The .38 Special (officially designated as the 38 Smith & Wesson Special) is a rimmed, straight walled centerfire revolver cartridge featuring a standard maximum bullet diameter of 0.357 inches (9.07 mm). The maximum case length is 1.155 inches (29.34 mm), and the maximum overall cartridge length is restricted to 1.550 inches (39.37 mm) to ensure compatibility with traditional revolver cylinders and lever action rifle mechanisms. The case utilizes a prominent rim diameter of 0.440 inches (11.18 mm) and maintains a uniform straight body diameter of 0.379 inches (9.63 mm) from the base to the case mouth. Ammunition for this caliber is most commonly found in 110 grain, 125 grain, 130 grain, and 158 grain bullet weights. Exact dimensions and operating pressures can vary slightly depending on manufacturing tolerances and whether you reference American SAAMI specifications or European C.I.P. standards.
.38 Special Dimensions
| Dimension | Specification | Source | Notes |
| Bullet Diameter | 0.357 in (9.07 mm) | SAAMI | Standard for jacketed bullets; lead bullets often measure 0.358 in. |
| Case Length | 1.155 in (29.34 mm) | SAAMI | Maximum case length; trim to length is typically 1.145 in. |
| Overall Cartridge Length | 1.550 in (39.37 mm) | SAAMI | Maximum overall length for standard revolver cylinders. |
| Rim Diameter | 0.440 in (11.18 mm) | SAAMI | Maximum rim diameter. |
| Case Diameter (Base) | 0.379 in (9.63 mm) | SAAMI | Maximum diameter measured at the base just above the rim. |
| Shoulder Diameter | N/A | N/A | Straight walled cartridge, no shoulder. |
| Neck Diameter | 0.379 in (9.63 mm) | SAAMI | Maximum external diameter at the case mouth. |
| Base Diameter | 0.379 in (9.63 mm) | SAAMI | Uniform with the neck due to the straight walled design. |
| Case Capacity | ~ 23.5 gr H2O | Industry Standard | Varies based on brass manufacturer and internal wall thickness. |
| Maximum Pressure | 17,000 psi | SAAMI | +P specification allows for 20,000 psi. C.I.P. maximum is 1,500 bar. |
Understanding The 38 Special Dimensions
- Bullet Diameter: The measurement across the widest point of the bullet. For the modern 38 Special, this is specified as 0.357 inches for jacketed bullets. Handloaders utilizing unjacketed cast lead bullets typically size them to 0.358 inches to ensure a tighter gas seal in the barrel.
- Case Length: The length of the empty brass cylinder from the base to the case mouth, maxing out at 1.155 inches. This extended case was originally designed in 1898 to accommodate a charge of black powder.
- Overall Cartridge Length (OAL): The total length of the fully assembled round from the base to the tip of the bullet, capped at 1.550 inches. OAL directly influences whether a cartridge will fit inside the cylinder of a revolver without binding the cylinder rotation.
- Rim Diameter: The diameter of the flange at the very bottom of the case measuring 0.440 inches. The revolver cylinder uses this prominent rim to securely hold the cartridge in place for headspacing and to extract the spent casings.
- Case Diameter: The width of the cartridge body. The 38 Special is a straight walled case, meaning the base diameter (0.379 inches) and the neck diameter (0.379 inches) are identical on unfired brass.
- Case Capacity: Measured in grains of water (gr H2O), this represents the internal volume of the empty case. Averaging roughly 23.5 grains of water, this case has a massive internal volume for its relatively low operating pressure.
- Maximum Pressure: The peak internal pressure generated when the cartridge is fired. Standard SAAMI limits set this at a very mild 17,000 psi, ensuring older revolvers do not suffer catastrophic failures. Modern +P ammunition is safely loaded to 20,000 psi for use in suitably rated firearms.
38 Special Cartridge Dimensions In Inches And Millimeters
For engineers, reloaders, and international shooters, below are the primary 38 Special measurements converted accurately between imperial and metric systems.
| Dimension | Inches (SAAMI Max) | Millimeters (Equivalent) |
| Bullet Diameter | 0.357 in | 9.07 mm |
| Case Length | 1.155 in | 29.34 mm |
| Overall Length | 1.550 in | 39.37 mm |
| Rim Diameter | 0.440 in | 11.18 mm |
| Base Diameter | 0.379 in | 9.63 mm |
| Neck Diameter | 0.379 in | 9.63 mm |
38 Special Case Dimensions
The 38 Special is defined by a rimmed, straight walled case. The official maximum case length is 1.155 inches (29.34 mm), with a nominal trim to length for reloaders sitting at 1.145 inches. The base diameter measures 0.379 inches, and the rim diameter extends significantly wider to 0.440 inches.
