The 6.8mm SPC cartridge was designed to address the deficiencies of the terminal performance of the 5.56×45mm NATO cartridge currently in service with the U.S. Armed Forces. The cartridge was the result of the Enhanced Rifle Cartridge program. The 6.8 SPC (6.8×43mm) was initially developed by MSG Steve Holland and Chris Murray, a United States Army Marksmanship Unit gunsmith, to offer superior downrange lethality over the 5.56 NATO/.223 Remington in an M16 pattern service rifle with minimal loss of magazine capacity and a negligible increase in recoil. The goal was to create a cartridge that would bridge the gap between 5.56 mm and 7.62×51mm NATO.
The program started the design by using a .30 Remington case, which was modified in length to fit into magazines that would be accommodated by the magazine wells of the M16 family of rifles and carbines that are currently in service with the U.S. Armed Forces.
In tests, it was determined that a 6.5 mm caliber projectile had the best accuracy and penetration, with historical data going back for decades of US Army exterior and terminal ballistic testing, but a 7 mm projectile had the best terminal performance. Further tests showed that a 6.8 mm caliber projectile was a compromise between the two, providing accuracy, reliability and terminal performance up to 500 meters. The combination of the cartridge case, powder load, and projectile easily outperformed the 7.62×39mm Soviet cartridge, with the new cartridge proving to be about 61 m/s (200 ft/s) faster. The resulting cartridge was named the 6.8 Remington Special Purpose Cartridge due to the size of its projectiles and the fact that it was based on the .30 Remington case.
In general, adapting an AR-style rifle to the new cartridge only requires the replacement of the barrel, bolt, magazine & muzzle device (if applicable) of the 5.56 mm-chambered rifle; but to further streamline and simplify the conversion process many parts manufacturers sell complete upper receiver assemblies chambered for 6.8 SPC alongside their conversion kits focusing on the key individual parts. While a complete 6.8 SPC assembly is a somewhat more expensive route, the conversion of an existing 5.56 mm/.223 rifle to 6.8 SPC using a complete upper assembly takes less than a minute on an AR-style rifle without the need for specialized tools or skills. In contrast, when swapping out the individual component parts, a significant level of gunsmithing experience, special tools, and time are generally required to detach the barrel from the rifle's upper receiver and the gas system, and conversely those same needs are required for the reassembly of the upper receiver with the new 6.8 SPC barrel. Also, there is the issue of having to readjust the sights if a new barrel is placed on an existing upper receiver.
The 6.8mm Remington SPC was designed to perform better in short barreled CQB rifles after diminished performance from the 5.56 NATO when the M16A2 was changed from the rifle configuration to the current M4 carbine. The 6.8 SPC delivers 44% more energy than the 5.56mm NATO (M4 configuration) at 100–300 metres (330–980 ft). The 6.8 mm SPC is not the ballistic equal of the 7.62×51mm NATO cartridge, but it has less recoil, has been said to be more controllable in rapid fire, and lighter, allowing operators to carry more ammunition than would otherwise be possible with the larger caliber round. The 6.8 mm generates around 2,385 J (1,759 ft·lbf) of muzzle energy with a 7.5-gram (115 gr) bullet. In comparison, the 5.56×45mm round (which the 6.8 is designed to replace) generates around 1,796 J (1,325 ft·lbf) with a 4.0 g (62 gr) bullet, giving the 6.8 mm a terminal ballistic