The 12.7 x 42 mm cartridge and the 50 Beowulf cartridge often show up together when researching ammunition, but are they really the same?

In a word, yes: the 12.7 x 42 mm and the 50 Beowulf are the exact same cartridge — but which one came first, and why is there a distinction in the first place? These are the more interesting questions.

Any time we talk about the 12.7 x 42 mm, we’re really talking about the 50 Beowulf. That’s partly because it was the original name, and partly because it sounds a lot more badass than the alternative.

Proving yet again the versatility of the Armalite Rifle platform, the .50 Beowulf round is an unstoppable beast of a cartridge designed for the AR-15. It’s named after a character from the oldest literary work written in the English language: Beowulf. It’s a tale about a 6th century Swedish hero who saves his country by slaying a giant monster named Grendel (ironically, the name of another badass cartridge) who is a descendent of Cain from the Bible.

Beowulf could arguably be called the English world's first superhero, so the 50 Beowulf cartridge is aptly named. It provides shooters with a truly heroic level of performance combined with absolute reliability. This is of paramount importance in a life or death scenario.

Let’s look at the history behind this cartridge, its performance, and what in the world 12.7 x 42 mm has to do with it.

The .50 Beowulf vs. the 12.7 x 42mm: What’s the Difference?

Developed by Bill Alexander (a British expat and founder of Alexander Arms), the .50 Beowulf is a proprietary large bore rifle cartridge specifically designed to provide unprecedented stopping power at close ranges. However, there seems to be some confusion among shooters about the designation of this cartridge. Complete rifles and upper receiver assemblies chambered for this cartridge (also manufactured by Alexander Arms) are designated as “.50 Beowulf.” On the other hand, rifles and upper receiver assemblies manufactured by other companies are designated 12.7 x 42mm. However, they still work with the same ammunition. So what’s the real difference? Why are there two different names?

The definitive answer: trademark laws.

Alexander Arms is the owner of the trademark name “.50 Beowulf.” Because it is a wildcat cartridge (with no established SAAMI dimensions or chamber pressure standards), other companies who also produce AR-15 rifles chambered for this cartridge have been forced to reverse engineer the cartridge and market their rifles using the designation 12.7 x 42mm instead of .50 Beowulf.

That’s it. There is no dimensional difference between the .50 Beowulf cartridge and the 12.7 x 42mm cartridge. The bullet diameter is 0.50 inches (12.7 mm) with a case length of 1.65 inches (42 mm). The distinction is just a matter of nomenclature.

While many vendors could likely obtain the rights to call their ammo products in this caliber “.50 Beowulf,” it’s a lot easier (and cheaper) to avoid legal hassles and just use the generic term.

What Is the Purpose of the .50 Beowulf?

With all that in mind, the .50 Beowulf is still a highly unusual rifle cartridge. However, it was specifically designed to fit in a standard size AR-15 magazine, whereas the .308 Winchester requires the much larger and heavier AR-10 rifle.

Being that the AR-15 has become the most popular sporting rifle in the United States, the .50 Beowulf’s rise in popularity among shooters seems only natural. That said, there is much more to love about this cartridge one you see it in action.

Stopping Power

The .50 Beowulf was originally developed to provide military and law enforcement personnel with an unprecedented level of stopping power at close to medium ranges. However, this same characteristic has also proven to be popular with hunters who choose to hunt dangerous game species at close ranges or in dense foliage. It has also become helpful for wilderness guides who need the ability to stop dangerous game in their tracks.

Penetration

However, it’s small size and superb stopping power aren’t it's only great qualities. The .50 Beowulf provides a level of performance that far surpasses 5.56 NATO or 7.62 x 51mm NATO rounds at close ranges. It even rivals the famed .50 Browning Machine Gun (aka .50 BMG) round. In fact, the .50 Beowulf delivers so much kinetic energy to soft targets that body armor is rendered useless.

When used against antagonists hiding behind cover, the .50 Beowulf can easily punch through cinder block or brick walls (as well as the assailant themselves). Beyond that, the Beowulf can penetrate laminated auto glass windshields and engine blocks of motor vehicles without deflecting. In a marine environment, carbines chambered for the .50 Beowulf make excellent boarding rifles, since the cartridge is capable of penetrating small to medium sized steel-hulled watercraft.

Balanced Performance

Unlike the .50 BMG, the .50 Beowulf can be easily carried and fired by a single shooter without generating excessive recoil or an excessively loud muzzle rapport.

