Rubberhead,
Double rifles perform better with the ammunition it was regulated with. Your rifle was not likely regulated in the US, and was likely regulated with a bullet heavier than 150 grains. If any of the documents came with it, they may ID the ammo used. Also they are sensitive( SxS more so than O/U)to the "bench procedure" used when shooting them. They should be held in both hands with the forward hand( not the rifle) resting on the bag. The bags should be built up high enough that you are sitting upright and able to move backward in recoil. You should fire the second barrel as soon as you can get back on target. For the next two shots, you should let the barrels cool to ambient temperature first, and do the same for the third set of shots. The best is to fire one set of two shots on one day and come back another day for the next set. The "drill" is not target shooting, rather it is to discover where two shots from cold barrels strike the target. To move the group to where you want it, you have to know where the first bullet( and maybe the second) will hit. Once you do that, you can shoot all you want. From your description, I would guess you could use 180 grain or heavier bullets in both barrels. If you insist on using 150 grain bullets, you might be able to do so by handloading them to a lower velocity. Of course, as always, I could be entirely wrong.
Mike
