The 338 ARC (Advanced Rifle Cartridge) is a modern AR-15 optimized centerfire rifle cartridge featuring a maximum case length of 1.490 inches (37.85 mm) and a maximum overall cartridge length of 2.260 inches (57.40 mm).Based on official SAAMI standards accepted in 2025, the case features a 0.441 inch rim diameter, a 0.441 inch base diameter, an approximate 0.420 inch shoulder diameter, and a maximum neck diameter of 0.365 inches.These precise dimensions allow the short, fat cartridge to safely feed heavy 338-caliber projectiles from standard AR-15 magazines. Published specifications can vary slightly depending on whether they represent maximum material conditions, nominal dimensions, or specific manufacturer tolerances across different standardization bodies.
338 ARC Case Dimensions
| Case Dimension | Inches | Millimeters | Specification Type | Source | Notes |
| Case Length | 1.490 | 37.85 | Maximum | SAAMI | Derived from the 6.5 Grendel parent case family. |
| Rim Diameter | 0.441 | 11.20 | Maximum | SAAMI | Utilizes a Type II 6.5 Grendel bolt face. |
| Rim Thickness | 0.059 | 1.50 | Maximum | Industry Std. | Controls extractor engagement on the bolt. |
| Base Diameter | 0.441 | 11.20 | Maximum | SAAMI | Measured just above the extractor groove. |
| Shoulder Diameter | 0.420 | 10.67 | Nominal | Industry Std. | Body to shoulder intersection. |
| Neck Diameter | 0.365 | 9.27 | Maximum | SAAMI | Outside case neck diameter. |
| Shoulder Angle | 30° | N/A | Half Angle | SAAMI | Standard Hornady ARC family geometry. |
| Overall Length | 2.260 | 57.40 | Maximum | SAAMI | Designed to fit standard AR-15 magazines. |
| Case Capacity | ~35.0 | 2.27 cm³ | Nominal (H2O) | Industry Std. | Varies by brass manufacturer and wall thickness. |
| Maximum Pressure | 52,000 | 358.53 MPa | Max Average | SAAMI (Piezo) | Established operating limit for AR-15 platforms. |
Understanding The 338 ARC Case Dimensions
To understand how the 338 ARC interacts with a rifle chamber, bolt, and magazine, you must analyze its discrete geometric components. Each dimension serves a specific engineering purpose to balance heavy projectile mass within a compact platform.
- 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. The 0.441 inch diameter ensures the cartridge centers securely on a Type II 6.5 Grendel bolt face.
- Base Diameter: This is the widest part of the main cartridge body, situated just ahead of the extractor groove. It governs how the cartridge seats into the rear of the chamber and stack within a magazine.
- 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 30 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 heavy 338-caliber 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 determines if the cartridge will fit inside a standard AR-15 lower receiver without modification.
- 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 for both subsonic and supersonic loads.
338 ARC Case Dimensions In Inches And Millimeters
Official SAAMI standardization utilizes inches, but metric parameters are frequently referenced by international shooters and brass manufacturers.
| Dimension | Measurement (Inches) | Measurement (Millimeters) |
| Maximum Case Length | 1.490 in | 37.85 mm |
| Maximum Rim Diameter | 0.441 in | 11.20 mm |
| Maximum Base Diameter | 0.441 in | 11.20 mm |
| Maximum Shoulder Diameter | 0.420 in | 10.67 mm |
| Maximum Neck Diameter | 0.365 in | 9.27 mm |
| Maximum Overall Length | 2.260 in | 57.40 mm |
338 ARC Case Length
The published maximum case length for the 338 ARC is 1.490 inches (37.85 mm).This incredibly short case length is required to leave enough physical room in the magazine for the massive 338-caliber bullets, some of which weigh up to 307 grains and make up nearly half the overall length of the loaded cartridge.
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 standard trim length slightly below the maximum. This nominal case length provides a buffer, ensuring consistent chamber fit, uniform crimping, and safe bullet release without pinching the case mouth into the rifling.
338 ARC Base And Rim Dimensions
The 338 ARC features a rimless, bottleneck design. Its base diameter measures 0.441 inches, and the rim diameter matches identically at 0.441 inches.
