The correct caliber should be stamped on the side. While you might be able to use .380 in a 9x18, it will be inaccurate, and you would certainly NOT want to try 9x18 in a .380.
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=== Update (12/10/2016) ===
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If it is 9mm Mak, you can usually find it at gun shows or online at gunbroker.com
The correct caliber should be stamped on the side. While you might be able to use .380 in a 9x18, it will be inaccurate, and you would certainly NOT want to try 9x18 in a .380.