Yes, I knew that about Thompson drums. Did you know the 10round drum (required by the 94AWB) was called the "X"?
Japanese machinegun design was ..interesting. Quite a fair example of how NOT to do things. I understand that several of the different machine guns required either lower pressure ammo than the standard rifle cartridge, and/or lubricated ammunition.
And since the ammo was apparently all dimensionally the same (approx.) it must have been quite a fun time for the logistics people.
Also know that the majority of SMG designs fire from the open bolt, and fire full auto only. Very few exceptions, and only one that I know of currently in production (and its a very popular exception) is the H&K MP5.
The MP 5 fires from a closed bolt, and is selective fire. This makes it both much more mechanically complicated and expensive than the simplest SMGs, such as the Sten and the M3/M3A1Grease gun. And that is about the only bad things you can say about the MP5, which has proven itself to be a world class act for many years now.
All else being equal (and it almost never is) bigger bullets tend to work better.