advantage over the 5.56 mm of 588 J (434 ft·lbf). One of the enigmatic features of this cartridge is it being designed for a short barrel carbine length rifle that the standard rifle length is (usually 41 cm (16 in)). The round only gains about 7.6–10.7 m/s (25–35 ft/s) for every 25 mm of barrel length past the standard 410-millimetre (16 in) barrel (all else being equal) up to barrel's length around 560–610 mm (22–24 in) with no gain/loss in accuracy. It also does very well in rifles with less than 410 mm (16 in) barrels. In recent developments (the period 2008-2012) the performance of the 6.8 SPC has been increased by approximately 61 to 91 m/s (200 to 300 ft/s) by the work of one ammunition manufacturer Silver State Armory LLC (SSA) and a few custom rifle builders using/designing the correct chamber and barrel specifications. Also, more recently, LWRC, Magpul and Alliant Techsystems (ATK) are currently introducing a new AR-15 designed for the 6.8 SPC which allows for a proprietary 6.8 Magpul P-Mags and an overall cartridge length of 5.9 centimetres (2.32 in). The Personal Defense Weapon (PDW) known as the Six8 is SPC II w 1:250 millimetres (10 in) twist and is able to use all current 6.8 SPC factory ammunition. See Gold Dot below for ATK's part.7.1-gram (110 gr) Nosler Accubond ; 870 m/s (2,840 ft/s)- Silver State Armory (SSA)
7.5-gram (115 gr) OTM ; 850 m/s (2,800 ft/s)- Remington Premier Match
5.5-gram (85 gr) Nosler E-Tip ; 940 m/s (3,100 ft/s) - Silver State Armory (SSA)
5.8-gram (90 gr) Nosler BSB ; 910 m/s (2,980 ft/s) - Silver State Armory (SSA)
7.1-gram (110 gr) Hornady BTHP TAP; 820 m/s (2,700 ft/s) - Hornady Law Enforcement "tactical" factory load
7.5-gram (115 gr) OTM (FMJ): 831 m/s (2,725 ft/s)} - Silver State Armory (SSA)
7.5-gram (115 gr) Boat Tail Hollow Point (BTHP): 850 m/s (2,800 ft/s)
7.5-gram (115 gr) Sierra Match King (SMK): 850 m/s (2,800 ft/s)
7.1-gram (110 gr) Hornady V-MAX : 850 m/s (2,800 ft/s)
7.1-gram (110 gr) SCHP; 850 m/s (2,800 ft/s) - Silver State Armory (SSA) "combat" factory load
7.1-gram (110 gr) BTHP OTM & Barnes TSX; 840 m/s (2,750 ft/s) - Wilson Combat factory load
5.5-gram (85 gr) Barnes TSX ; 970 m/s (3,180 ft/s) - Silver State Armory (SSA) "tactical" factory load
5.8-gram (90 gr) Speer Gold Dot 930 m/s (3,050 ft/s)- Federal(ATK) "tac/mil" load.
7.1-gram (110 gr) Nosler Accubond ; 850 m/s (2,800 ft/s)- Silver State Armory (SSA)
7.5-gram (115 gr) OTM ; 820 m/s (2,700 ft/s)- Remington Premier Match
6.2-gram (95 gr) Barnes TTSX ; 880 m/s (2,880 ft/s) - Doubletap
5.8-gram (90 gr) Bonded Defense JSP ; 910 m/s (2,980 ft/s) - Doubletap
6.5-gram (100 gr) Nosler Accubond ; 855 m/s (2,805 ft/s) - Doubletap
7.1-gram (110 gr) Nosler Accubond ; 830 m/s (2,710 ft/s) - Doubletap
7.5-gram (115 gr) Full Metal Jacket Boat Tail ; 806 m/s (2,645 ft/s) - Doubletap
5.8-gram (90 gr) Speer® TNT ; 910 m/s (2,980 ft/s)- Silver State Armory (SSA)
7.1-gram (110 gr) Nosler Accubond ; 820 m/s (2,700 ft/s)- Silver State Armory (SSA)
7.5-gram (115 gr) OTM ; 800 m/s (2,625 ft/s)- Remington Premier Match
5.5-gram (85 gr) Barnes TSX ; 920 m/s (3,030 ft/s) - Silver State Armory (SSA)
5.5-gram (85 gr) Nosler E-Tip ; 900 m/s (2,950 ft/s) - Silver State Armory (SSA)
5.8-gram (90 gr) Nosler BSB ; 870 m/s (2,840 ft/s) - Silver State Armory (SSA)
7.1-gram (110 gr) Hornady V-MAX : 810 m/s (2,650 ft/s)
7.5-gram (115 gr) Sierra Match King (SMK): 810 m/s (2,650 ft/s)
7.5-gram (115 gr) OTM (FMJ): 785 m/s (2,575 ft/s)} - Silver State Armory (SSA)
5.5-gram (85 gr) Barnes TSX ; 940 m/s (3,070 ft/s) - Silver State Armory (SSA) "tactical" factory load
5.8-gram (90 gr) Speer Gold Dot 880 m/s (2,900 ft/s)- Federal(ATK) "tac/mil" load.
