They do not measure "about the same" at all. First, the loaded case mouth diameter on the 9 mm is .390" while the .380 is just .373". This means that a modified 9mm case will likely not chamber safely without neck turning. Second, the case diameter above the rim is .391" on the 9 mm, just .374" on the .380. It will be difficult to size the tough 9mm cases down this much. Even if you do and cut the case back to .380 length, the internal capacity will be much different. That means that you can't safely use published .380 ACP load data. Don't do it.
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