Thanks for the private recommendations on filler.  The biggest challenge is finding a workable bullet given the choked bore.  Still cannot find any reference on a known cartriodge.  Big bore for a light gun.  I will get it sorted.

Otto
I will try to register on GGCA, was registered ages ago but forgot password.  Asked for a new one, tried it, still no go.  Going to register under my other e-mail. 
If it is easy to post photos on ASSRA, where are the actual instructions on how to do it?
It is an interesting rifle.
Thanks
Gigg

Gigg,
First thing I do is to size the picture, so that it isn't too large. You can do this in whatever program, that you use for pictures and you want to end up with a picture of not more than 100kb. My email gives me the picture size under attachments and is another way to check the size of the picture.
I then move the picture to my desktop. If I use email to check the picture, I just save the email picture to the desktop.
Now follow the directions above from oneatatime. Reply---Attachments---choose file on the desktop---Post message.
If things aren't as you want them, you can click "modify" and correct the mistake.
If you need help getting the picture the right size, PM me and I can do it for you.
Otto

It worked! Nifty rifle. Any chance it is a 24 gauge? What is the base diameter of the chamber?

Gigg,
Good job of posting a picture of your rifle. 
I can't find any 28 gauge based cartridges in my reference books.
I have a 16 gauge double rifle. It came with some loaded rounds and a couple of bullet molds but no load information. So, not much help in answering your original question. 
The GGCA is probably your best source for information on the rifle itself. You might also try Double Gun Forum, especially for load information.
Both the GGCA and the DGF will want the following:
A photo of the entire rifle, as well as the one that you posted here.
A photo of the marks under the forearm.This is the most important picture. It will be helpful in identifying the proof date and original caliber/cartridge.
A photo of your chamber cast, along with your measurements and observations from firing the rifle, using the 28 gauge shotgun shells.
I seem to recall that one of the custom gunmakers made a 28 gauge single shot rifle, maybe 10 years ago. I'll see if any of my friends can recall the name of the gunmaker, as it's likely that he has loading information.
If you select your last post and hit modify, at the top right side, you can correct the diameters in your last post and then hit save.
Otto

Gigg,
The German proof law of 1893 may be of some help to you. In addition to requiring proof testing of smoothbore barrels, there was an additional table for proof loads for rifled barrels.
This information is located under the forearm, usually on the bottom flat. 
As time went by, information as to bullet type/weight and powder type was added. In 1912-1913 the "R" for rimmed cases was added.
I'm sure that the experts at the GGCA forum will be able to give you additional information on your rifle and if your lucky, maybe some loading information.
Otto

Found a .575494 mold even though they are now discontinued, turned the base plug to create a slight indent similar to the German bullets.  Cast 400gr from soft lead, German bullets were 380gr.  Loading with 4ogr ffg, 20.2 cc cream fo wheat, SPG lube.  Next step to the range soon as soon as it stops raining.
Gigg