Years ago, I had a magazine article with directions and pictures for making up shotshells for the 444 Marlin. I was going to get to it one day, but never did. Of course, I can't find the article at this time.
However, the guy suggested a 410 shotshell cup. That is a fit for the interior of the 444 Marlin, but I would look at the 28 gauge or 20 gauge shotshell cup to see what the dimensions are.
Now, you have just fired up my interest again. I have absolutely no use for a shotgun, but it would be fun to play with loading and a little shooting. When Winchester came out with the Model 94 in the 410 shotshell, I had to take two asprin and take a nap for that hummm to go away and it still knaws on me when I see one.
Let us know if you persue the shotshell loading.
Thanks,
Harold Clark