The .300 Winchester Short Magnum is a high performance short action centerfire rifle cartridge featuring a maximum case length of 2.100 inches (53.34 mm) and a maximum overall cartridge length of 2.860 inches (72.64 mm). Based on official SAAMI standards, the case features a 0.535 inch rim diameter, a 0.555 inch rebated base diameter, a 0.538 inch shoulder diameter, and a maximum neck diameter of 0.344 inches. Published specifications can vary slightly depending on whether they represent maximum material conditions, nominal dimensions, or specific manufacturer tolerances across different standardization bodies.
300 WSM Case Dimensions
| Case Dimension | Inches | Millimeters | Specification Type | Source | Notes |
| Case Length | 2.100 | 53.34 | Maximum | SAAMI | Standard trim to length is 2.090 in. |
| Rim Diameter | 0.535 | 13.59 | Maximum | SAAMI | Rebated rim design. |
| Rim Thickness | 0.054 | 1.37 | Maximum | SAAMI | Controls bolt face engagement. |
| Base Diameter | 0.555 | 14.10 | Maximum | SAAMI | Measured just above the extractor groove. |
| Shoulder Diameter | 0.538 | 13.67 | Maximum | SAAMI | Body to shoulder intersection. |
| Neck Diameter | 0.344 | 8.74 | Maximum | SAAMI | Outside case neck diameter. |
| Shoulder Angle | 35° | N/A | Half Angle | SAAMI | Steep shoulder promotes powder burn efficiency. |
| Overall Length | 2.860 | 72.64 | Maximum | SAAMI | Allows feeding from short action magazines. |
| Case Capacity | ~80.0 | 5.18 cm³ | Nominal (H2O) | Industry Std. | Varies by brass manufacturer and wall thickness. |
| Max Pressure | 65,000 | 448.16 MPa | Max Average | SAAMI (Piezo) | C.I.P. specifies 63,817 psi (440 MPa). |
Understanding The .300 WSM Case Dimensions
To understand how the .300 WSM interacts with a rifle chamber, bolt, and magazine, you must analyze its discrete geometric components. Each dimension serves a specific engineering purpose.
- Case Length: Measured from the base of the rim to the case mouth, case length determines how far the brass extends into the chamber before contacting the end of the neck clearance.
- Rim Diameter and Thickness: The rim provides the gripping surface for the rifle extractor. Because the .300 WSM uses a rebated rim, the rim is slightly narrower than the main case body, allowing it to fit standard magnum bolt faces.
- Base Diameter: This is the widest part of the main cartridge body, situated just ahead of the extractor groove. The unusually wide base gives the cartridge its magnum powder capacity without requiring a long action.
- Shoulder Diameter and Geometry: The shoulder acts as the physical stop that prevents the cartridge from pushing too far into the barrel. Its diameter and steep 35 degree angle dictate chamber fit and headspacing.
- Neck Diameter: The neck holds the bullet. The outside diameter must clear the chamber walls to allow the brass to expand and release the projectile safely upon firing.
- Overall Cartridge Length (OAL): The measurement from the case base to the tip of the seated bullet. It dictates magazine fit and whether the bullet will prematurely engage the rifling lands.
- Case Capacity: The internal volume of the case, generally expressed in grains of water (gr H2O). This dictates the maximum powder charge the cartridge can safely hold.
.300 WSM Case Dimensions In Inches And Millimeters
Official standardization bodies use different base measurement systems. SAAMI specifications rely on inches, whereas European C.I.P. standards utilize metric parameters.
| Dimension | Measurement (Inches) | Measurement (Millimeters) |
| Maximum Case Length | 2.100 in | 53.34 mm |
| Maximum Rim Diameter | 0.535 in | 13.59 mm |
| Maximum Base Diameter | 0.555 in | 14.10 mm |
| Maximum Shoulder Diameter | 0.538 in | 13.67 mm |
| Maximum Neck Diameter | 0.344 in | 8.74 mm |
| Maximum Overall Length | 2.860 in | 72.64 mm |
.300 WSM Case Length
The published maximum case length for the .300 WSM is 2.100 inches (53.34 mm). However, operating precisely at this maximum can cause safety issues if the case stretches during firing and pinches inside the chamber throat, which drastically spikes chamber pressures.