Furthermore, when using a rifle chambered for the .50 Beowulf as an entry weapon, frangible ammunition can be used to blow the hinges off doors without the need to worry about ricochets or backsplash endangering the shooter. Plus, the large .50 caliber diameter provides instant stopping power against assailants at close ranges while minimizing over-penetration (when loaded with relatively lightweight hollow point bullets).

Great for Hunting

The .50 Beowulf also makes an excellent cartridge for hunting medium to large size game species, since it has muzzle energies comparable to those of the .308 Winchester. It’s well suited to harvesting game species such as wild boar, black bear, mule deer, elk, and moose at ranges out to 150 yards. Plus, the large .50 caliber diameter combined with the heavy weight of the bullet provides a very high sectional density that translates to a much higher inertia than the .30 caliber bullets fired by the .308 Winchester.

This is why the .50 Beowulf is an excellent choice for hunting dangerous game in dense foliage. Heavy .50 caliber bullets are less easily deflected by intervening tree limbs than the lighter .30 caliber bullet fired by the .308 Winchester.

.50 Beowulf Specifications

Although the concept of a large caliber conversion for the AR-15 rifle is not new, most of the cartridges designed for a purpose, such as the .450 Bushmaster or the .458 SOCOM, generate excessive recoil that may damage the lugs on the bottom of the upper receiver. After all, the AR-15 was originally designed to fire the much smaller 5.56 x 45mm NATO cartridge.

Recoil Control

In order to prevent damage to the upper and lower receivers, Alexander designed the .50 Beowulf cartridge with a much lower chamber of 33,000 psi (a significant improvement over the 62,000 psi of the 5.56 NATO). This, in turn, creates a bolt carrier assembly recoil velocity that is equal to the 5.56 NATO, with a felt recoil that's only slightly higher. While the recoil of the 5.56 NATO feels like a .22 Magnum or a .22 Hornet, the .50 Beowulf feels more like a 20 gauge shotgun.

Reliable Design

Because the .50 Beowulf is based on the .50 Action Express pistol cartridge, it has a rebated rim with tapered straight walls that make extraction from the chamber extremely reliable. It also fires a much larger bullet than either the 5.65 NATO or the 7.62mm NATO cartridges, with the bullets measuring a full ½ inch in diameter. They can weigh as little as 300 grains and as much as 400 grains.

Speed

With the muzzle velocities ranging from 1,800 fps to 1,900 fps, the .50 Beowulf is significantly slower than most rifle cartridges. It is instead more comparable to that of the .50 Action Express pistol cartridge, which serves as the parent cartridge for the .50 Beowulf. This cartridge has muzzle velocities ranging from 1,250 fps to 1,550 fps.

Despite its diminished muzzle velocity, the .50 Beowulf produces muzzle energies that range from 2,200 ft./lbs. to 2,800 ft./lbs. (comparable to the .308 Winchester). However, due to the lighter weight and lesser velocity of the .50 Beowulf, compared to the .308 Winchester, you will commonly see a faster bullet drop (4 inches at 150 yards — assuming a 100 yard zero) and thereby a less effective range. So while the .50 Beowulf is an excellent cartridge for use at close to medium ranges and does an excellent job of retaining its downrange energy, it’s not well-suited for long range shots due to its highly arched trajectory.

Final Thoughts

Over the years, the .50 Beowulf cartridge has become a popular choice among many shooters. With compact AR-15 rifles chambered for the .50 Beowulf and a host of other benefits, it is no wonder this cartridge gained the attention of both the military and law enforcement operations.

With the AR-15 rifle also becoming increasingly popular among hunters, the .50 Beowulf is fast gaining popularity with those who hunt big and/or dangerous game. Those who need a large, heavy, bullet with a high degree of kinetic energy and deep penetration to take down their targets won’t be disappointed.

Besides that, the .50 Beowulf is available in a wide range of bullet weight and types — frangibles, full metal jackets, soft points, hollow points, and more. Combine that with the ability to reload these cartridges using dies (and load data) produced by Alexander Arms, and you’ve got one hell of a cartridge.

Best of all, you probably already have an AR-15 in your gun cabinet (if not, you’re long overdue). Shooting .50 Beowulf through your AR-15 is just a simple conversion away.

If you’re interested in trying out the incredible power of the .50 Beowulf (or the 12.7 x 42 mm, for those still confused), get yourself a box from Pro Armory. We have great deals on ammo and accessories — all in stock and ready to ship to you for the lowest price possible.

Browse Pro Armory today to find the best rounds for all your shooting needs.