The 50 Beowulf (designated officially as the 12.7x42mm) is a heavy-hitting, large-bore rifle cartridge developed in 2001 by Bill Alexander of Alexander Arms.Designed specifically to maximize terminal ballistics and stopping power at short to medium ranges from the standard AR-15 platform, it uses a massive 500-thousandths-inch projectile.By utilizing a fat, slightly tapered straight-walled case with a rebated rim, the 50 Beowulf delivers devastating energy transfer for tactical applications, vehicle barrier penetration, and big-game hunting while fitting inside standard AR-15 upper receivers with only a barrel, bolt, and magazine swap.
50 Beowulf Case Dimensions Overview
| Dimension Property | Specification (Imperial) | Specification (Metric) |
| Parent Case | 50 Action Express (Lengthened) | 50 Action Express |
| Bullet Diameter | 500 thousandths in | 12 mm and 7 tenths |
| Case Length | 1 inch and 650 thousandths | 42 mm |
| Case Trim Length | 1 inch and 640 thousandths | 41 mm and 70 hundredths |
| Overall Length (COAL) | 2 inches and 125 thousandths (Max) | 54 mm |
| Rim Diameter | 447 thousandths in | 11 mm and 35 hundredths |
| Rim Thickness | 45 thousandths in | 1 mm and 14 hundredths |
| Base Diameter | 535 thousandths in | 13 mm and 59 hundredths |
| Neck Diameter | 525 thousandths in | 13 mm and 34 hundredths |
| Case Capacity | Approx 64 grains water | 4 cubic cm and 15 hundredths |
| Maximum Average Pressure | 33-thousand psi | 228 megapascals |
Understanding the 50 Beowulf Case Dimensions
- Parentage & Lineage:Developed from a lengthened 50 Action Express pistol case design, modified extensively to function reliably in a rifle platform;
- Rebated Rim Architecture:Features a rim diameter narrower than its case body, matching the footprint of the 6.5 Grendel to fit standard AR-15 bolt faces;
- Slightly Tapered Straight Wall:Unlike true straight-wall cases, it incorporates a subtle body taper to ensure smooth feeding and extraction from high-capacity rifle magazines;
- Heavy Projectile Focus:Engineered to fire massive bullets ranging from 200 grains up to 600 grains, with 335-grain to 400-grain loads being the most common choices.
50 Beowulf Case Dimensions in Inches and Millimeters
| Measurement Point | Max (Inches) | Min (Inches) | Max (Millimeters) | Min (Millimeters) |
| Case Length | 1 and 650 thousandths | 1 and 600 thousandths | 41 and 91 hundredths | 40 and 64 hundredths |
| Trim-To Length | 1 and 640 thousandths | None | 41 and 65 hundredths | None |
| Rim Diameter | 447 thousandths | 437 thousandths | 11 and 35 hundredths | 11 and 10 hundredths |
| Base Diameter | 535 thousandths | 530 thousandths | 13 and 59 hundredths | 13 and 46 hundredths |
| Neck Diameter | 525 thousandths | 520 thousandths | 13 and 34 hundredths | 13 and 21 hundredths |
| Bullet Diameter | 500 thousandths | 498 thousandths | 12 and 70 hundredths | 12 and 65 hundredths |
| Overall Length (COAL) | 2 and 125 thousandths | 2 and 000 thousandths | 54 mm | 50 and 80 hundredths |
50 Beowulf Case Length
The maximum case length of the 50 Beowulf is 1 inch and 650 thousandths (42 mm), with a recommended trim-to length of 1 inch and 640 thousandths (41 mm and 65 hundredths).Because the cartridge operates at a conservative maximum pressure limit of 33-thousand psi to protect AR-15 bolt lugs, brass stretching is typically minimal during normal firing cycles. Handloaders should still measure and trim cases periodically to maintain consistent crimp uniformity.
50 Beowulf Base and Rebated Rim Dimensions
The 50 Beowulf utilizes an unusual rebated rim design that allows a massive case body to work with standard AR-15 bolt dimensions:
- Rim Diameter:447 thousandths in (11 mm and 35 hundredths)
- Base Diameter: 535 thousandths in (13 mm and 59 hundredths)
- Rim Thickness: 45 thousandths in (1 mm and 14 hundredths)
This configuration enables standard rifle bolts to lock onto the smaller rim while enclosing a robust, high-capacity powder chamber.
50 Beowulf Neck and Body Taper Dimensions
- Body Taper: Features a gradual taper along its straight-walled body to prevent binding and ensure effortless extraction in semi-automatic actions;
- Neck Diameter: Measures 525 thousandths in at the case mouth to tightly grip heavy 50-caliber bullets;
- Headspacing: Headspaces on the case mouth against the forward chamber ledge, requiring strict adherence to overall case trim specifications.
50 Beowulf Case Capacity and Pressure
The internal case capacity of the 50 Beowulf averages approximately 64 grains of water, providing an abundant volume for fast-to-medium rifle propellants. This volume pairs exceptionally well with powders like Hodgdon Lil’Gun, IMR 4198, and Accurate 1680, driving heavy 335-grain bullets to muzzle velocities near 1-thousand 800 feet per second while remaining safely at or below 33-thousand psi.
50 Beowulf Dimensions vs Similar Cartridges
| Cartridge | Bullet Diameter | Case Length | COAL | Max Pressure |
| 50 Beowulf | 500 thousandths | 1 and 650 thousandths | 2 and 125 thousandths | 33-thousand psi |
| 458 SOCOM | 458 thousandths | 1 and 575 thousandths | 2 and 260 thousandths | 35-thousand psi |
| 450 Bushmaster | 452 thousandths | 1 and 700 thousandths | 2 and 260 thousandths | 38-thousand 500 psi |
| 50 Action Express | 500 thousandths | 1 and 286 thousandths | 1 and 610 thousandths | 36-thousand psi |
| 45-70 Government | 458 thousandths | 2 and 105 thousandths | 2 and 550 thousandths | 28-thousand psi |
Chamber and Case Dimensions
Chamber specifications for the 50 Beowulf require specialized barrel extensions and heavy-duty bolts to handle the outward thrust generated by its extra-wide 535-thousandths-inch base diameter. Because “50 Beowulf” is a registered trademark of Alexander Arms, independent ammunition and barrel makers frequently produce identical dimensions under the metric designation 12.7x42mm.
Common Questions
- Can I form 50 Beowulf brass from 50 Action Express cases?No! Although the Beowulf traces its conceptual roots to the 50 Action Express, the Beowulf case is significantly longer (1 inch and 650 thousandths versus 1 inch and 286 thousandths for the 50 AE), meaning direct case conversion is impossible and factory-formed brass must be used.
- What is the standard twist rate for 50 Beowulf dimensions?A 1 in 20 twist rate is standard for factory barrels, optimized to stabilize heavy bullet weights ranging from 300 grains up to 500 grains.
- Does the 50 Beowulf require special magazines?While it fits into standard AR-15 magazine bodies, it feeds in a single-stack layout. Standard 5-56 millimeter magazines often require modified followers or dedicated single-stack Beowulf magazines to prevent feeding jams.