6.2-gram (95 gr) Barnes TTSX: 790 m/s (2,600 ft/s) - Wilson Combat factory load
7.1-gram (110 gr) SCHP; 810 m/s (2,650 ft/s) - Silver State Armory (SSA) "combat" factory load
7.1-gram (110 gr) Hornady BTHP TAP; 780 m/s (2,550 ft/s) - Hornady Law Enforcement "tactical" factory load
7.1-gram (110 gr) BTHP OTM & Barnes TSX; 790 m/s (2,600 ft/s) - Wilson Combat factory load
9.1-gram (140 gr) Berger VLD; 732 m/s (2,401 ft/s) - Silver State Armory (SSA) factory load. (Discontinued)
Typical trajectory information from carbines with drop and velocity calculated at sea level with a 91 metres (100 yd) zero.
When the LWRC Six8 was being developed, Alliant Techsystems was contracted to develop a new 6.8×43mm round for the weapon. Unlike smaller commercial firms, ATK is a large ammunition supplier that delivers products for the U.S. Army, so it had large resources and manufacturing capabilities at its disposal. Commercial cartridges varied in case capacity and thickness, but LWRC wanted a thick and durable case for military uses. A 90 gr (5.8 g) load was developed specifically for a high muzzle velocity and low felt recoil from the Six8's 8.5 in (220 mm) barrel. Effective range would be over 300 yd (274 m) and the bullet would still have enough energy to penetrate intermediate barriers. Three 90 gr loads were constructed for testing that included Gold Dot, Monolithic Hollow Point, and FMJ. The Gold Dot bullet was selected with a .035 in (0.89 mm) jacket and a bonded core. The propellant was designed for reduced muzzle flash stable performance at temperatures between -29.2 to 125.6 degrees F. Muzzle velocity averaged at a 200 ft/s (61 m/s) difference at the required temperature extremes from the 8.5 in barrel. From a 24 in (610 mm) barrel, the round produced a group of 1.56 in (40 mm) at 200 yd (183 m).
By late 2004 the 6.8×43mm SPC was said to be performing well in the field against enemy combatants in Special Operations. However the cartridge was not used by conventional US military personnel. It was not adopted for widespread use due to resistance from officials. The 6.8 SPC was designed for better terminal effectiveness at the shorter ranges of urban combat experienced in Iraq. When fighting in Afghanistan began to intensify, engagements began taking place at greater distances. Experiments suggested that the relatively short 6.8 mm bullets became ineffective at longer ranges. In 2007, both the U.S. SOCOM and the U.S. Marine Corps decided not to field weapons chambered in 6.8 mm due to logistical and cost issues. An unnamed LWRC representative said in January 2014 that the US military is once again taking a look at the 6.8 SPCII after all the commercial development in the last 10 years.
While there are many rumors of evaluations of the cartridge by several major Federal and local law enforcement agencies, the US Drug Enforcement Administration has allowed individual agents to purchase the M6A2 D-DEA - which uses the 6.8mm Remington SPC - as an authorized alternative to their duty weapon. In 2010 the Jordanian state-owned arms manufacturer KADDB announced that they would be producing 6.8 mm rifles and carbines for the Jordanian Army. There is also a contract between LWRC, Magpul, Alliant Techsystems and The Saudi Royal guard for around 36,000 Six8 PDWs & undisclosed amount of ATK/Federal XD68GD (90gr Gold Dot 'training' ammo) and proprietary Magpul 6.8 Pmags specifically for the LWRC Six8. See above under the subsection 'Development' Section. The 6.8 is also being produced in a Squad Automatic Weapon or SAW by U.S. Machine Gun Armory. The MGA SAW™ is fully compatible with the United States Department of Defense model designations: M249 and MK46. It is currently being shipped to US allies and is under testing with the US military.
There are several different chambers for the 6.8 SPC which yield different results. They are:
- Original Murray 6.8×43 ERC developed in 2002.
- Murray DMR chamber, which was meant to address improved accuracy expectations for the ERC Special Purpose Rifle program in SOCOM.
