What is the difference in a 380 and a 40 caliber bullet?
The .380 ACP has an actual bullet diameter of .355 inches,
versus the .4005 inch diameter of the .40 S&W bullet.
Additionally, the cartridge length of the .40 is considerably
longer - 22mm, versus the 17mm cartridge length of the .380 ACP.
The .40 is a considerably more powerful round, and typically
requires a delayed blowback system of operation, whereas most .380
pistols are unlocked breech direct blowback pistols.