Hi Ed - Oh yeah - 22" burn it - not as efficiently as 24" or 26", but you'll not be spitting 40 gr. of unburnt powder out the tubes. Note that it will kick somewhat - maybe more than that! HA!
: Belgium ribs are usually brazed together, not welded nor soldered. Scratch a rib where it contacts the barrel, on the undersize. A brass coloured line means they're brazed on.
: Setting up the barrels so they shoot together is more involved than aligning up the bore axis's. They must converge the proper amount to regulate with a specific load.
: I was lucky with my 12 bore double as it regulated with both the 5 1/2 and 7 dram BP loads, and also shot close enough with smokeless to be useful - same velocity as 7 dram black powder load, but 1/3 the recoil.
: If you're going to separate the tubes, Moody should be able to rifle them then, if the ribs aren't brazed on, that is.
: Ned Robert's book has several different pictures of rifling benches, some with spindles, some with a barrel used for a guide. The extra barrel is the easiest, just takes a long table. You can use any muzzleloading barrel as a guide for the slow twist. As I said, I'd go with something slower than 66" and faster than 100". A GM .50 or .54 cal. barrel would be about perfect at 80".