.38 refers to the type of firearm

Firearm

A firearm is a weapon that launches one, or many, projectile at high velocity through confined burning of a propellant. This subsonic burning process is technically known as deflagration, as opposed to supersonic combustion known as a detonation. In older firearms, the propellant was typically...

 and firearm cartridges

Cartridge (firearms)

A cartridge, also called a round, packages the bullet, gunpowder and primer into a single metallic case precisely made to fit the firing chamber of a firearm. The primer is a small charge of impact-sensitive chemical that may be located at the center of the case head or at its rim . Electrically...

.

The .38 is considered a large caliber firearm cartridge; Anything larger than .32 is considered a large caliber, and one of the most common handguns manufactured in the United States

United States

The United States of America is a federal constitutional republic comprising fifty states and a federal district...

. The .38 caliber revolver is the standard sidearm of police in the United States, seconded by the .357 Magnum revolver.

Handgun cartridge table

CAUTION: The following information is only as accurate as sources quoted, and the transcription thereof. DO NOT use these figures as the basis for any loading data without independent verification.

Cartridge name	Bullet diameter	Case length	Cartridge length	Type	Source
.380 ACP .380 ACP The .380 ACP pistol cartridge is a rimless, straight-walled pistol cartridge developed by firearms designer John Browning. The cartridge headspaces on the mouth of the case. It was introduced in 1908 by Colt, and has been a popular self-defense cartridge ever since...  (.380 Auto, 9 mm Kurz, 9mm Browning Short, 9x17 mm)	0.356 in (9 mm)	0.68 in (17.3 mm)	0.98 in (24.9 mm)	Rimless	Barnes 1997, p. 274
.38 Casull	0.356 in (9 mm)	0.933 in (23.7 mm)	-	Rimless	http://www.municion.org/38/38Casull.htm
.38 Short Colt .38 Short Colt The .38 short colt was originally a heeled bullet cartridge intended for cartridge conversions of the .36 cal cap & ball revolvers from the American Civil War-era.Later, this cartridge was fitted with an inside-lubricated bullet in the 125–135 grains range....	0.357 in (9.1 mm)	0.762 in (19.4 mm)	1.052 in (26.7 mm)	Rimmed	Barnes 1997, p. 274
.38 Long Colt .38 Long Colt The .38 Long Colt is a cartridge introduced by Colt's Manufacturing Company in 1875, and was adopted as a standard military pistol cartridge by the United States Army in 1892 for the Colt New Army M1892 Revolver. It is slightly more powerful than the .38 Short Colt, or .38 SC. The .38 Long Colt...	0.357 in (9.1 mm)	1.03 in (26.2 mm)	1.32 in (33.5 mm)	Rimmed	Barnes 1997, p. 274
.38 Special .38 Special The .38 Smith & Wesson Special is a rimmed, centerfire cartridge designed by Smith & Wesson. It is most commonly used in revolvers, although some semi-automatic pistols and carbines also use this round...	0.357 in (9.1 mm)	1.16 in (29.5 mm)	1.55 in (39.4 mm)	Rimmed	Barnes 1997, p. 274
.38 AMU (.38 Army Marksmanship Unit)	0.357 in (9.1 mm)	1.15 in (29.2 mm)	-	Rimless	http://www.municion.org/38sp/38Amu.htm
.38 ACP .38 ACP The .38 ACP also known as the .38 Auto was introduced at the turn of the 20th century for the Browning designed Colt M1900. The cartridge headspaces on the rim. It had first been used in his Model 1897 prototype, which Colt did not produce...  (.38 Auto, 9x23 mmSR)	0.358 in (9.1 mm)	0.9 in (22.9 mm)	1.28 in (32.5 mm)	Semi-rimmed	Barnes 1997, p. 274
.38 Super .38 Super The .38 Super or .38 Super Automatic is a pistol cartridge that fires a diameter bullet. The Super was introduced in the late 1920s as a higher pressure loading of the .38 ACP or .38 Auto. The old .38 ACP propelled a bullet at . The improved .38 Super Auto pushed the same bullet at...	0.358 in (9.1 mm)	0.9 in (22.9 mm)	1.28 in (32.5 mm)	Semi-rimmed	Barnes 1997, p. 274
.38 Super Comp	0.358 in (9.1 mm)	0.896 in (22.8 mm)	1.28 in (32.5 mm)	Rimless	Starline cartridge dimensions
.38 S&W .38 S&W The .38 S&W is a revolver cartridge developed by Smith & Wesson in 1877. Though similar in name, it is not interchangeable with the later .38 Smith and Wesson Special due to a different case shape and slightly larger bullet diameter....  (.38 Colt New Police, .380 Rim, .38 S&W Corto, .380-200 British Service)	0.359 in (9.1 mm)	0.78 in (19.8 mm)	1.2 in (30.5 mm)	Rimmed	Barnes 1997, p. 274
.380 Revolver Short [British Webley pattern from 1860s, copied by Colt to produce .38 Short Colt]	0.375 in (9.5 mm)	0.7 in (17.8 mm)	1.1 in (27.9 mm)	Rimmed	Barnes 1997, p. 274
.380 Revolver Long [British Webley pattern from 1860s]	0.375 in (9.5 mm)	1 in (25.4 mm)	1.4 in (35.6 mm)	Rimmed	Barnes 1997, p. 274
.38-40 Winchester .38-40 Winchester The .38-40 Winchester is actually a .40 caliber cartridge shooting .401 caliber bullets. The cartridge that was introduced by Winchester in 1874 and is derived from their .44-40 Winchester. This cartridge was introduced for rifles, but in its reintroduction for Cowboy Action Shooting it has seen... (.38-40 WCF)	0.401 in (10.2 mm)	1.3 in (33 mm)	1.59 in (40.4 mm)	Rimmed	Barnes 1997, p. 92