6.8 SPC vs 5.56: Upping the Lethality of the M4 Carbine

The lethality of the 5.56 NATO/223 Remington rifle cartridge has been a topic of heated debate since its adoption by the US military during the Vietnam War. When fired from a 20 inch barrel, the terminal performance of the 5.56 is quite impressive, however problems arise when a shorter barrel length is used.

During Operation Iraqi Freedom, the US Army started fielding the M4 carbine with a 10.5” barrel, as a short barrel was ideal for close quarters battle (CQB) as it was more maneuverable. However, the shorter barrel negatively affected the terminal ballistics of the 5.56 and soldiers later reported enemy combatants taking multiple hits while remaining combat effective.

The search for a new rifle cartridge was on, as US Special Operations Command (SOCOM) wanted something that could outperform the 5.56 in CQB conditions with a short barreled rifle (SBR).

One such cartridge was the 6.8 Remington Special Purpose Cartridge (SPC). Developed as a joint operation by Remington and the Army Marksmanship Unit, the 6.8mm SPC showed a lot of promise as it had impressive terminal effects at CQB ranges while experiencing a full powder burn in an SBR.

Although the 6.8SPC was not adopted by the US military, it has found a home in civilian AR platform rifles as a hunting cartridge and self-defense round.

In this article, we will evaluate the 6.8 SPC vs 5.56 NATO to help you understand the differences between the two and give you a clearer idea of which cartridge is best for your new AR-15.

What is the difference between 6.8 SPC and 5.56?

The difference between 6.8 SPC and 5.56 is the bullet diameter each cartridge fires. The 6.8 SPC fires a heavier 0.277” diameter bullet while the 5.56 NATO fires a lighter 0.224” diameter bullet.

A Note on Nomenclature

Please note that within this article we will refer to the 223 Remington (223 Rem) and the 5.56x45mm NATO round interchangeably. There are differences between the two and you can read about them in this article: .223 vs 5.56

In short, a 223 Rem can safely be fired from a rifle or handgun chambered in 5.56, however the opposite is not true.

Cartridge Specs

When evaluating centerfire rifle cartridges, it’s a good idea to analyze the cartridge specs to gain more knowledge of each.

The most obvious difference between the 6.8 Remington SPC and 5.56 NATO are the bullets that each cartridge fires. The 5.56 fires a 0.224” diameter bullet while the 6.8 fires a 0.277” diameter bullet. The 0.277” bullet is the same diameter as those fired by the 270 Winchester, 277 SIG Fury, and 270 WSM while the 0.224” diameter bullet is also fired by the 224 Valkyrie, 22-250, and 22 Hornet.

In terms of bullet weight, the 6.8 SPC fires heavier bullets than the 5.56. Typical factory loads for 5.56 range between 35 and 77 grains, however the 55 gr and 62 gr are the most common. Ammo for the 6.8 SPC can range between 75 and 120 grains, with the 115 grain bullet being the most popular by far.

Although the 6.8 fires heavier bullets, both the 5.56 and 6.8 SPC have the same maximum overall length of 2.26”. This is due to the design of the AR platform, as it can only handle cartridges 2.26” or shorter.

The 6.8 SPC has a slightly shorter case length at 1.687” vs 1.76” for the 5.56. However, the 6.8 SPC is wider than the 5.56 with a base diameter of 0.422” and 0.377”, respectively. The wider base and rim diameter requires a larger bolt when converting an AR-15 to fire 6.8 SPC.

This wider case allows for more case capacity, which the 6.8 SPC needs to fire its heavier bullets at acceptable muzzle velocities. The 6.8 SPC has a case capacity of 34.8 gr compared to 28.5 gr for 5.56.

Although it can house almost 22% more powder, the 6.8 SPC and 5.56 NATO have virtually the same maximum chamber pressure of 55,000 psi per SAAMI specs.

6.8 SPC vs 6.8 SPC II – What’s the Difference?

Although the 6.8 SPC seemed to be a potent and elegant answer to the 5.56 NATO terminal performance issue, there were some problems with its implementation.

When you’re looking to purchase a new upper receiver or barrel for 6.8, you might notice that the barrel is stamped as 6.8 SPC II. Many new shooters wonder what the difference between 6.8 SPC and 6.8 SPC II is, and the answer is in the design of the chamber.