Because the 38 Special was originally adopted in the late 19th century as a black powder cartridge, its case is physically much larger than necessary for modern smokeless powder. Modern smokeless powder charges often fill only a small fraction of the 23.5 grains of water capacity. This excess internal volume requires reloaders to exercise extreme caution. It is exceptionally easy to accidentally double charge a 38 Special case with fast burning pistol powder, which will severely damage the firearm upon ignition.
38 Special Bullet Diameter
The standardized maximum bullet diameter for modern 38 Special ammunition is 0.357 inches (9.07 mm).
The name 38 Special is a historical leftover. The cartridge was developed from the 38 Long Colt, which originally used a heeled bullet that measured roughly 0.380 inches in diameter to match the outside diameter of the brass case. When ammunition manufacturers transitioned to modern inside lubricated bullets that drop inside the case mouth, the bullet diameter was reduced to 0.357 inches, but the original 38 caliber naming convention was retained for marketing and legacy purposes.
The standard 38 Special barrel features a groove diameter of 0.355 to 0.357 inches and a bore or land diameter of 0.346 inches. Firing a 0.357 inch bullet down this barrel forces the copper jacket or soft lead to engrave tightly into the rifling.
38 Special Overall Length
The SAAMI maximum overall cartridge length (OAL) for the 38 Special is 1.550 inches (39.37 mm). This number is a hard limit designed to guarantee that the bullet tip will not protrude past the front face of a standard revolver cylinder.
Typical factory cartridge length varies based on the bullet profile. A classic 158 grain lead round nose load often measures exactly at the maximum 1.550 inches. Modern jacketed hollow points often weigh less and are physically shorter, resulting in an OAL closer to 1.450 inches. If a reloader exceeds 1.550 inches, the nose of the bullet will bind against the forcing cone of the barrel and physically prevent the cylinder from indexing.
38 Special Chamber And Cartridge Dimensions
The relationship between the 38 Special cartridge and the firearm chamber is perfectly adapted for revolvers. The 38 Special cartridge headspaces entirely on its 0.440 inch rim.
This means the protruding rim of the brass casing physically stops against the rear cylinder face. Because it headspaces on the rim, the overall case length does not dictate how deep the cartridge drops into the chamber. This design allows revolver cartridges to employ a heavy roll crimp. A roll crimp physically bends the case mouth into a groove on the bullet known as a crimp groove or cannelure. This heavy crimp is mandatory for holding the bullet firmly in place, ensuring reliable ignition and preventing bullets from backing out of their cases under recoil.
38 Special Dimensions vs Similar Cartridges
To understand the 38 Special size profile, it is helpful to compare it against other popular handgun cartridges. Note that dimensional similarities do not imply ammunition interchangeability.
| Cartridge | Bullet Diameter | Case Length | Overall Length | Rim Diameter | Maximum Pressure | Source |
| 38 Special | 0.357 in | 1.155 in | 1.550 in | 0.440 in | 17,000 psi | SAAMI |
| 357 Magnum | 0.357 in | 1.290 in | 1.590 in | 0.440 in | 35,000 psi | SAAMI |
| 38 Short Colt | 0.358 in | 0.762 in | 1.050 in | 0.440 in | 12,000 psi | SAAMI |
| 38 Long Colt | 0.358 in | 1.031 in | 1.320 in | 0.440 in | 12,000 psi | SAAMI |
| 9mm Luger | 0.355 in | 0.754 in | 1.169 in | 0.394 in | 35,000 psi | SAAMI |
38 Special vs 357 Magnum Dimensions
The 357 Magnum was developed directly from the 38 Special in the 1930s by lengthening the case and strengthening the web. Their dimensional relationship is one of the most famous safety features in the firearms industry.
- Bullet and Rim Dimensions: Both cartridges use the exact same 0.357 inch bullet diameter, the exact same 0.379 inch base diameter, and the exact same 0.440 inch rim diameter.
- Case Length Difference: The 357 Magnum case (1.290 inches) is intentionally manufactured exactly 0.135 inches longer than the 38 Special case (1.155 inches).
- Pressure Differences: Standard 38 Special operates at a very mild 17,000 psi. The 357 Magnum generates a massive 35,000 psi.
Because of this specific dimensional design, a short 38 Special cartridge can be loaded and fired safely in any 357 Magnum revolver, providing shooters with a low recoil practice option. However, the elongated 1.290 inch case of the 357 Magnum physically prevents it from fully chambering in a weaker 38 Special revolver. This deliberate dimensional offset protects shooters from catastrophic firearm failures.
