15 years ago, I bought a little Savage 24D 20Ga/.22WM combo with open (separate) barrels. It was a nice little gun, but the hits were abolutely unpredicatable.
I sighted it in the same day as I bought, and shot a fox with the rifle barrel in the evening. When I a week later shot at a crow, I didn't hit it at all. Back at the hut, I took a target paper, put it at 50 yards and fired a control shot. I missed the back spot with almost a foot.
At home I examined the gun cautiously, and found that the barrels could move in the muzzle joint. It was enough to grasp the gun by the barrels to destroy the sighting-in. Having discovered this, I gave it back to the seller.
I have had other combination guns, but with joined solded barrels. There has never been any problem with them.
Pete