This 0.441 inch rim diameter is inherited from the 6.5 Grendel, which traces its roots back to the Russian 7.62x39mm and the PPC family of benchrest cartridges.It dictates that the 338 ARC requires a specialized Type II Grendel bolt face with a 0.136 inch bolt face depth.A standard 5.56 NATO bolt will not work, as its 0.378 inch bolt face is far too small to accept the 338 ARC rim.
338 ARC Shoulder And Neck Dimensions
Above the straight taper of the body, the case steps down to grip a large 0.338 inch bullet. The shoulder diameter at the intersection of the case body is approximately 0.420 inches. From there, the brass angles inward at a steep 30 degree half angle to meet the neck.
The neck diameter reaches a maximum allowable SAAMI specification of 0.365 inches. To safely release a 0.338 inch bullet, the chamber neck must be slightly wider than the cartridge neck. A loaded cartridge typically measures around 0.363 inches at the neck depending on the brass wall thickness. The 30 degree shoulder prevents the short, fat column of powder from burning inefficiently while providing a positive headspace datum.
338 ARC Case Capacity
Case capacity heavily influences internal ballistics, powder selection, and pressure generation. The nominal, industry accepted capacity for the 338 ARC family is approximately 35.0 grains of water, or roughly 2.27 cubic centimeters.
This restricted powder volume is an intentional design choice. Large capacity cases struggle to reliably ignite small pinches of powder used for subsonic ammunition. By keeping the case capacity small and the bullet heavy, the 338 ARC fills the case efficiently with fast burning powders, creating highly consistent subsonic velocities and low standard deviations for suppressed shooting.
338 ARC Case Dimensions vs 300 Blackout
The 300 Blackout is the primary competitor to the 338 ARC in the realm of suppressed AR-15 platforms.
| Dimension | 338 ARC | 300 Blackout | Source |
| Case Length | 1.490 in | 1.368 in | SAAMI |
| Rim Diameter | 0.441 in | 0.378 in | SAAMI |
| Base Diameter | 0.441 in | 0.376 in | SAAMI |
| Bullet Diameter | 0.338 in | 0.308 in | SAAMI |
| Overall Length | 2.260 in | 2.260 in | SAAMI |
| Typical Subsonic Weight | 307 gr | 220 gr | Factory Data |
Both cartridges share the exact same 2.260 inch maximum overall length, allowing them to fit standard AR-15 lowers. However, the 338 ARC uses a much wider 0.441 inch base compared to the 0.376 inch base of the 300 Blackout. The 300 Blackout uses a standard 5.56 bolt, while the 338 ARC requires a Grendel Type II bolt.Because the 338 ARC can accommodate bullets nearly 90 grains heavier, it delivers approximately 1.5 times the muzzle energy of a subsonic 300 Blackout round.
338 ARC Case Dimensions vs Similar Cartridges
The 338 ARC exists in a growing family of specialized cartridges designed to maximize the ballistic potential of the AR-15 and compact platforms.
| Cartridge | Case Length | Base Diameter | Shoulder Angle | Neck Diameter | Overall Length |
| 338 ARC | 1.490 in | 0.441 in | 30° | 0.365 in | 2.260 in |
| 6mm ARC | 1.490 in | 0.441 in | 30° | 0.276 in | 2.260 in |
| 22 ARC | 1.525 in | 0.441 in | 30° | 0.256 in | 2.260 in |
| 6.5 Grendel | 1.520 in | 0.441 in | 30° | 0.293 in | 2.260 in |
| 8.6 Blackout | 1.850 in | 0.473 in | 30° | 0.365 in | 2.800 in |
338 ARC vs 6.5 Grendel Case Dimensions
The 338 ARC is a direct descendant of the 6.5 Grendel parent case.
They share identical base diameters (0.441 inches), rim diameters (0.441 inches), and the signature 30 degree shoulder angle. The primary dimensional differences reside in the neck and case length. The 338 ARC features a neck diameter blown out to 0.365 inches to hold a 0.338 caliber bullet, whereas the 6.5 Grendel features a narrower neck diameter for a 0.264 caliber bullet.To make room for the incredibly long 338 caliber subsonic projectiles within the confines of an AR-15 magazine, Hornady shortened the 338 ARC case length to 1.490 inches compared to the Grendel’s 1.520 inches.
