From Mak.com:
What caliber should I get, .380 or 9x18 Makarov?
By all means, get one in the original caliber of 9x18 Makarov, unless you are already familiar with the .380 ACP and reload for it. 9x18 Makarov ammunition availability is no longer an issue. In fact, you can often get surplus 9x18 Makarov ammo cheaper than .380 ACP! Note that you can now buy aftermarket replacement barrels in both 9x18M and .380 ACP (see accessories page). So if you made the unfortunate mistake of buying a .380 ACP Makarov, there's still hope for you (just kidding). Also, if you have .380 ACP ammo hanging around, feeling lonely, you can retrofit a barrel in .380 ACP and you're all set. May require some mechanical skill and a barrel press. See our Makarov Accessories page for how to order barrels. Barrel replacement is not a simple procedure, but can be done by mechanically inclined people. My general rule of thumb is that if you can change the oil on your car, you can do this. Read more about how to do this here.