So I could buy some of the .380 bullets and just load them and use them?
I wouldn’t without a gunsmith’s exam and okay. Some Makarov’s imported in the ‘90s were chambered in .380. I honestly don’t know if the 9x18 Mak’s can shoot both. I know I wouldn’t.
The .380 and 9mm Mak are close enough that it might work but I wouldn’t try it except in an emergency.
The Mak is more powerful, the case a tiny bit longer and the bullet a couple of thousandths larger. Accuracy would normally be a bit worse because of the undersized bullet.
You would have to rebarrel.