Knowing which spec 6.8 Remington SPC chamber your rifle has means a lot. The 6.8 Remington SPC has been around for a few years, but few shooters realize that not knowing which chamber spec their rifle is can lead to a dangerous mishap. Here’s how to tell the difference.
While it was still relatively new, Remington’s 6.8 mm Special Purpose Cartridge (SPC) gained acceptance in more military and law enforcement roles after its introduction in 2004. Once a new cartridge hits the market, it’s common for shooters and manufacturers to tinker with original designs to raise accuracy and performance. During a five-year span after its introduction, the 6.8 SPC underwent the inevitable evolution, too.
Reviewing cartridge history, the 6.8 mm Remington SPC was designed to increase performance in short barreled CQB rifles after diminished performance from the 5.56 NATO when the AR15 was changed from the rifle configuration to the current M4 carbine.
More Energy
The 6.8 SPC delivers 44% greater energy than the 5.56 mm NATO cartridge in the M4 configuration at 100 to 300 meters. The 6.8mm SPC is not the ballistic equal of the 7.62x51mm NATO cartridge; however, it has less recoil, is 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 approximately 1,759 ft-lb of muzzle energy with a 115-grain bullet. In comparison, the 5.56x45mm round (which the 6.8 is designed to replace) generates around 1,325 ft-lb with a 62-grain bullet, giving the 6.8mm a terminal ballistic advantage over the 5.56mm of 434 ft-lb.
6.8 SPC Built for SOCOM
The 6.8 mm Remington SPC (or 6.8x43mm) is a rifle cartridge collaboratively developed from members of US SOCOM. Based upon the .30 Remington cartridge, it is midway between the 5.56x45mm NATO and 7.62x51mm NATO in bore diameter and velocity. It is particularly adaptable to current 5.56 mm NATO firearms, the cartridge overall length being comparable.
Though ballistically similar to the 1950s-era .280 British, improved propellant powders allow the 6.8 mm to have a smaller case. The “standard” SAAMI (Sporting Arms and Ammunition Manufacturers Institute) 6.8 mm SPC has a muzzle velocity of 2,625 feet per second from a 24-inch barrel using a 115-grain Hornady OTM bullet.
Something I noticed when reloading
When I began loading the 6.8 SPC, one of the first things I noticed was the difference in large primers in Remington brass versus small rifle primers in all other loads. My curiosity centered around the benefits of using a large rifle primer over a small rifle primer. After some digging, I learned that large rifle primers offered about 50 fps more velocity over small primers. But, small rifle primers in a 6.8 SPC rifle cartridge yield lower standard deviation in velocities than for large primer 6.8 SPC rifle cartridges. It is not uncommon for a small primer case to have SDs in the low teens to single digits, whereas a large primer case will give a SD in the 20s or the 30s.
Standard deviation is a simple measure of the variability or dispersion of a data set, or in the case of ballistics, a group of bullet holes or the difference in velocity between shots. A low standard deviation indicates that all of the bullet holes are very close to target, while high standard deviation indicates that bullet holes are “spread out” over a large range. In regards to velocities, the closer the velocity from a single shot is from the mid-point of the string of shots, the lower the standard deviation. In simple terms, the lower the standard deviation of velocity, the more likely a load will group tighter.
Over the past five years, several firearms manufacturers have modified their 6.8 SPC chamber dimensions, too. I was surprised at the time that a large number of shooters, as well as gun retailers still lacked knowledge of the newer, modified 6.8 SPC chambers in the guns they were selling.
Spec 1 vs. Spec 2
Two standardized, yet different, chambers for the 6.8 SPC exist. Each yields different results. Only the rifles chambered with the newer specified chamber, also known as 6.8mm Spec II chamber, can safely use the higher-powered and higher-pressure ammunition. All other rifles should only be used with standard cartridge pressure loadings. Knowing which 6.8 Remington SPC spec chamber your rifle has means a lot.
In cartridge developments between 2004 and 2008, the performance of the 6.8 SPC increased. Velocities jumped approximately 200 fps by the work of one ammo manufacturer. A few custom rifle builders joined the party using updated chamber and barrel specifications. The velocities obtained now from an 8-inch barrel are equal to what a 16-inch barrel produced four years previously. The new 16-inch barrel velocities equaled to what a 24-inch barrel produced four years prior.
Reaches Further
By increasing the velocity of 6.8 SPC ammo 200 fps, the range of terminal effectiveness increases approximately 100 yards. Or, an 8-inch barrel now has the same effective range as a 16-inch barrel had in 2004. A 6.8 loses approximately 25 fps per inch of decreased barrel length. A 6.8 SPC Spec 1 zeroed at 100 yards drops 3 inches more at 300 yards than a Spec 2. The Spec 2 is traveling 200 fps faster.
The ammunition manufacturer making the most headway into 6.8 SPC Spec 2 loading was Silver State Armory. They offered ammunition loaded at the standard Spec 1 chamber pressure as well as the higher-velocity Spec 2. “The original SAAMI 6.8 SPC chamber has 50 thousandths of an inch of free-bore, and Spec 2 has 100 thousandths free-bore,” said Art Kalwas, president of Silver State Armory. “The extra free-bore drops pressure 5,000 to 6,000 psi with no loss of velocity. It’s similar to the differences between a .223 and a 5.56 NATO chamber, or .308 Winchester and 7.62 NATO.”
“There are guns out there that can shoot anything, but there are a lot of guns out there that are more selective of ammo,” Kalwas said. “A lot of people had gunsmiths extend the throats on their Spec 1 chambers to allow use of 6.8 Spec 2 ammo.”
A Change in the Industry
Gun makers were divided in their thinking, with some adhering to the Spec 1 chamber dimensions. Model 1 Sales, LMT (Lewis Machine & Tool) and Ruger manufactured rifles with 6.8 SPC Spec 1 chambers, consequently they can only shoot standard commercial loads. “Everybody else has gone to Spec II,” Kalwas said. “Barrett, Noveske, POF, LWRC, Rock River, Remington, DPMS, Spikes Tactical, Stag Arms and a few others have adopted Spec 2 chambers.”
Another topic of discussion is accuracy. Some think chambers with longer throats are less accurate than shorter Spec 1 chambers. Rifles with short throats force the bullet to “jump” a shorter distance to where the rifling begins. This can increase accuracy. The difference in accuracy potential for the 6.8 SPC may be of little consequence, owing to its CQB role. The difference between a sub-MOA gun and one that keeps rounds under 2 MOA matters less in short-range tactical scenarios.
Manufacturers have different philosophies on barrel twist rates for 6.8 SPC, too. Noveske builds barrels with 1:12-inch twist. POF, Spikes Tactical, and Barrett build their 6.8 SPC guns with 1:10-inch twist rates. “A good way to go is a 1:11-inch twist for the 16-inch to 20-inch barreled guns,” Kalwas said. “Below 16-inch barrels, I would go with 1:10-inch twist.”
| 6.8 SPC Spec 1 Manufacturers | 6.8 SPC Spec 2 Manufacturers |
| Model 1 Sales | Barrett |
| LMT (Lewis Machine & Tool) | Noveske |
| Ruger | POF |
| LWRC | |
| Rock River Arms | |
| Remington | |
| DPMS | |
| Spikes Tactical | |
| Stag Arms |
A lot of used 6.8 SPC rifles moveed into the secondary market. Gun buyers need to learn the difference between Spec 1 and Spec 2 guns. A Spec 2 chamber and fed a diet of Spec 2 ammo puts more fight-stopping power on target.
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