Compared: The 6.8mm Rem. SPC and 7.62x39 Soviet
By Chuck Hawks
As Ruger now offers their Ranch Rifle in 6.8mm Remington SPC, perhaps a comparison article with that cartridge and the 7.62x39 Soviet service cartridge is in order. The 6.8mm SPC was designed specifically for the American M-16 and the 7.62x39 was designed specifically for the Red Army's SKS and AK-47 assault rifles, the favored weapons of terrorists around the world, but both cartridges have subsequently been adapted to other firearms, including bolt action sporters and the popular Ruger Ranch Rifle. The Ranch Rifle is not a military arm, but it was inspired by the US M1 Carbine, which was.
6.8mm Remington SPC
The 6.8mm SPC was designed as a replacement for the less effective 5.56mm NATO round, particularly for Special Forces use. (SPC stands for "Special Purpose Cartridge.") "6.8mm" is Army speak for ".270" and the 6.8mm uses standard .277" diameter bullets, the same as the famous .270 Winchester. The US military, and particularly Special Forces troops, have found the 5.6mm NATO cartridge's little .224", 55-62 grain bullets lacking in stopping power, hence the desire for a larger caliber cartridge that can shoot heavier bullets.
The 6.8mm SPC is based on a shortened, blown-out and necked-down .30 Remington case loaded to a maximum cartridge overall length of 2.260", the longest cartridge that the M-16 action can digest. Remington factory ballistics call for a 115 grain bullet at a muzzle velocity (MV) of 2625 fps and muzzle energy (ME) of 1759 ft. lbs. from a 24" barrel. The Ruger Ranch Rifle, like most military style carbines, comes with a short 18.5" barrel and that entails significant velocity loss. However, bolt action 6.8mm SPC hunting rifles usually have longer barrels.
Because of the 6.8mm's limited case capacity, reloaders find it difficult to equal the factory load ballistics within safe pressure limits. The SAAMI maximum average pressure (MAP) for the 6.8mm SPC is 55,000 piezo psi. Due to the popularity of the .270 Winchester cartridge, 6.8mm SPC reloaders have a plethora of .277" bullets from which to choose. Bullets weighing no more than 130 grains are most suitable for the small 6.8mm SPC cartridge, with 115-120 grain bullets being ideal.
The 7.62x39 was designed during WW II as an intermediate range (between pistol and main battle rifle) cartridge to counter the German's then new and successful 8x33mm assault rifle and cartridge. The 7.62x39 is no longer the front line military cartridge of Russia, but it remains the service standard in other countries, including Red China.
This is not a true 7.62mm (.30 caliber) cartridge. Like other Russian/Soviet cartridges, it was designed for .303" bore diameter rifles and to use .311" diameter bullets. (Note: Ruger 7.62x39 rifle barrels are produced with a bore diameter of .300" and shoot .308" diameter bullets to facilitate reloading by American shooters.) Because 7.62x39 ammunition has been produced in many countries, sometimes without (to put it mildly) rigorous quality control, the ballistics of the cartridge are somewhat flexible. Typical, good quality, American factory loads drive a 123-125 grain bullet at a MV of about 2300-2365 fps (depending on brand) with approximately 1445-1552 ft. lbs. of ME from a 24" barrel. Short barreled assault rifles will produce considerably lower velocities.
For the hunter and recreational shooter the 7.62x39 is hard to categorize. It is an "in-between" cartridge. It is noisy and lacks the flat trajectory necessary for a varmint cartridge, yet it is not powerful enough to be an efficient big game cartridge. With good bullet placement, it will admittedly kill the smaller species of deer and antelope, but there are a great many better cartridges for the deer hunter. It is probably at its best for use on tough little animals like javelina and small feral pigs that might run 40-70 pounds, a rather limited application.
Because of the 7.62x39's small case capacity, reloaders cannot meaningfully exceed the factory load velocity within the specified MAP, which is 50,000 cup. Most US bullet makers have introduced 120-125 grain .31" diameter bullets for reloaders that are specifically intended for use in the 7.62x39 cartridge, although the selection of bullet weights and styles is limited compared to the choices in .277" and .308" bullets.