Because brass flows forward under the extreme pressures of firing and resizing, a fired and full length resized case will frequently measure longer than a factory new case. Therefore, handloaders universally utilize a trim length of 2.090 inches. This nominal case length provides a 0.010 inch buffer, ensuring consistent chamber fit, uniform crimping, and safe bullet release.
.300 WSM Base And Rim Dimensions
The .300 WSM features a rebated rim, bottleneck design. Its base diameter measures a massive 0.555 inches, but the rim diameter is rebated (reduced) down to 0.535 inches.
This 0.535 inch rim diameter allows the cartridge to utilize the same bolt face diameter as traditional belted magnums like the .300 Winchester Magnum or 7mm Remington Magnum. The 0.555 inch base diameter provides a massive internal powder column while keeping the cartridge short enough to fit inside a standard 2.860 inch short action magazine. The rim thickness (0.054 inches) determines how well the bolt extractor claw can grip the cartridge during extraction.
.300 WSM Shoulder And Neck Dimensions
Above the straight taper of the body, the case steps down to grip a .308 caliber bullet. The shoulder diameter at the intersection of the case body is 0.538 inches. From there, the brass angles inward at a very sharp 35 degree half angle to meet the neck.
This steep 35 degree shoulder is a hallmark of the short magnum design. It promotes efficient powder burning by keeping the powder column short and wide. The neck diameter reaches a maximum allowable SAAMI specification of 0.344 inches. To safely release a 0.308 inch bullet, the chamber neck must be slightly wider than the cartridge neck.
.300 WSM Case Capacity
Case capacity heavily influences internal ballistics and pressure generation. The nominal industry accepted capacity for the .300 WSM is approximately 80.0 grains of water, or roughly 5.18 cubic centimeters.
However, case capacity is not a static specification. It is a physical measurement that fluctuates between brass manufacturers. Because external dimensions are strictly controlled by chamber sizes, any variation in the thickness of the brass walls directly alters the internal volume. Brands known for heavy duty brass often feature thicker web structures, which reduces case capacity. A reduction in case capacity causes standard powder charges to produce higher pressures, which is why load data must be adjusted when switching brass brands.
.300 WSM Case Dimensions vs .300 Winchester Magnum
The .300 WSM was designed to mimic the ballistic performance of the legendary .300 Winchester Magnum while fitting into a lighter, shorter rifle action.
| Dimension | .300 WSM | .300 Winchester Magnum | Source |
| Case Length | 2.100 in | 2.620 in | SAAMI |
| Rim Diameter | 0.535 in (Rebated) | 0.532 in (Belted) | SAAMI |
| Base Diameter | 0.555 in | 0.513 in (Above Belt) | SAAMI |
| Shoulder Angle | 35° | 25° | SAAMI |
| Overall Length | 2.860 in | 3.340 in | SAAMI |
| Case Capacity | ~80.0 gr H2O | ~90.0 to 94.0 gr H2O | Standard Data |
The .300 Winchester Magnum uses a traditional belted case that requires a standard long action (3.340 inch overall length). The .300 WSM achieves similar velocities by utilizing a much wider 0.555 inch base and a steeper 35 degree shoulder, allowing it to pack magnum power into a 2.860 inch short action footprint. The .300 WSM also headspaces entirely on its shoulder, whereas the .300 Winchester Magnum originally headspaces on its belt.
.300 WSM vs .270 WSM and 7mm WSM Case Dimensions
The .300 WSM is the parent case for the rest of the Winchester Short Magnum family.
