Custom built AR-15 chambered in 6.8SPC from Fighting Sheep Dog

Custom built AR-15 chambered in 6.8SPC from Fighting Sheep Dog

In today’s Caliber Check, we’ll look at the 6.8SPC cartridge (SPC = Special Purpose Cartridge).

Originally designed by Special Ops and the US Army Marksmanship Unit, the 6.8 was supposed to be that “happy medium” between the 5.56x45mm of an AR and the 7.62x51mm (aka .308) of an M14.

Of course, the parent cartridge is the .30 Remington (as mentioned below), which is why you can often use a 5.56 magazine to shoot 6.8 rounds in a pinch (you just have to figure out how many rounds the magazine will hold and still fire consistently).

Per Wikipedia:

The 6.8 mm Remington Special Purpose Cartridge (aka 6.8 SPC, 6.8 SPC II & 6.8×43mm) is a rifle cartridge that was developed by Remington Arms in collaboration with members of the U.S. Army Marksmanship Unit, United States Special Operations Command to possibly replace the 5.56 NATO cartridge in a Short Barreled Rifle(SBR)/Carbine.

Based upon the .30 Remington cartridge, it is midway between the 5.56×45mm NATO and 7.62×51mm NATO in bore diameter and muzzle energy. It uses a .277″ (7.0358 mm) diameter bullet, the same as that used in the venerable .270 Winchester hunting cartridge. It is particularly adaptable to current 5.56 mm NATO firearms, sharing the same cartridge overall length (2.260″).

6.8 Ammo in a 5.56 Magazine

6.8 Ammo in a 5.56 Magazine

PROS and CONS of a 6.8SPC-based firearm:

  • While the 6.8SPC round is comparable in price to a .308 round, it cannot beat the steadily-lowering prices of the 5.56/.223.
  • The 6.8SPC has 80% of the power of a .308 cartridge with half the recoil (and even less recoil than a .243 round reviewed elsewhere on this site), making it easier to control when firing.
  • Converting your AR from a 5.56 to a 6.8 is EASY! You only need to replace the barrel, bolt and magazine of the 5.56mm-chambered rifle. (Many companies make it even easier, selling fully assembled uppers chambered in 6.8SPC.) PLUS, it takes literally a minute or less to convert from 5.56 to 6.8 (and you don’t even need a special tool – just 2 pins!)

“The minimum chamber size is smaller than the maximum ammo size. The maximum ammo size is 1.350″ and the min chamber size is 1.346”. They either need to make the “go” gauges 1.350 and call that minimum chamber size or make the ammo and go gauges 1.346 or less. The no-go gauges are not as large as max chamber size. There should be .006 difference from “go” to “no-go” and the ammo should be equal to or less than the “go”. If you rifle checks out with a “go” gauge but your factory ammo will not chamber call the ammo manufacturer.” – Per ARPerformance.com

But the BIGGEST pro comes in the area of hunting. The smallest caliber that most states allow for hunting “medium sized game” (think coyote, hog, elk, black bear, deer, etc) are cartridges that fall below .243 of an inch (6mm). The 6.8SPC cartridge allows hunters the use of the AR platform with stellar performance out to 300-400+ yards!

(And let’s be honest, if a gunsmith decided to build HIS rifle in a 6.8SPC…don’t you think there’s probably something to it???)

John Young siting in the custom 6.8 build

John Young siting in the custom 6.8 build

Here’s the technical specs (again, from Wikipedia):

The 6.8 SPC delivers 44% more energy than the 5.56 mm NATO (M4 configuration) at 100–300 metres (330–980 ft). 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 around 1,759 ft·lbf (2,385 J) of muzzle energy with a 115-grain (7.5 g) bullet. In comparison, the 5.56x45mm round (which the 6.8 is designed to replace) generates around 1,325 ft·lbf (1,796 J) with a 62-grain (4.0 g) bullet, giving the 6.8mm a terminal ballistic advantage over the 5.56mm of 434 ft·lbf (588 J). sne of the enigmatic features of this cartridge is (it being designed for a short barrel carbine length rifle) that the standard rifle length is 16 inches (41 cm). The round only gains about 25–35 feet per second (7.6–11 m/s) per inch of barrel length past the standard 16 inch barrel (all else being equal) to around 22–24 inches (56–61 cm) with no gain/loss in accuracy. It also does very well in rifles/pistol with less than 16″ barrels.

To sum it up – for the money, you can’t go wrong with a 6.8SPC chambered AR – especially if you’re a hunter. With half the recoil of the 5.56 at about the same price as the .308, it’s like having your cake and shooting it, too!

Suggested reloading components:

  • Hornady small primer brass for the best accuracy, it is more consistant in weight and neck thickness.
  • CCI -#41 and 450 small primers, Wolf small magnum primers for a less expensive option(apx $18/1000 from Wideners)
  • H322 powder for the best accuracy, 29.5 is max case capacity without compressing the load.
  • RE 10X for 100-115gr bullets where more velocity is wanted- 29-29.5 gr but work up to it.
  • RE 7 for 85-95gr bullets- 29.2 gr is standard but work up to it.
  • H335 is a good all-around ball powder for good velocities over a wide weight range of bullets.
  • AA2200 and 1200R are new powders on the market , many shooters having good luck with them.
  • 8208 and N530 work with heavier bullets 120-130gr
  • Sierra 110gr Prohunter bullet for great accuracy and terminal performance on deer
  • Barnes 85, 95 and 110 TSX for the best terminal performance on hogs
  • Speer 90gr TNT for great accuracy and peformance on varmints and yotes.

– From ARPerformance.com