Reloading .380acp (9mm Browning) with 9mm Parabellum brass.

Name Bullet Diameter Case Length Rim Base Shoulder Neck OAL
.380 ACP 9.02 (.355) 17.30 (.680) 9.50 (.374) 9.50 (.374) N/A 9.50 (.373) 25.00 (0.984)

9×19mm Parabellum (9mm Luger) 9.02 (.355) 19.15 (.754) 9.96 (.392) 9.93 (.391) N/A 9.65 (.380) 29.69 (1.169)
 
Thinner walls mean the outside diameter of the casing is smaller in diameter than a 9mm. Being a larger diameter, a 9mm will not fit in a .380 chamber, and no you can't size down a 9mm case to fit (I know that's what you're thinking) the case would just return to its original size when you insert the bullet because the bullets are the same diameter. Isn't physics a wonderfull thing?
 
Thinner walls mean the outside diameter of the casing is smaller in diameter than a 9mm. Being a larger diameter, a 9mm will not fit in a .380 chamber, and no you can't size down a 9mm case to fit (I know that's what you're thinking) the case would just return to its original size when you insert the bullet because the bullets are the same diameter. Isn't physics a wonderfull thing?

I wasn't thinking when I replied to Ganderite post. If I resized a Parabellum casing, the bullet Dia would have to be smaller.
 
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