338 ARC vs 8.6 Blackout Case Dimensions
While both the 338 ARC and the 8.6 Blackout fire heavy 338 caliber bullets designed for subsonic suppression, they operate on completely different host platforms.
The 8.6 Blackout uses a shortened 6.5 Creedmoor parent case with a 0.473 inch base and a 1.850 inch case length. Its larger dimensions and 2.800 inch overall length require an AR-10 sized large frame rifle. The 338 ARC was engineered specifically to bring 338 caliber subsonic performance to the much smaller, lighter AR-15 platform by utilizing the compact 0.441 inch Grendel case head and a 2.260 inch overall length.
338 ARC Chamber And Case Dimensions
A rifle chamber must be fractionally larger than the cartridge it accepts. The relationship between 338 ARC case dimensions and chamber dimensions centers heavily on headspacing.
Headspace for the 338 ARC is measured from the bolt face to a specific datum line on the 30 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. Proper cartridge dimensions ensure safe alignment, efficient extraction, and reliable bullet entry into the fast 1:8 twist rifling.
338 ARC Case Forming And Resizing
For reloaders looking to source brass, the 338 ARC can theoretically be formed from high quality 6.5 Grendel brass.
This process involves forcing the Grendel brass through a specialized sizing die that expands the neck diameter up to 0.365 inches. However, because the 6.5 Grendel case is slightly longer at 1.520 inches, the brass must be carefully trimmed down to the 338 ARC maximum length of 1.490 inches. Neck turning may also be required to ensure the brass walls do not become too thick to safely chamber after the aggressive expansion process.
338 ARC Manufacturer Specifications vs Actual Measurements
There is a documented difference between absolute SAAMI standards 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 338 ARC 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.
Common Questions
What are the 338 ARC case dimensions?
The key maximum case dimensions for the 338 ARC include a 1.490 inch case length, a 0.441 inch rim diameter, a 0.441 inch base diameter, a 0.420 inch shoulder diameter, and a 0.365 inch neck diameter.
What is the 338 ARC case length?
The maximum SAAMI case length for the 338 ARC is 1.490 inches (37.85 mm).
What is the 338 ARC rim diameter?
The 338 ARC rim diameter is 0.441 inches (11.20 mm), requiring a Type II 6.5 Grendel bolt face.
What is the 338 ARC base diameter?
The base diameter of the 338 ARC, measured just above the extractor groove, is 0.441 inches (11.20 mm).
What is the 338 ARC shoulder diameter?
The nominal shoulder diameter of the 338 ARC is approximately 0.420 inches (10.67 mm).
What is the 338 ARC neck diameter?
The maximum outside neck diameter of the 338 ARC is 0.365 inches (9.27 mm).
What is the 338 ARC case capacity?
The nominal case capacity of the 338 ARC is roughly 35.0 grains of water, or 2.27 cubic centimeters. Actual capacity varies depending on the brass manufacturer wall thickness.
What is the 338 ARC overall length?
The maximum overall cartridge length for the 338 ARC is 2.260 inches (57.40 mm) under SAAMI specifications.
How does 338 ARC compare with 300 Blackout case dimensions?
Both cartridges share a 2.260 inch overall length. The 338 ARC uses a wider 0.441 inch base and a slightly longer 1.490 inch case compared to the 300 Blackout 0.376 inch base and 1.368 inch case. The 338 ARC is designed to push much heavier 338 caliber bullets.
How does 338 ARC compare with 6.5 Grendel case dimensions?
Both cartridges share the exact same 0.441 inch base, rim diameter, and 30 degree shoulder angle.The 338 ARC simply features a wider neck for a 338 caliber bullet and a slightly shorter 1.490 inch case length to accommodate the long projectiles within AR-15 magazines.
What is the 338 ARC maximum pressure?
The SAAMI maximum average pressure for the 338 ARC is 52,000 psi (piezo method), optimized for safe and reliable operation in gas impingement AR-15 rifles.