I recall from several years ago that somehow one round of my 9mm Luger (9x19) accidentally got mixed in with my 9x18 Mak ammo and fired just fine from my Bulgarian Makarov.The 380 (17mm)and 9x18 (18mm) are straight wall cases.
The 9mm is a tapered case.
The 9mm and Makarov case have the same size base.
So reloaders can trim the 9mm case back to 18mm and reload it as a 9x18 Mak.
I did that a lot. When I first got into Makarovs the ammo was hard to get and expensive.