9.3x57,
Those were some of my concerns as well, so I regulated this using the 20 gauge Lyman cast hourglass shaped slugs, loaded into Winchester AA hulls with AA shotwads and 21.0 grains of Universal Clays powder. I can make them out of dead soft lead because the slugs are protected from the rifling by the wads. I also shot some double round ball loads in it with 15% reduction in the powder, (I found that the two round balls weighed 15% more the the slugs I was shooting in it).
Usually, one of the round balls would go about where the sights were aimed but the other would vere off to the side a bit.

The Lyman slugs were specially prepaired and weighed 385 grains when poured with soft lead. The two .535 round balls weighed 450 grains. Either load would be devistating to anything hit with it. The loading data comes with the Lyman mold to make the slugs and I found that by cutting off the crimped area and roll crimping them, I got a better and more uniform load. I bought the roll crimper from Ballistic Products in Minn. I also roll crimped the round ball loads, this way I can see instantly what load I have in my hand. All these components are readily available and should be for some time. Anyone who thinks they can just go and buy the new sabot slugs, or any other for that matter, and have them be regulated in their double gun and shoot to the point of aim, is in for a big surprise. Rolling your own is manditory if you want to shoot a double barrel slug gun. I have found that they are just as picky, maybe even more so, than any other double rifle. You must pick a load and stay with it. Mine as solder tightly together and there is no moveable regulating wedge on it.

This is the second 20 gauge slug gun I have built and I have the third one about 3/4 done. Here in Iowa, we still have some shotgun only seasons and most of our shots are 50 yards or less in timber, so I figured this was a natural answer to a limitation put on us by the State. I first checked with my local game warden to make sure it would be legal to hunt with. He told me that 20 gauge was the minimum size and that rifled barrels of 20 gauge were legal, so off I went and built the first one. They make a great deer hunting tool. Lightweight and fast shooting.