The 6.8 SPC II chamber is designed to handle full-power 6.8mm SPC loads while the original 6.8 SPC chamber had a design flaw that could potentially cause pressure issues.

In their haste to bring the 6.8 SPC to market and capitalize the hype surrounding the new round, Remington botched the release of the cartridge with inaccurate chamber design specifications.

The original chamber designs called for 0.050” of free bore. However, when combined with a 1:10 twist rate barrel, pressure issues began to creep up. Initial reports claimed that the rounds were only around 200 psi above the maximum 55,000 psi pressure limits, but if the military learned anything from Vietnam, it was to follow specs to the letter.

Remington sought to remedy the higher pressure issue the best way they saw fit, which was to underload the ammo instead of fixing the free bore measurement. The resulting “light” 6.8 SPC ammo failed to meet expectations, and the US Army scrapped the project.

However, in all the excitement over the new 6.8 SPC, several civilian firearm manufactures clambered to release rifles chambered in the new cartridge before SAAMI had officially standardized the 6.8 SPC round. With numerous faulty rifles in circulation, the only action SAAMI could take was to sanction the round.

This is not to say that original 6.8 SPC rifles won’t function properly, but there is the potential for a critical pressure failure when using full power rounds.

The answer was the 6.8 SPC II.

The 6.8 SPC II added an additional 0.050” of free bore to the chamber and decreased the barrel twist rate to 1:11. This completely resolved the pressure issues experienced with the original design. All current rifles chambered in 6.8 SPC are technically chambered in 6.8 SPC II.

Although most factory loads will be loaded light (due to potential liability issues of original 6.8 SPC rifles still running loose in the wild), ammo for 6.8 SPC II rifles can be loaded to the full potential that the round was designed for.

Recoil

The 5.56 NATO has less recoil than the 6.8 SPC, although most shooters would describe the 6.8 as having light to moderate recoil.

Recoil is an important consideration when purchasing a new rifle as a round with heavy recoil will be more difficult to control and will slow your rate of follow up shots.

Recoil is affected primarily by muzzle velocity (FPS), powder charge, bullet weight, and rifle weight.

As the 6.8 SPC fires heavier bullets, it will generally have higher free recoil than the 5.56.

For example, a 6.8 SPC 120 gr SST fired at 2,400 fps from a 6-pound AR-15 will have about 9 ft-lbs of free recoil compared to about 4 ft-lbs for a Hornady 55 gr FMJ M193 5.56 NATO round traveling at 3,200 fps from the same rifle. In comparison, a SS109 62 grain bullet for 5.56 NATO round will have around 5 ft-lbs of free recoil.

It should be noted that although the 6.8 SPC has a little over double the free recoil of the 5.56, most shooters would classify the 6.8 as having low recoil. However, for recoil sensitive shooters, the 5.56 is the better option.

Muzzle Velocity, Kinetic Energy, and Trajectory

The 5.56 is well-known for its incredibly high muzzle velocity and flat trajectory, however the 6.8 SPC dominates in terms of kinetic energy delivered to the target.

For the purpose of this comparison, the two most common 5.56 NATO rounds, the 55 gr M193 and 62 gr SS109/M855, will be compared to the Hornady 6.8 SPC 120 gr SST factory load. All loads will have a 100-yard zero.

Looking at the ballistics table, we can see that the 5.56 dominates in terms of muzzle velocity. The M193 load leaves the barrel at 3240 fps and the M855 at 3060 fps. This high muzzle velocity allows the 55 and 62 grain bullets to reach their targets faster, meaning that gravity has less time to affect their flight. This translates into a flatter trajectory.

At 500 yards, we see that the M193 and M855 have experienced -54.8” and -57.8” of bullet drop, respectively. Compared to the 120 gr Hornady SST that has dropped -78.8” by 500 yards, clearly showing how the 5.56 has a more favorable trajectory at longer ranges.

However, what the 5.56 lacks is muzzle energy, and this is where the 6.8 SPC really shines. At the muzzle, the 120 gr SST is packing 25% more muzzle energy than both 5.56 rounds. Furthermore, the 6.8 maintains its kinetic energy more effectively at range. For example, at around 250 yards the 6.8mm SPC still has around 1,000 ft-lbs of kinetic energy while both 5.56 rounds drop below this threshold around 100 yards.

