The 6.8mm Rem SPC cartridge: Part one

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Case-heads. Left to right: 5.56/223 (0.378in dia.); 6.8SPC (0.422in); 7.62x39mm (0.445in)

Cartridges keep rolling in with a new arrival every year or two, most recently the .300 AAC Blackout, 6.5mm Hornady Creedmoor, and .28 Nosler, covering the entire range from subsonic midget to barrel burning elk-slayer. Launched at SHOT 2002, the 6.8mm Rem SPC was the first significant newcomer of this millennium. Eh? 6.8mm isn’t new, simply the metric designation of the longstanding .270 calibre with its 0.277in diameter bullets. SPC – Special Purpose Cartridge, its raison d’être was to improve terminal ballistics from short-barrelled AR-type carbines favoured by US Special Forces. ‘Rem’ is of course Remington. More often than not, it’s called the 6.8SPC.

While a 21st century innovation, its case is based on one that first saw the light of day 110 years ago as the .30 Remington. Its development is one more initiative in 70-year process to produce a standard NATO cartridge that combines good manners in lightweight selective (semi plus full-auto) fire assault rifles with lethality and acceptable range. The first result was the overly powerful American-designed 7.62x51mm (.308 Win), forcing rifle designers to beef up actions and barrels while restricting firearms to semi-auto fire. Soviet bloc countries adopted the smaller 7.62x39mm M43 and designed the hugely successful AK47 series around it.

M4 carbine in 5.56mm form – its 14.5in barrel causes a major reduction in effective range. (US Army photograph)

M4 carbine in 5.56mm form – its 14.5in barrel causes a major reduction in effective range. (US Army photograph)

Before NATO countries had completed 7.62mm rifle issue, the American Army started playing fast and loose with another beau, the small-calibre high-velocity model. The Vietnam War saw a hurried adoption of Gene Stoner’s AR15 and its 5.56x45mm (.223 Rem) cartridge as the M16A1 and M193 respectively, a tiny 55gn bullet given 3,200fps MV. The combination of high velocities, short-range firefights, and frangible projectile saw the ‘Mattel Toy’ rifle and ‘Mouse Cartridge’ combination confound critics by inflicting horrendous injuries. Such was its lethality that the USA was accused of using inhumane ammunition.

NATO then endeavoured to find a sub-calibre replacement for the 7.62x51mm, finally adopting an enhanced 5.56 in 1980, designated M855 in the US military. Using the 62gn FN SS109 bullet, MVs dropped by 150fps compared to the M193, but usable ranges increased. However, it too needs high terminal velocities to produce lethality, quoted as 2,600fps and above, so the effective range is much shorter than its 500-600 metre hit capability. That’s from the M16A2 rifle with its 20in (508mm) barrel, and the US Army has since adopted the short (14.5”/370mm) barrel M4/M4A1 carbine as its primary infantry personal firearm, reducing MVs by another 150fps. Special Forces choose shorter barrel variants, down to 10.5”/266mm. Criticisms of 5.56 cartridges’ performance reappeared.

L-R: German WW2 7.92x33;  Soviet 7.62x39 M43; British 7x43 Mk1z; 7.62x51 NATO (.308 Win)

L-R: German WW2 7.92×33; Soviet 7.62×39 M43; British 7×43 Mk1z; 7.62×51 NATO (.308 Win)

5.56 v 7.62

Conflicts covering Vietnam to Afghanistan have pitted the 5.56 against the 7.62x39mm M43 with its 123gn, 0.311in diameter bullet at a nominal 2,350fps MV. Many, rarely combat veterans, claim the 7.62 M43 is more lethal and provides superior ‘barrier penetration’. This is unlikely overall, but M855 terminal velocity is crucial and the M43 has one advantage – its lower charge weight-to-bore ratio reduces the effect of barrel length changes on MVs.

The M4A1 was originally intended for limited issue, primarily Special Forces. User feedback from Iraq revealed problems with its effective range – depending on the source, 90 to 150 metres is quoted. Cutting barrels back further to 10.5in/266mm reduced MVs to barely above 2,600fps, so maximum lethality was restricted to the shortest range firefights. The M262 cartridge was developed as a stopgap solution, loading the 77gn Sierra HPBT MatchKing bullet (OTM in military speak), which is more frangible than the SS109 and works with terminal velocities around 2,200fps. It was only available in limited quantities and cost four times as much as the M855. As US involvement in Afghanistan grew, finding alternatives became urgent, and most unusually, USSOCOM (US Special Operations Command) authorised a bottom-up approach with a small team of serving Special-Ops non-coms and armourers given a small budget and authority to experiment with larger calibre alternatives, seeking support from the American firearms industry as needed.

Rivals. Left to right: 7.62 M43; 5.56/223 with 77gn Sierra MK to its left; 6.8SPC with Remington 115gn FMJ to its right

Rivals. Left to right: 7.62 M43; 5.56/223 with 77gn Sierra MK to its left; 6.8SPC with Remington 115gn FMJ to its right

M4/16 platform

Any new cartridge had to fit the existing M4/M16 platform – that is, only the barrel, bolt and magazine could be changed. It had to stay within the 5.56’s 2.26in overall length, the magazine well unable to accommodate anything longer. Case-head diameter is a crucial factor as Stoner built his original AR15 around the .222 Remington case with its 0.378in diameter rim carried into the .223/5.56mm. 7.62x39mm AR15 conversions are available, but the cartridge’s 0.445in diameter rim only just fits the revised bolt face and even so the locking lugs are left so thin they risk premature fatigue cracking.

It did allow the use of the cartridge as a control, a key objective being any new design had to offer superior ballistics. It also allowed the testing of Alexander Arms’ efficient 6.5mm Grendel (a 7.62×39/PPC based number), but this design retains the M43’s case head with consequent reliability issues. (A ‘rebated-rim’ version was successfully tested but later dropped.)

The AR-15 / M16 bolt-face can just accommodate the 6.8’s case-head and retain sufficient metal in the locking lugs to remain reliable

The AR-15 / M16 bolt-face can just accommodate the 6.8’s case-head and retain sufficient metal in the locking lugs to remain reliable

The development team identified 6.5 to 7mm calibres as offering the best combination of firearm handling, MVs, external ballistics and lethality. This duplicates the results of extensive British War Office work by its ‘Ideal Calibre Panel’ in the late 1940s, which developed and tested 6.8 and 7mm assault rifle cartridges before adopting the 7x43mm (.280/30 British) and proposing it as the NATO standard, a fatter number than could be accommodated here – a pity the US Army vetoed it with its insistence on a powerful thirty! Remington took the old slimmer .30 Rem (a rimless .30-30WCF introduced for semi-auto and pump sporters), shorted and strengthened it to produce a small cartridge with 13 per cent greater case capacity than the 5.56. It was extensively tested in 6.5, .270, and 7mm versions. 6.5 gave the best accuracy and flattest trajectory, 7mm the highest retained energy, but the middle calibre came out best overall. Hornady, Remington, and Sierra designed 110-115gn bullets for it and the 6.8SPC was born as a limited use special-ops tool and new SAAMI registered civilian sporting cartridge. Next month: rifles, chambers, components and ballistics.

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