- The Remington SAAMI submitted specifications. It was supposed to have a 1.3 mm (0.050 in) freebore, 45° cone angle, 7.1 mm (0.278 in) ⌀ freebore. The reamers and PTG prints had an 80° neck to freebore cone angle, which was a result of a mistake in the reamer drawing submitted, and was never corrected by the reamer maker or Remington during the process of tooling up for the testing protocols that eventually drove the SAAMI submission.
- SPC II is current standard chamber used by most barrel manufactures. It has been said to be very close to the original Enhanced Rifle Cartridge Program chamber. It has a 2.9 mm (0.114 in) freebore, 45° cone angle, 7.1 mm (0.278 in) ⌀ freebore, 7.84 mm (0.3085 in)neck.
- 6.8 ARP(6.8×43mm renamed )(DMR has been replaced by 6.8×43/6.8 ARP, both are/were created by AR Performance). It has a 2.4 mm (0.095 in) ⌀ freebore, 45° cone angle, 7.05 mm (0.2775 in) dia, and a 7.84 to 7.85 mm (0.3085 to 0.309 in) neck.[Proprietary chamber]
- Noveske Mod 1 designed by Noveske Rifleworks LLC. It has been said to have a 2.5 mm (0.100 in) Freebore.[Proprietary chamber]
Only the rifles chambered with the newer specified chamber (6.8mm Spec II, Noveske Mod 1 and 6.8 ARP chambers) can safely use the higher pressure military/tactical and near max-maximum handloaded ammunition. Those rifles using the Original SAAMI specs should only be used with the standard commercial cartridge pressure (Specified by SAAMI).
Several twist rates are available for 6.8 SPC barrels. The slowest twist rates are approximately 1:12", while twist rates between 1:9.5" and 1:11" are common. The de facto standard twist rate for 6.8 SPC is currently 1:11". Bison Armory employs a twist rate of 1:7" in some of their 16" and shorter barrels. This is done in order to stabilize 180 to 200 grain bullets for subsonic performance similar to the 300 AAC Blackout. Bison Armory undertook simulation, pressure testing, and field testing, to determine that the 1:7" twist rate does not significantly increase chamber pressure, does not decrease muzzle velocity, and is safe to shoot all factory ammunition when combined with the SPC II chamber.
The first major manufacturer to offer a 6.8mm Remington SPC chambered version of the AR-15 was Barrett Firearms Company, offering the Barrett M468 and later the REC7. By 2007, most major manufacturers of AR-15 type rifles for the civilian gun market were offering rifles in this caliber. Dedicated AR upper receiver assemblies chambered for the round are produced by a number of smaller firms, including Daniel Defense. Ruger Firearms no longer produces a 6.8 mm for their Ruger SR-556 piston-driven AR-15 variant. The Stag Arms Hunter and Tactical models utilize the newer chamber(SPC II) and specified twist rates to accommodate higher pressure loadings, as well as upper receivers in Left-Handed configurations. Rock River Arms has a LAR-6.8 X Series rifle and uppers. Microtech Small Arms Research offers their version of the Steyr AUG in 6.8. Robinson Armament Co. offers the XCR-L in 6.8, which can be easily converted between 6.8, 5.56, and 7.62×39. Bushmaster has promised its ACR customers a 6.8 barrel conversion ability since the launch of the ACR (4 years ago as of April 2014), it also announced a 6.8 ACR offering alongside its .223/5.56 initial offering in April 2010, but has yet to deliver a 6.8 Remington SPC ACR to the market. Given the track record of regularly promised, yet undelivered 6.8 offerings for the ACR, a significant number of ACR industry experts, firearms enthusiasts and owners believe Bushmaster will never release 6.8 products for the ACR (nor any other caliber beyond .223/5.56). Ruger Firearms chambered their Mini-14 Ranch Rifle in this round for several years; however, it has been discontinued.
Remington also makes a bolt-action rifle chambered for 6.8 SPC, a 610-millimetre (24 in) barrel Model 700. Ruger no longer produces their M77 Hawkeye Compact rifle with a 420-millimetre (16.5 in) barrel weighing in at 2.7 kilograms (6.0 lb). Browning offers their A-Bolt rifle in 6.8 SPC, and Thompson/Center offers barrels chambered for 6.8 SPC for the Encore and Contender G2.