Andreas,
I would like to ad a little of my expereince.
A couple of points:
1- Shoot from off hand or at leat not resting the rifle directly. I found that what works for me is resting the back of my forward hand on an horizontal post as a rest.
2- Shoot at two targets, one for each barrel, so you know where each barret hit from aiming point.
3- You should try at least 3 right-left to know where it shots.

You need to know exactly how the rifle shot from a free recoiling position as when hunting.

Hope this helps.
Martin