DHerr - you can get wads from TRACKOFTHEWOLF.COM for your 28 bore. I concurr about using black powder due to the small bore size of this gun. It is much easier with larger bore guns, to convert to smokeless duplication loads. With the small bore size, pressure using smokeless climbs quickly.
: Looking at 28 bore modern shotshell loads will give load and wad columns in plastic as well as card wads that develope pressures in the 8,000 to 11,000 psi range.
: You are on your own as I haven't developed loads for one of these guns myself, but it wouldn't be difficult. There is no "formula" for conversion as the bore guns use very fast, shotgun-type powders for dupication,nitro to black loads. The smaller the bore, the slower the powders, and therein lies the problem. New ground.