Calibre's Calibre: 6.8 SPC

Since the creation of the M16, and the adoption of the small 5.56mm NATO round it fires, much has been made about the lethality of the round’s small bore projectile. After all, in civilian circles, the average 40-62 grain .22-calibre round both .223 Remington and 5.56mm NATO uses is generally considered only marginally acceptable for anything larger than varmint-size game. Of course, the idea behind hunting rounds and Hague Convention-compliant combat ammunition is very different, but nonetheless the lessons learned on the modern battlefield have only served to fan the flames of those that claim the standard issue 5.56 round is ineffective.

And right around the turn of the millennium, those flames were enough to incite the pentagon into researching the viability of a replacement round that could serve as a new intermediate cartridge, offering a compromise between the 5.56mm NATO and 7.62mm NATO rounds. According to Christopher R. Bartocci’s recounting of the round’s creation in the book The Black Rifle II: The M16 into the 21^st Century, “[I]n early 2002, soldiers of the 5th Special Forces Group (Abn), headed by MSG Steven Holland, received approval to initiate a Proof of Concept to develop a new capability that would increase incapacitation, lethality, and range over the existing 5.45x39mm, PRC 5.8x39mm, 7.62x39mm and 5.56x45mm NATO cartridges. This initiative was a grassroots effort aimed at providing better combat power for Special Operations Forces and soldiers of the Light Infantry, to include USMC MEU-SOC.”

Interestingly, those impressive and far-reaching goals were put to the firearms industry with the understanding that any company involved in the development of the new round would be expected to do so at no expense to the government. The assessment of the prototype cartridges would be carried out by the US Army Marksmanship Unit, but the costs of development would be borne by the private sector companies involved; an idea that appealed to Remington. Viewing the program as an opportunity to help in the Global War on Terror, Remington agreed to develop and manufacture the round, using bullets provided by Sierra and Hornady. Powders were produced by Western Powder, Accuracy Arms, and St. Marks Powder Company.

The round started with a .30 Remington case, which boasted the same case head and extractor rim dimensions at 5.56mm NATO to increase parts compatibility, but featured a shortened case to allow the round to be loaded into standard mil-spec magazines and magazine wells. From there, the Special Purpose Cartridge team experimented with 6.5mm, 6.8mm, and 7mm projectiles, before settling on the 6.8mm round as the projectile that provided the best mix of terminal and in-flight ballistics, reliability, and accuracy.

The result? A round that fired bullets weighing roughly twice as much as the 5.56 NATO’s, at right around 2,500 feet per second (roughly 600 fps slower than the 62-grain SS109 and M855A1 loads), that demonstrated far better terminal performance, largely due to that increase in mass. At the muzzle, even a light 115 grain 6.8 SPC load develops 1,700 ft-lbs of energy, as compared to the roughly 1,300 ft-lbs provided by most military 5.56mm NATO loads.

However, as promising as 6.8 SPC looked, it was ultimately superceded by the progression of the military’s specialized Mk 262 round, which was being developed concurrently alongside 6.8 SPC. Mk 262 involved the use of a super-long 77 grain bullet in a standard 5.56mm NATO case, which provided much of the same terminal ballistic performance of the 6.8 SPC round, but far better accuracy potential and long-range performance. This was due primarily to the design of the bullets used, and the constraints imposed by the magazine well dimensions of the mil-spec M4 rifle.

By using a heavier bullet, the 6.8 SPC round required a substantial powder load in order to effectively reach out and obtain the 450 metre performance the US Army was looking for. As a result of the substantial thrust required to pop those 115+ grain pills downrange, the 6.5 SPC rounds were developing 55,000 PSI and putting 6,537 ft-lbs of force into the bolt. 5.56mm NATO boasted higher pressures at 58,000 PSI but only 5,432 ft-lbs of bolt thrust.

All that powder meant the bullets used needed to be relative stumpy. The ogive, which is the curved portion of the bullet’s nose, is shorter and more aggressively curved. This gives a heavier bullet, but poor aerodynamic performance, as opposed to a round with a longer nose and a more gradual and graceful ogive. Conversely, the 77-grain bullets installed in Mk 262 rounds required less powder, and so could be loaded further into the case. This provided the necessary space to stretch the bullets out for a far more slippery ogive than the stocky 6.8 SPC projectiles. This gave the Mk 262 its stellar long-range performance, without giving up too much in the way of terminal ballistics.

However, in civilian circles, the 6.8 SPC’s greater emphasis on lethality has made it a popular medium-game hunting choice in the US, where the AR-15 is considered a successful hunting rifle. The ease with which a rifle can be altered to fire 6.8 SPC (it requires nothing more than a barrel or upper change), it’s performance on deer-sized game, and additional laws requiring such game be shot with a calibre larger than .22 have all made this once-special forces hopeful into a respected sporting round.