The exact .357 Magnum bullet diameter is 0.357 inches, which converts to 9.07 millimeters. While .357 Magnum ammunition is available with different bullet weights, the cartridge’s nominal projectile diameter remains exactly the same across all factory loads. This measurement refers specifically to the width of the bullet itself, which differs from the internal dimensions of the barrel. Specifically, a standard .357 Magnum utilizes a narrower bore diameter of 0.346 inches and a groove diameter of 0.355 inches to ensure a tight gas seal upon firing.
357 Magnum Bullet Diameter
| Specification | .357 Magnum |
| Nominal Bullet Diameter | 0.357 inches |
| Bullet Diameter In Inches | 0.357″ |
| Bullet Diameter In Millimeters | 9.07 mm |
| Groove Diameter | 0.355 inches |
| Bore Diameter | 0.346 inches |
| Typical Bullet Weights | 110 gr to 180 gr |
| Case Length | 1.290 inches |
| Overall Cartridge Length | 1.590 inches |
| Maximum Pressure | 35,000 PSI |
What Is The 357 Magnum Bullet Diameter?
The 357 Magnum bullet diameter is the physical width of the unfired projectile seated in the cartridge case. Introduced in 1935 by Smith & Wesson and Winchester, the cartridge was designed to fire a .357-caliber bullet at much higher velocities than earlier revolver rounds to penetrate the early ballistic vests used by bootleggers and gangsters. The bullet must be wide enough to securely engage the barrel’s rifling while maintaining a precise fit within the cylinder chambers of the revolver.
357 Magnum Bullet Diameter In Inches
According to the official Sporting Arms and Ammunition Manufacturers’ Institute (SAAMI) specifications, the 357 Magnum bullet diameter in inches is standard at 0.357 inches. While manufacturing tolerances allow for very slight variations depending on whether the bullet is a jacketed hollow point or a cast lead projectile, 0.357″ is the targeted nominal measurement for nearly all standard factory ammunition.
357 Magnum Bullet Diameter In Millimeters
For those using metric measurements, the 357 Magnum bullet diameter in mm is 9.07 mm. Because 1 inch equals 25.4 millimeters, multiplying a 0.357-inch bullet by 25.4 yields exactly 9.0678 mm, which naturally rounds to 9.07 mm.
357 Magnum Bullet vs Bore Diameter
A common point of confusion is the difference between the bullet’s width and the barrel’s internal dimensions. The 357 Magnum bore diameter is 0.346 inches. The bore diameter measures the distance directly across the “lands” (the raised portions of the rifling inside the barrel). The unfired 357 Magnum projectile diameter (0.357″) is deliberately wider than the bore so the rifling can aggressively bite into the bullet jacket, imparting the stabilizing spin required for accuracy.
357 Magnum Groove Diameter
The 357 Magnum groove diameter measures 0.355 inches. The groove diameter measures the distance across the deepest cuts inside the rifled barrel. When the 0.357-inch bullet is fired, it swages (squeezes) slightly to completely fill the 0.355-inch grooves. This creates a perfectly tight pressure seal that traps the high-pressure powder gases directly behind the bullet, preventing velocity loss.
357 Magnum Bullet Weights
While the cartridge diameter is fixed, you can select vastly different projectile masses depending on your application. The 357 Magnum bullet weight ranges from lightweight 110-grain defensive rounds to heavy 180-grain hard cast lead rounds used for hunting deep-woods game. Changing the weight alters the bullet’s length, energy, and velocity, but the diameter remains completely fixed at 0.357 inches.
Bullet Diameter vs Other Cartridges
Shooters must be careful when comparing caliber names, as the marketed name does not always reflect the exact physical measurement. Here is how the 357 Magnum bullet size compares to other popular rounds:
| Cartridge | Bullet Diameter | Diameter In mm |
| .357 Magnum | 0.357″ | 9.07 mm |
| .38 Special | 0.357″ | 9.07 mm |
| .350 Legend | 0.357″ | 9.07 mm |
| .357 SIG | 0.355″ | 9.02 mm |
| 9mm Luger | 0.355″ | 9.02 mm |
| .380 ACP | 0.355″ | 9.02 mm |
Notice that the modern .357 SIG actually fires a narrower 0.355-inch bullet (the exact same diameter as a standard 9mm), whereas the older .38 Special fires a 0.357-inch bullet, despite being called a “.38.” Never assume a bullet’s true diameter based purely on the numerical name stamped on the box.
