EDITED TO ADD- I'm still trying to figure out how the magazine works on the 100mm Ground type machine gun, and what the cartridge might look like. It's not as clear to me as the MMP78.
Well according to the MG manual the magazine "slides" into/onto the machine gun horizontally but maybe that's just a mechanic of the model? The MSiA version "pegs" in. I'm not really sure how it would feed though or even that it could hold a respectable amount of ammunition. http://dalong.net/review/mg/m40/p/m40_m0009.jpghttp://dalong.net/review/mg/m96/p/m96_mb0019.JPGhttp://dalong.net/review/mg/m96/p/m96_mb0018.JPG
You can see a really small round toward the bottom of the first scan. Looks like more of a rifle casing than a pistol one I guess? Certainly doesn't look like a scaled up 10mm round from a Colt Delta Force Elite
That round isn't to scale if it's really supposed to be a 100mm. My working theory is that the magazine is helical feed like a Calico or Bizon SMG.
I'm betting Okawara, Katoki, and others aren't as worried about how these things work as we are... My big pet peeve is the Kaempher/Efreet type shotgun, which I've seen listed as either 42mm or 90mm. It's reasonable that the shot could be 42mm each, but the bore should be significantly larger than 90mm.