Texasjack,
Just a couple notes.The 16 ga. slugs marked Magnum should be ok in your drilling for occasional use, they are loaded to meet pressure specs.The prohibition is likely against using 3" shells(scarce but made at one time).You should really be getting better groups.My experience with drillings and other combination guns shows they usually shoot very well( the barrel bundles are very stiff).When I lived near Waffen Frankonia,I tried all my guns with S&B ammo. Only one performed as well with it as with RWS. I suggest you try a different bench technique, letting the barrel cool between shots, and if that doesn't work, try different ammo(the brass will be OK for reloading). Also the front trigger is a single set trigger, if it has a screw.A lot of them have been adjusted so they won't hold,under the misimpression that the screw is for adjusting the normal pull.I reccomend readjusting it and try again. The DRP is not likely for weichlot, softsolder was a commonly used material in public domain.It was likely for some other feature.
Mike