It's also important to note that the test barrel length used for the 5.56 rounds was 20” which is similar to what the United States Marine Corps uses in their M16’s. However, the test barrel for the 6.8 SPC was 16”, which shows how well the 6.8 can outperform the 5.56 NATO cartridge in a short barrel.

Ballistic Coefficient and Sectional Density

The heavier bullet fired by the 6.8 SPC will have a higher ballistic coefficient and sectional density than the lighter bullets fired by the 5.56 NATO.

Ballistic coefficient (BC) is a measure of how aerodynamic a bullet is and how well it will resist wind deflection. Sectional density (SD) is a way to evaluate the penetration ability of a bullet based on its external dimensions, design, and weight.

The heavier bullets fired by the 6.8 Remington SPC are more effective at resisting wind drift and will, therefore, have a higher BC compared to the 5.56. Although the light bullets of fired by the 5.56 are excellent for long range shooting, they are more easily deflected by the wind.

If we consider the same bullets examined in the previous section, the 6.8 SPC 120 gr Hornady SST has a BC of 0.400 while the 5.56 M193 and M855 have a BC of 0.243 and 0.274, respectively.

When it comes to penetration, the heavier and wider bullets fired by the 6.8SPC will penetrate deeper than the thinner 0.224” diameter bullets fired by the 5.56. The 6.8mm SPC 120 grain bullet will have a SD of 0.223 compared to 0.157 for the 5.56 M193 55 gr round and 0.177 for the M855 62 grain bullet.

Hunting

Although the US military did not officially adopt the 6.8 SPC as a duty round (though there are some unverified anecdotal reports that Special Forces has used the cartridge), the 6.8 has found a home in the hunting community.

The 6.8mm SPC has enough kinetic energy to take down a whitetail out to around 300 yards and is extremely popular in areas with increasing feral hog populations. The low recoil, fast follow-up shot capability, and high magazine capacity of the 6.8 makes it a potent option for hogs and other large varmints that can wipe out an entire field of crops in a single night.

The 5.56 NATO/223 Remington has been a favorite cartridge for varmint hunters for decades. Its lightweight bullets make short work of coyotes, groundhogs, and other thin-skinned varmints and its flat trajectory makes it great for long range shots.

Many hunters debate the efficacy of the 5.56 NATO for use on whitetail and other deer-sized animals. Some detractors of the cartridge site battlefield reports of the ineffectiveness of the 5.56 NATO cartridge of two-legged varmints. However, it is important to remember that the military is required to use FMJ bullets in combat due to the Hauge Convention. Civilian hunters are, thankfully, not restricted by the Hague Convention and can use expanding bullets for while hunting.

Several states allow 5.56/223 Rem for deer hunting, and some of the heavier 75 and 77 grain factory loads from Federal, Nosler, and Hornady are extremely effective on deer. However, most states have a minimum caliber requirement of 0.243” for deer, so this makes the 6.8 SPC a better choice in those areas.

Overall, the 6.8 makes a better deer cartridge as it fires heavier bullets and will make a wider wound channel to increase blood loss. This aids in a cleaner kill and reduces the potential for prolonged suffering of the game animal. That being said, proper shot placement is paramount in any hunting situation, so practicing your marksmanship skills is critical for making a clean and ethical harvest.

The 5.56 NATO is an excellent varmint cartridge and is best suited for this role, however heavier factory loads can be used for larger game animals if your state or province allows for their use.

Ammo and Rifle Cost/Availability

The 5.56 NATO is one of the most popular centerfire rifle cartridges in North America, making rifles and ammo extremely easy to come by.

The hype surrounding the initial release of the 6.8 SPC was high, but the botched chamber designs really hindered the cartridge’s mainstream acceptance. Ammo had to be (criminally) underloaded, inhibiting the cartridge’s true potential.

In terms of ammo availability, the 5.56/223 Rem outstrips the 6.8 SPC to the tune of around 9:1. The demand for 5.56 ammo has led to a massive supply, and there is a wide variety of bullet types and weights to choose from to meet all your shooting needs.

