I’m working on a story in which some foreign mercenaries are brought into an American city in the latter 1920s as hired muscle. Rather than having them equipped with Thompsons or BARs, it’s a point of plot that they brought their own weapons with them. What non-American submachine guns were available between 1925 and 1930?
One possibility is the Bergmann MP18.
Another is the Beretta 1918 SMG Or this link. Looks like SIG made a copy of the aforementioned MP18.
If you want to be innovative, you can have your mercs use pintle-mountedVillar-Perosas…
Surprisingly, it doesn’t look like there were a whole lot of them before the late 30’s.
The French were making the MAS 1924 for their military in that period.
You’ve also got the Mauser M1932 Schnellfeuer, which was a full-auto version of the “Broomhandle” C96 Mauser. The “Official” full-auto Mauser versions didn’t appear until 1932 but Astra in Spain and various places in China were making their own full-auto C96s well before that- from the late 1920s at least.
Otherwise, the already mentioned MAS-1924, Bergmann MP18, and the very similar Beretta M1918 are pretty much it, from what I can tell.
For something closer to the BAR, there was a version of the Lewis Gun which had a wooden foregrip and was designed to be used as an “Automatic Rifle” rather than a machine-gun (at least in the WWI context). If you’ve seen The Mummy, there’s a scene where the mysterious guardian character grabs the Lewis Gun from a crashed biplane and proceeds to use it in an “Automatic Rifle” capability throughout subsequent parts of the film.