38 Special SAAMI vs C.I.P. Dimensions
Cartridge dimensions and safe pressure limits are governed by SAAMI in the United States and C.I.P. in Europe.
While the physical dimensional specifications for the 38 Special are functionally identical between the two bodies to ensure global compatibility, the way they measure and report pressure differs. SAAMI lists the maximum average pressure for standard 38 Special at 17,000 psi, measured via conformal piezoelectric transducers. C.I.P. lists the maximum pressure for the exact same cartridge at 1,500 bar, which translates to approximately 21,755 psi. These are not contradictory safety standards. They are the result of different testing methodologies, transducer placement locations, and historical pressure measurement conversions.
38 Special Bullet Weight And Dimensions
The 38 Special is widely loaded in several common bullet weights, which alter how the internal dimensions of the cartridge are configured.
- 110 to 125 grain: Lighter bullets commonly used in modern jacketed hollow point defensive ammunition. These shorter bullets leave ample case capacity and are typically loaded to +P pressures to achieve maximum velocity.
- 130 grain: The traditional standard weight for full metal jacket target ammunition, offering a balanced profile and low recoil.
- 158 grain: The historical standard weight for the caliber. These longer lead bullets take up more space inside the brass case and provide excellent momentum and penetration characteristics.
Bullet weight does not change the standardized external brass case dimensions. However, because a heavier bullet is physically longer, the manufacturer must adjust the seating depth to ensure the loaded round still fits inside the cylinder.
Why 38 Special Dimensions Matter
Understanding the specific 38 Special dimensions is essential for several practical reasons.
- Cylinder Binding: The strict 1.550 inch overall length defines whether a cartridge will function in a revolver. Handloaders who seat their bullets too shallow will lock up their firearm the moment the cylinder tries to rotate past the forcing cone.
- Crimping Consistency: Because a heavy roll crimp is required to prevent bullets from backing out under recoil, cases must be trimmed to uniform lengths. A long case will buckle in the crimping die, while a short case will not receive adequate crimp tension.
- Lever Action Compatibility: Many shooters pair their revolver with a lever action rifle chambered in 357 Magnum, which can also fire 38 Special ammunition. These rifles rely heavily on exact overall cartridge lengths to transfer rounds smoothly from the magazine tube to the chamber without jamming.
Manufacturer Specifications vs Independent Measurements
If you use calipers to measure factory 38 Special ammunition from companies like Federal, Winchester, or Hornady, the numbers will rarely match the SAAMI maximum specifications exactly.
SAAMI and C.I.P. publish maximum allowable dimensions to ensure absolute safety and compatibility across all firearm brands. Ammunition manufacturers purposefully produce their cases slightly smaller than these maximums. For example, measuring a factory cartridge and finding a case length of 1.145 inches or an OAL of 1.535 inches does not mean the ammunition is defective. It means the manufacturer built in a reliable negative tolerance margin to guarantee the ammunition will load easily into dirty match grade revolver cylinders.
Common Questions
What are the dimensions of the 38 Special?
The key maximum dimensions are a bullet diameter of 0.357 inches, a case length of 1.155 inches, and an overall cartridge length of 1.550 inches.
What is the 38 Special case length?
The maximum SAAMI case length is 1.155 inches (29.34 mm).
What is the 38 Special overall length?
The maximum overall length (OAL) is 1.550 inches (39.37 mm).
What is the 38 Special bullet diameter?
Modern 38 Special bullets measure 0.357 inches in diameter for jacketed projectiles and 0.358 inches for cast lead projectiles.
What is the 38 Special rim diameter?
The rim diameter measures 0.440 inches (11.18 mm).
What is the 38 Special case diameter?
The 38 Special is a straight walled cartridge, meaning the case diameter is uniformly 0.379 inches from the base to the neck.
How do 38 Special and 357 Magnum dimensions compare?
They share the exact same bullet diameter and base diameter, but the 357 Magnum is intentionally manufactured with a longer 1.290 inch case to prevent it from chambering in older 38 Special firearms.
Why is it called the 38 Special if the bullet is 0.357 inches?
The name is a historical holdover from its predecessor, the 38 Long Colt, which originally utilized a heeled bullet matching the outside diameter of the brass case.
What is the maximum overall length of the 38 Special?
The absolute maximum allowed by SAAMI is 1.550 inches to ensure the bullet does not bind against the front of a standard revolver cylinder.
What is the 38 Special maximum pressure?
The SAAMI maximum average pressure is 17,000 psi for standard loads and 20,000 psi for +P overpressure loads.