Cheap surplus FMJ 5.56 ammo from Winchester, Remington UMC, and Fiocchi can be had for around $0.60/round while premium hunting ammo like Nosler Ballistic Tip, Hornady Black, and Federal Fusion can be had for under $2/round. Home defense and law enforcement ammo is also available, like Speer Gold Dot and Winchester PDX-1 for around $2/round.

Buying in bulk is always smart, make sure to check out our stock of 5.56 bulk ammo.

For the 6.8 SPC, there are fewer options available in terms of ammo and the price is generally higher. Cheap FMJ plinking ammo will usually run you around $1.50/round while premium hunting ammo typically starts around $2.50/round and goes up from there.

In terms of rifle availability and price, the 5.56 NATO cannot be beat as the AR-15 is the most popular rifle in America. The vast majority of AR-15’s produced are chambered in 5.56, so your rifle options are extensive.

When looking for a bolt-action 5.56 rifle, you’ll quickly discover that most of them are chambered in 223 Rem and not 5.56. As the 223 Remington is marketed primarily as a varmint round, it holds a very large market share in the bolt-action rifle space. However, remember that it is not safe to fire 5.56 ammo in a 223 rifle as we discuss in this article HERE.

If you really want a bolt action rifle in 5.56, there are a few options available like the Ruger American Ranch, the Ruger GUNSITE Scout, and Mossberg MVP Patrol.

The beautiful thing about the AR platform is how modular it is. As the 6.8 SPC was developed for the AR-15, performing a conversion is as simple as changing the barrel, bolt, and magazine. Many shooters prefer to have a separate upper receiver for 6.8, allowing them to change calibers on their favorite AR by simply pushing out two pins. In terms of upper receivers, the LWRC Six8 and PSA Mid-Length receivers are both excellent options.

As the 6.8 SPC was designed with the AR-15 in mind, there are currently no bolt-action rifle options available for the cartridge.

Reloading

In terms of reloading, both cartridges are excellent options if you like to roll your own ammo.

For the 5.56 NATO, there is a plethora of surplus brass and bullets available on the primary and secondary markets. Manufacturers like Hornady, Winchester, Federal, Nosler, Barnes, and Sierra all make a wide variety of bullets for 0.224” caliber as it is extremely popular.

In terms of reloading dies, understand that there are only 223 Remington dies available since the 5.56 and 223 Rem cases have identical dimensions.

The 6.8 SPC will be a little more challenging in terms of components, as brass cases are not as plentiful as 5.56 cases. Bullets for 0.277” caliber should be fairly easy to locate as it has been a popular hunting caliber for many decades with the rise in popularity of the 270 Winchester. Furthermore, as the US Army just adopted the 277 SIG Fury cartridge as their new battle round, 0.277” diameter bullets may be in higher demand and easier to source in the near future.

Final Shots: 5.56 vs 6.8 SPC

The 5.56x45mm NATO has been one of the most popular centerfire rifle cartridges in North America since its adoption during the Vietnam War. Its low recoil and high muzzle velocity make it an incredibly soft and flat shooting cartridge that is great for varmint hunting, long range target shooting, or general plinking.

However, the lighter bullets fired by the 5.56 leave something to be desired when it comes to kinetic energy and lethality, as combat reports from Iraq and Afghanistan critique the 5.56’s combat effectiveness.

Adapting new cartridges to the AR-15 rifle is nothing new, as rounds like the 300 Blackout and 6.5 Grendel have all seen moderate success in civilian shooting and hunting circles.

Although the 6.8 Remington SPC cartridge had a rocky start to its life due to faulty chamber designs, the rectified 6.8 SPC II has proved to be an excellent choice for home defense as well as hog and whitetail hunting. Combine an effective range of 300 yards, a 25% boost in kinetic energy over the 5.56, and the capability to maintain its lethality in a 16” carbine, the 6.8 Remington Special Purpose Cartridge gives shooters a massive upgrade in terminal ballistics by simply swapping an upper receiver.

The lower cost of ammo and high levels of availability of 5.56/223 Rem ammo make it an appealing choice for most AR-15 owners. However, for those shooters who want more terminal ballistics from their carbine or the ability to hunt medium sized game should seriously consider upgrading to the 6.8mm SPC.

No matter which cartridge you choose, make sure you stock up on ammunition here at Ammo.com and I’ll see you on the range!