I had to adjust my model 48's rear aperature far left also. It had a dramatic power loss ,less than 400 fps. I sent it to RWS for repair. They replaced the breech seal and now it's back up to normal power and the rear sight is now centered and dead on.
Apparently the repair changed the point of impact.
I do not have the explanation but originally the sight started out centered. About the time I had the power loss I had to adjust the sight to the left. After the repair it's point of impact is back to dead center
kind of like putting your thumb over the end of a garden hose, the water always shoots to one side. So think of the blast of air coming from the 48's transfer port, and try to picture one side leaking. The effect would be one side of the pellet getting hit with the energy and the other side maybe not so much. I think it would have that effect. Just a guess.
I've been giving this some thought, if there was a leak there should be a drop in fps, but there isn't, and the hose with thumb over theory sounds good but I'm trying to imagine that the barrel and rifling would correct this phenom, also the rear of the pellet has a conical indent which would focus the blast of air to the center. Hhhhuuuummmm.......I will check the fps through my crony and post them just to be sure.
OK, 10 shots with exact diabolo, 8.2 gr I believe, average 1017fps, that sounds about right. scope back on top shooting straight, looks like the scope stays. I'll put some swanky target sights on my 34.
