do this, but rarely 6". But I'm betting this is not a wood stock. Having the action screws tightened properly, and in the right sequence can have an effect and would be my guess. The front action screw should be tightened 1st, and be the tightest. The rear screw is next and it should be slightly less tight. If there is a middle screw it should just be snug enough that it will not come out. Torque specs can be found for specific rifles.
BTW, once a rifle is zeroed I don't touch the barrel for cleaning unless it gets wet and wouldn't think about removing the action from the stock until after hunting season. Even after cleaning I wouldn't trust the zero to be the same until I had put 10-12 rounds through the rifle. Most rifles are a little less accurate for the 1st few rounds after cleaning anyway. Accuracy won't decline until around 200-300 rounds with most guns anyway before they need cleaning.