We will compare these two cartridges based on velocity, kinetic energy, ballistic coefficient (BC), trajectory, bullet cross-sectional area, sectional density (SD), killing power and recoil. At the end of the comparison, there will be some comments about rifle and cartridge availability and concluding remarks. As the basis of comparison, we will use common Remington factory loads shooting expanding hunting bullets. Our comparison load for the 7.62x39 will use the Remington 125 grain Pointed Soft Point (or spitzer) bullet and our comparison load for the 6.8mm SPC will use the Remington 115 grain Core-Lokt Ultra Bonded, a similarly shaped spitzer (pointed) bullet.
Velocity is important because higher velocity flattens trajectory and makes hitting at longer ranges easier. Velocity is also an important factor in computing kinetic energy. Here are the velocities in (feet per second) for our comparison loads from the muzzle to 300 yards.
As can be seen from those figures, the 6.8mm SPC operates at considerably higher velocity than the 7.62x39. How this will affect trajectory, energy and killing power we will soon find out.
Kinetic energy is important because it powers bullet expansion, penetration and is an important factor in killing power. Here are the energy figures (in foot-pounds) for our cartridges from the muzzle to 300 yards. Bear in mind that a minimum of 800 ft. lbs. at bullet impact is generally regarded as the minimum required for hunting CXP2 game such as deer.
As can be seen from these figures, the 6.8mm SPC is a considerably more powerful cartridge than the 7.62x39mm, putting about as much energy on target at 300 yards as the 7.62x39 does at 200 yards. For the big game hunter looking for a minimum recoil rifle/cartridge combination, a 6.8mm SPC hunting rifle with a 24" barrel would definitely be the better choice. Unfortunately, such rifles are hard to find.
Ballistic coefficient is a measure of how well a bullet passes through the air; essentially, how streamlined it is. Sectional density (the ratio of a bullet's mass to its diameter) plays a role here, as a longer, skinnier bullet usually flies through the air more efficiently than a shorter, fatter bullet. The higher the BC, the better in terms of achieving a flat trajectory and retaining velocity and energy downrange. Here are the ballistic coefficients of our two bullets.
Because the similarly shaped 6.8mm SPC's .277" bullet is almost as heavy as the 7.62x39's .311" bullet, it causes less drag and flies through the air better. We will next see how this affects trajectory. Note that both of these bullets are relatively short and light for their caliber and neither sports a particularly impressive BC in the greater scheme of things. The Remington 100 grain Core-Lokt PSP bullet used in the .243 Winchester cartridge, for example, sports a .356 BC and a 160 grain Hornady LeverEvolution (FTX) bullet for the .30-30 cartridge has a .330 BC.
Trajectory is important because the flatter a cartridge shoots the easier it is to hit with as the range increases. A flat trajectory, for example, has long been the 5.56mm NATO service cartridge's primary advantage over the 7.62x39. Most of the time this made little difference in the close range jungle fighting so often encountered during the war in Vietnam, but it can matter in the open terrain of the Middle East. Likewise, the woods hunter has less need for a flat shooting cartridge than the plains or mountain hunter. Here are the "long range" trajectories for our two cartridges as shown in the 2008 Remington catalog, calculated for a scoped rifle with a line of sight 1.5" over bore.
As can be seen from these trajectories, the 6.8mm SPC out-ranges the 7.62x39mm by about 50 yards. The 7.62x39 is limited by both its remaining energy and its trajectory to about 200 yard shots, while the greater energy and flatter trajectory of the 6.8mm SPC allows for shots out to about 250 yards on the smaller species of CXP2 game.
Bullet Cross-Sectional Area
The cross-sectional area of a bullet is important because, given equal ratios of bullet expansion, the fatter bullet creates a wider wound cavity and destroys more tissue. Here are the cross-sectional areas of our two bullet diameters in square inches.
It should be obvious that a .303 bullet has a greater frontal area than a .270 bullet, so the foregoing numbers should come as no surprise. Greater bullet diameter is, in fact, the 7.62x39's primary advantage over the 6.8mm SPC.
Sectional density is the ratio of a bullet's weight to its diameter. SD is calculated by dividing a bullet's weight (in pounds) by the square of its diameter (in inches). SD matters because, other factors such as bullet expansion being equal, the bullet with the greater SD will penetrate deeper, creating a longer wound cavity and destroying more tissue. With directly comparable cartridges, bullet cross-sectional area and SD tend to cancel out in terms of total tissue destruction. The fatter bullet creates a larger diameter, but shorter wound cavity and the bullet with superior SD plows a longer, but narrower wound cavity. Naturally, to harvest game, any bullet must have sufficient penetration to reach the vitals.