- .270 WSM: Shares the exact same 2.100 inch case length, 0.555 inch base, and 35 degree shoulder as the .300 WSM. It is simply necked down to accept a .277 caliber bullet.
- 7mm WSM: Unlike the .270 WSM, the 7mm WSM does not share the exact same case dimensions. Winchester pushed the shoulder of the 7mm WSM forward slightly, resulting in a slightly longer case length of 2.100 inches but a different distance to the shoulder datum. This was intentionally done to prevent a 7mm WSM cartridge from being accidentally chambered in a .270 WSM rifle, which could cause a catastrophic failure.
.300 WSM Chamber And Case Dimensions
A rifle chamber must be fractionally larger than the cartridge it accepts, otherwise the round would not feed. The relationship between .300 WSM case dimensions and chamber dimensions centers heavily on headspacing.
Headspace for the .300 WSM is measured from the bolt face to a specific datum line on the 35 degree case shoulder. When the firing pin strikes, the case expands to seal the chamber. If the case dimensions are too small relative to the chamber, the brass will stretch aggressively to fill the void, thinning the material near the base. If the case dimensions are too large, the bolt will not close. Proper cartridge dimensions ensure safe alignment, efficient extraction, and reliable bullet entry into the rifling.
.300 WSM Manufacturer Specifications vs Actual Measurements
There is a documented difference between absolute standards (SAAMI/C.I.P.) and the dimensions of a physical cartridge sitting on a reloading bench.
SAAMI lists maximum material conditions. Ammunition manufacturers intentionally produce brass slightly smaller than these maximums to guarantee their cartridges will chamber in any rifle, regardless of how tightly the manufacturer cut the chamber. A fired .300 WSM case expands to the exact dimensions of the specific rifle chamber it was fired in. When a handloader runs this fired brass through a full length sizing die, the machine compresses the base, shoulder, and neck back down to factory nominal tolerances, allowing it to chamber easily once again.
Common Questions
What are the .300 WSM case dimensions?
The key maximum case dimensions for the .300 WSM include a 2.100 inch case length, a 0.535 inch rebated rim diameter, a 0.555 inch base diameter, a 0.538 inch shoulder diameter, and a 0.344 inch neck diameter.
What is the .300 WSM case length?
The maximum SAAMI case length for the .300 WSM is 2.100 inches (53.34 mm). The standard trim to length for reloading is 2.090 inches.
What is the .300 WSM rim diameter?
The .300 WSM features a rebated rim with a maximum diameter of 0.535 inches (13.59 mm).
What is the .300 WSM base diameter?
The base diameter of the .300 WSM, measured just above the extractor groove, is 0.555 inches (14.10 mm).
What is the .300 WSM shoulder diameter?
The maximum shoulder diameter of the .300 WSM is 0.538 inches (13.67 mm).
What is the .300 WSM neck diameter?
The maximum outside neck diameter of the .300 WSM is 0.344 inches (8.74 mm).
What is the .300 WSM case capacity?
The nominal case capacity of the .300 WSM is roughly 80.0 grains of water, or 5.18 cubic centimeters. Actual capacity varies depending on the brass manufacturer and wall thickness.
What is the .300 WSM overall length?
The maximum overall cartridge length for the .300 WSM is 2.860 inches (72.64 mm) under SAAMI specifications.
How does .300 WSM compare with .300 Winchester Magnum case dimensions?
The .300 WSM is an unbelted short action cartridge with a 2.100 inch case length and a 0.555 inch base.The .300 Winchester Magnum is a belted long action cartridge with a 2.620 inch case length and a 0.513 inch base above the belt. Both fire the same .308 caliber bullets.
What is the .300 WSM maximum pressure?
The SAAMI maximum average pressure for the .300 WSM is 65,000 psi (piezo method). Under C.I.P. testing methods, the maximum pressure is rated at 63,817 psi (440 MPa).