If some care is given to components and charges, there shouldn't be a problem with odd-named cases. The capacity is what is important. A comparrison could be made using an innert filler.
: I do strongly suggest a modern loading manual that has a good variety of shotshell hull makes listed in the data.
: Shotshell length is very important. The 3" chambers could be problems as that much room isn't needed and the extra length must be taken up with wads which add weight to the 'ejecta'. Due to extra wadding, etc, 3" hulls generally don't give much if any increase in velocity over 2 3/4" hulls. Increased case capacity makes abig difference in rifle calibres, but not in shotshells, possibly due to the very low presures used.
: I would sort out a variety of loading data for various brands of cases, with a given ball of slug weight, then pick the smallest charge used in the lot of cases, and start 1 gr. UNDER that amount. That should be quite safe as most data for reloading is listed well under maximum pressures. For expample, we know that modern shotshells are loaded to 11,000 LUP and Lyman's listings in their manuals run from 7,000 to 10.500 LUP normally. The important aspect, is the pressures are listed in good manuals and data from manuals without pressure data, shouldn't be used when developing slug or RB loads. It is important to know what sort of cussion you have. I am not suggesting increasing data over what the manuals say, just that it is nice to know that THIS data shows 8,400LUP, so it is very safe in my gun, not on the hairy edge of the scale where a small mistake in loading will possibly blow the gun apart. I thought I would add that as a warning - it is imortant to take care and go slowly.
: I have heard of shotguns with 3" chambers that were exceptionally accurate using 2 3/4" hulls.
: Keep me posted of load developement - got me excited now, and I may have to look around for another suitable double for modification to a SxS 12 bore ball gun. They have a charm, feel and handling characteristics that cannot be met with any other barrel system.