The 6.8 SPC cartridge provides a significant performance jump over the 5.56 mm, while still fitting on a standard-size AR lower. The unit shown here is a 6.8 SPC upper receiver and magazine from DPMS.

 

    The intermediate cartridge has a long, complex history. First pioneered by the Germans in World War II, soon thereafter adopted whole-heartedly by the Soviets, experimented with by the British and belatedly accepted by the West in the 1960s, there have been a lot of ups and downs in its story. In fact, the newest member of the family, the 6.8x43 mm SPC cartridge, continues the tradition by hearkening back to the roots of the concept—albeit through an ultra-modern cartridge designed for today’s most advanced rifle platforms.

Striking a Balance

The strength of the 6.8 SPC is its similar overall length to a 5.56 mm, allowing it to fit into the magazine well of a standard AR lower. Obviously, one must be extremely vigilant regarding the use of proper ammunition when using a lower marked for 5.56 mm.

     So what is exactly is the intermediate cartridge? To understand its role, one must consider the types of military small arms that existed in the pre-World War II era. Military small arms were made up primarily of either traditional full-size rifles with commensurate full-size cartridges or sub-machine guns chambered in handgun cartridges. The former offered long-range performance, but a slow rate of fire, while the latter provided a large volume of fire but extremely short range.

     The solution was the intermediate cartridge, a form of shortened rifle cartridge designed to bridge the gap between those two types of small arms, offering rifle performance out to several hundred yards while still providing a high-volume of fire at closer ranges. The concept was pioneered by the Germans during World War II with the 7.92x33 mm Kurz cartridge chambered in the StG44—a radically advanced gun and cartridge combination.

Follow the Leader

The 6.8 SPC (left) propels a 115-grain bullet at around 2,600 fps, providing a serious potential jump in downrange performance when compared to a 5.56 mm (right)

     The intermediate cartridge thrived in the post-war era, with the Soviet Union adopting the 7.62x39 mm cartridge in the SKS and AK-47 series of rifles. In addition, the British experimented for a short time with a similar concept with their highly advanced but short-lived .280 British intermediate cartridge. Due to pressure from the United States, however, NATO ultimately adopted the full-power 7.62x51 mm cartridge.

     Belatedly, the West did come around to the intermediate-cartridge concept in the 1960s with the M16, chambered in 5.56x45 mm. Ironically, although the United States may have been initially resistant to the intermediate-cartridge, they raised the stakes significantly with this new round, which featured an extremely lightweight bullet propelled at exceedingly high velocities.

     In fact, the high-velocity approach of the 5.56 mm has now become the standard, with the M16 series itself having the distinction of being the longest-serving military service rifle in our country’s history. But, that tenure has not been without controversy—namely regarding the stopping power potential of the lightweight round.

The Solution

    This very criticism led to the development in 2004 of the 6.8x43 mm SPC (special purpose cartridge) at the behest of the U.S. Army Special Operations Command. The requirements for the new cartridge were stringent, calling for a round that offered greater downrange energy and lethality over the standard 5.56 mm, while also being adaptable to current AR-pattern rifles and carbines.

     Answering the call was Remington, which worked in close conjunction with Army Special Forces as well as the Army Marksmanship Unit to develop the new 6.8 SPC cartridge. Interestingly, Remington based the case of the new cartridge on one of its own obsolete rounds, the rimless .30 Rem.

      The resulting round had a similar overall length to that of a 5.56 mm, a requirement of the military’s request, but propelled a 115-grain bullet at around 2,600 fps. When compared against a 77-grain 5.56 mm at around 2,750 fps, the 6.8 SPC offers a significant jump in performance and energy. And, interestingly, as opposed to the ultra-high-velocity approach of the 5.56 mm, the 6.8 SPC more closely follows the ballistics approach of earlier intermediate cartridges with reasonably heavy bullets at respectable velocities.

 

 

Cartridge	Bullet Weight	Muzzle Velocity	Muzzle Energy
7.92x33 mm	125 grains	2,250 fps	1,400 foot-pounds
7.62x39 mm	125 grains	2,350 fps	1,525 foot-pounds
.280 British	140 grains	2,400 fps	1,800 foot-pounds
5.56x45 mm	77 grains	2,750 fps	1,300 foot-pounds
6.8x43 mm	115 grains	2,600 fps	1,725 foot-pounds
Weights, velocities and energies are representative estimates. Velocities from an approximately 16-inch barrel.

 

To convert a standard 5.56 AR to 6.8 SPC, one needs a complete 6.8 SPC upper and matching magazine. Swapping out the parts can be done in seconds. The unit shown is a DPMS 6.8 SPC upper receiver assembly mated to a standard DPMS AP4 lower assembly.

     However, the 6.8 SPC cartridge has advantages beyond its ballistics. Due to its similar overall length to that of the 5.56 mm, it is possible to employ the cartridge in an AR platform. To upgrade to this new cartridge, a new upper receiver and magazine can be used with an existing 5.56 mm lower receiver. The result is a rifle that offers a new class of power and performance in a familiar, easily adaptable package.

The Price of Admission

      I recently had the opportunity to try out a 6.8 conversion unit from DPMS for myself. As I already owned a DPMS AP4 carbine in 5.56 mm, all I needed to switch my carbine to the new chambering was a complete upper receiver and matching magazine. Once I had them in hand, the “conversion” took all of about 20 seconds. I pushed two pins, swapped out the upper and swapped out the magazine. While DPMS’s proprietary steel magazine features the same overall dimensions of a 5.56 mm magazine, it has a smaller capacity of 25 rounds due to the larger size of the 6.8 SPC cartridge.

      Once I had it together, it was hard to tell the difference between the 6.8 SPC and the original 5.56 mm carbine. The weight, size and handling characteristics were practically the same. Upon firing, the difference was more noticeable. Perceived recoil and muzzle flip were more pronounced, although not gratuitously so. This kind of performance bump cannot be expected without some sort of side effect, but it is well worth it for what you gain.

An Easy Answer

     So, one of the most recent members into the family of the intermediate cartridge actually hearkens back to the earliest days of the concept. And, it does so in a configuration easily adaptable to contemporary firearm designs. For those looking for an easy upgrade to their 5.56 mm AR offering greater downrange performance as either a tactical carbine or a good medium-sized-game hunting rifle, the 6.8 mm SPC is definitely worth a look.

	Although they feature similar overall dimensions, the steel 6.8 SPC magazine (bottom) holds 25 rounds total due to the larger size of the cartridge.

—Michael O. Humphries