What is the difference between a .380 cal bullet and a .38?

There are several different types of .38 cartridges. The main thing which separates the .380 from the rest of them is that the .380 is intended for use in an unlocked breech pistol, whereas most of the other rounds are too powerful to be used in such a pistol. The .380 has a bullet diameter if .355 inches, and the metric designation for the cartridge is 9x17mm.

The most common .38 cartridge would be the .38 Special, which has a bullet diameter of .357 inches, has a longer case, and the most obvious difference is that it uses a rim, as opposed to the rimless design of the .380 ACP.

You may also have the .38 Super in mind, which is a cartridge intended for autoloading pistols, and still remains popular with competition firers. It has a bullet diameter of .356 inches, has a 22.83mm case length (vs. the 17.3mm case length of the .380), and much higher chamber pressures than the .380 ACP, making it unsuitable for the simple, unlocked breech pistols of the type commonly chambered for cartridges such as the .380 ACP.