Bullet Weight
Ammunition manufacturers have developed multiple specialized loads for the .357 Magnum over its near-century of service, with the 125-grain and 158-grain being the most iconic.
| Bullet Weight | Example Load | Bullet Type | Advertised Velocity |
| 110 grain | Hornady Critical Defense | FTX | 1,500 fps |
| 125 grain | Federal Premium | JHP | 1,450 fps |
| 135 grain | Speer Gold Dot | JHP | 1,240 fps |
| 158 grain | Winchester Super-X | Jacketed Soft Point | 1,235 fps |
| 180 grain | Buffalo Bore | Hard Cast Lead | 1,400 fps |
Velocities represent typical factory test barrel data, generally from a 4-inch vented barrel.
Dimensions
To understand how the cartridge safely fits into the cylinder of a modern revolver, here are the complete SAAMI-verified 357 Magnum dimensions:
| Dimension | Measurement |
| Bullet Diameter | 0.357″ |
| Groove Diameter | 0.355″ |
| Bore Diameter | 0.346″ |
| Case Length | 1.290″ |
| Overall Length | 1.590″ |
| Base Diameter | 0.379″ |
| Neck Diameter | 0.379″ |
| Rim Diameter | 0.440″ |
| Maximum Pressure (SAAMI) | 35,000 PSI |
357 Magnum vs 38 Special
Because both cartridges share an exact 0.357-inch bullet diameter and were historically developed together, many shooters wonder how the 357 Magnum vs 38 Special compare.
Despite sharing a bullet diameter, they perform very differently due to their dimensions and internal pressures:
- Case Length:The .357 Magnum case (1.290 inches) was intentionally designed to be approximately 0.135 inches longer than the .38 Special case (1.155 inches).
- Safety: This extra length is a critical safety feature. It physically prevents a high-pressure .357 Magnum cartridge from chambering inside an older firearm designed only for the lower-pressure .38 Special.
- Pressure and Velocity:The .357 Magnum operates at a maximum average pressure of 35,000 PSI, while the standard .38 Special is limited to just 17,000 PSI. This allows the Magnum to push the exact same bullet significantly faster, resulting in much higher muzzle energy.
While you cannot safely put a .357 Magnum into a .38 Special firearm, you can safely fire .38 Special ammunition through a .357 Magnum revolver.
357 Magnum Bullet Diameter: Exact Measurement
The exact .357 Magnum bullet diameter is 0.357 inches, or 9.07 millimeters.
While the actual projectile measures 0.357 inches across, it is forcefully driven through a revolver barrel with a 0.346-inch bore diameter and a 0.355-inch groove diameter. This intentional sizing discrepancy forces the copper jacket or lead casting of the bullet to squeeze tightly into the barrel’s rifling grooves. This physical engagement creates a perfect pressure seal behind the bullet, preventing the 35,000 PSI of expanding gases from escaping past the projectile while simultaneously ensuring the bullet stabilizes in flight.
FAQs
What is the bullet diameter of 357 Magnum?
The nominal bullet diameter is 0.357 inches.
What is 357 Magnum bullet diameter in mm?
In metric measurements, the diameter translates to 9.07 mm.
Is 357 Magnum actually .357 inches?
Yes, it is true to its name. The actual projectile measures 0.357 inches across.
Is 357 Magnum the same diameter as 38 Special?
Yes. Both cartridges use the exact same 0.357-inch diameter bullet, but the .357 Magnum utilizes a longer case and operates at much higher pressures.
What is the bore diameter of 357 Magnum?
The barrel’s internal bore diameter (measured land-to-land) is 0.346 inches.
What is the groove diameter of 357 Magnum?
The barrel’s internal groove diameter (measured groove-to-groove) is 0.355 inches.
What bullet weights are available for 357 Magnum?
Factory loads typically range from lightweight 110-grain defensive rounds to heavy 180-grain hunting rounds, with 125-grain and 158-grain being the most common.
What is the case diameter of 357 Magnum?
The case is completely straight-walled. It maintains a diameter of 0.379 inches from the base all the way up to the neck.
How does 357 Magnum bullet diameter compare with 9mm?
A standard 9mm Luger bullet measures 0.355 inches, making the .357 Magnum (0.357 inches) slightly wider.
Is 357 Magnum the same as 357 SIG?
No. They are completely different cartridges for different firearm platforms. The .357 Magnum is a rimmed revolver cartridge that fires a 0.357-inch bullet. The .357 SIG is a bottlenecked semi-automatic pistol cartridge that fires a narrower 0.355-inch bullet.