Making the barrels heavier - or reducing the weight of the ball should bring them closer together, shouldn't it?

In the 12 bore double I worked with, I used the same ball I load in my 14 bore muzzleloader. The mould is .684", an Italian mould that looks like a Lyman mould. It casts a pure lead ball of .682" which weighs 482gr., while the mould throws a WW ball of .684" weighing 466gr.

My double shotgun put the tubes together - they'd been shortened to 26" with sights let into the top rib. I even mounted a scope on it for load development. It regulated with both smokeless and black powder- presumably, the excessive recoil of the black powder (in comparrison to smokeless) had little to do with the regulation, merely the velcoity of the balls seemed critical to the regulation of the barrels. Both the 7 dram 2F black powder load and my smokeless loads put out the same speed - 1,500fps.

The smokeless felt like it had only 1/2 the recoil.

Because we are attempting to maintain penetration and killing power, I think going to a foster (hollow) slug or foster styled slug is a bad idea. If a reduction to 1oz is necessary, then the use of cup-shaped wads such as I used to keep the ball in the middle of the bore for accuracy, would also work with a 16 bore ball. The 16 bore ball measures .662". This 1oz. ball will give even higher speeds than 14 bore or 12 bore balls, which should being the barrels closer together, shouldn't it?

I would not consider separating the tubes and attempting to re-regulate them until I'd exhausted all other methods first. A 16 bore ball of WW alloy, driven to 1,800 or 1,900fps will most certaily smash both shoulders of an elk. Those speeds should be achievable with Longshot, Herco or other slow shotgun powders like Blue Dot. IMR's SR4759 or H4227 or IMR4227 can make even higher speeds, but I've not used them - yet.