Most Makarov's are chambered in 9x18. There are some being chambered in 9x19, but those are very few and very very far between in western markets outside of Europe at present. You are correct that they are NOT interchangable. The 9x19 will NOT fit into the smaller 9x18 chamber and firing a 9x18 in a chamber designed for 9x19 will result in case failure at a minimum and possibly a more explosive failure at worst. It should be stamped ON THE BARREL what ANY firearm is chambered for. Using ONLY what it was designed to fire is the safest way to go, period.

 

9x19 mm fires a .355" diameter bullet. 9x18mm Makarov fires a .366" diameter bullet. DO NOT attempt to interchange these two rounds.