The 6.8mm SPC bullet is superior in this important category. Most experts would agree that for reliable penetration on CXP2 game, a bullet with a SD of at least .200-.205 is required. The 115 grain 6.8mm bullet surpasses this minimum, but the 125 grain 7.62x39 bullet does not.
There are many ways to estimate killing power, most of which are not particularly accurate. Admittedly not perfect, but displaying a positive correlation with results in the field, is the Guns and Shooting Online Rifle Killing Power Formula. This uses the bullet's retained energy, sectional density and cross-sectional area and is usually calculated for 100 yards. Here are the killing power numbers at 100 yards for the 6.8mm SPC and 7.62x39 (taken from the "G&S Online Rifle Killing Power Formula and List" on the Tables, Charts and Lists page), along with a few other common cartridges listed for comparison.
These figures show that the 6.8mm SPC is superior to the 7.62x39 and about equal to the .243 in killing power at 100 yards. The 7.62x39, while superior to the .223, is clearly inferior to all of the other cartridges listed. Note, in particular, the discrepancy in killing power between the 7.62x39 and the .30-30, a cartridge to which it is sometimes compared. (There is a detailed comparison of these two cartridges on the Rifle Cartridge page.)
Recoil is bothersome to all shooters and a rifle that kicks less can be shot more accurately, particularly as the number of rounds expended mounts. Accurate bullet placement is, by far, the most important factor in killing power. Here are the recoil figures for our two cartridges, in foot-pounds of energy, taken from the "Rifle Recoil Table" on the Tables, Charts and Lists page.
The 7.62x39 delivers one foot-pound less recoil energy in a rifle that is 1/2 pound lighter, a significant difference. However, neither the 6.8mm SPC nor the 7.62x39 are hard kicking cartridges in the greater scheme of things and very few shooters will be bothered by either.
Most shooters used to the recoil of rifle cartridges such as the .30-30 Win. will find the 6.8mm SPC and 7.62x39 to be mild cartridges. For comparison, in 7.5 pound rifles a typical .30-30 Win./170 grain load delivers 10.8 ft. lbs. of recoil energy
As we have seen, the 6.8mm SPC is superior to the 7.62x39 in most areas. It shoots flatter and hits harder without generating objectionable recoil. It is also a more accurate cartridge and the 6.8mm SPC has done well in military style match shooting.
Because of its low recoil, it would be a good cartridge for which to chamber an ultra-light bolt action rifle. It is a fine choice for the new or recoil shy deer and antelope hunter and would be perfectly adequate as a small predator cartridge, although the lighter .223 Remington will serve just as well for that purpose. Zero a 6.8mm SPC rifle to hit 3" high at 100 yards and it should have a useful MPBR (+/- 3") of about 250 yards for hunting CXP2 game, with killing power to match.
Should the 6.8mm SPC be adopted by the US military, it will undoubtedly be a commercial success. However, as I write these words, it seems unlikely that Congress will vote the funds to make that possible. That means that the 6.8mm SPC will likely have to depend on the civilian shooter for its success. So far, it has not set any sales records and suitable hunting rifles are very rare.
For the foreseeable future, factory loaded ammunition for the 7.62x39 will be cheaper and much more available than 6.8mm ammo. Someone looking for a utility rifle who will only shoot factory loads might want to consider that. It will be interesting to see how the 6.8mm SPC fares in the Ruger Ranch Rifle.
On the other hand, the 6.8mm SPC is a better choice for the reloader because it is a more versatile cartridge and there are more suitable hunting bullets available in .270 caliber than in .303 caliber. In addition, much 7.62x39mm factory loaded ammo is imported from Third World countries and uses steel cases and Berdan primers, so it cannot be reloaded.
In summation, the 6.8mm SPC is clearly superior to the 7.62x39 Soviet cartridge for sporting purposes. For the hunter, reloader and recreational shooter, the 6.8mm SPC seems like the way to go. A rifle in 7.62x39mm might suit the person shopping for a bargain basement utility rifle who intends to shoot only inexpensive factory loaded ammunition.
Copyright 2008, 2012 by Chuck Hawks. All rights reserved.