New .243 Service Rifle Cartridge, A Proposal
NOTE: This article is obsolete, as U.S. soldiers have let it be known that the 6.8mm SPC is their cartridge of choice. It functions in M-16 length actions. I am all in favor of giving the boys in uniform what they want and now support the universal adoption of the 6.8mm SPC by the U.S. military.
During my high school ROTC days, and later while on active duty with the United States Air Force, I had the opportunity to fire the M-1 Garand service rifle in .30-06 Spfd., the M-1 carbine in .30 Carbine, and the M-16 rifle in 5.56mm NATO. I also qualified with the S&W .38 Special service revolver, which was the U.S.A.F. sidearm of choice at the time. As a civilian, I have had some experience with sundry military caliber bolt action rifles, a .45-70 single shot rifle, the .45 Colt Single Action Army revolver, the .45 ACP M-1911 pistol, and the 9x19 Beretta 92 pistol, all of which types have been service standard at one time or another. This meager exposure to military small arms certainly does not make me an expert (although I did qualify as an "Expert" shooter, the top shooting classification in the Air Force), nor does my lifelong interest in the (civilian) sporting use of firearms.
However, like many shooters and gun enthusiasts, I am always interested in firearms related topics. One subject that has been hotly debated ever since the 5.56mm NATO (.223 Rem.) cartridge replaced the 7.62mm NATO (.308 Win.) as service standard for infantry rifles is the formers suitability as a military cartridge. This naturally brings up the question, if one is not a fan of the 5.56mm, of what its replacement should be.
As hunting cartridges, the .223 Rem. is best described as a varmint (ground hog) cartridge, and the .308 Win. as an "all-around" (antelope, deer, elk) cartridge. Since enemy soldiers are approximately the size of deer, not groundhogs, the .308 is the obvious choice between the two. But the U.S. military, led by the USAF (which was the first service to adopt the .223 cartridge and the M-16 rifle to fire it), decided otherwise. Apparently the overriding factor was the greatly reduced recoil of the .223, which allowed a very high rate of fire and (most of all) didn't intimidate the inexperienced conscripts that formed the bulk of the U. S. military at the time. Comparing both cartridges in 7.5 pound rifles, the .223 firing a 62 grain bullet at a 3,020 fps. generates only 4.07 ft. lbs. of recoil energy; the .308 firing a 150 grain bullet at 2,800 fps. generates 17.72 ft. lbs. of recoil energy. (All recoil figures are taken from the "Rifle Recoil Table" on my Guns & Shooting Page.)
The 5.56mm NATO and the M-16 rifle first proved themselves in the bloody and often short-range fighting of the Vietnam War. (Ironically, so did its main competition in the world military small arms market, the Russian 7.62x39 cartridge and AK-47 rifle, only on the other side.) But just because the handy .22 caliber M-16 rifle proved generally superior to the larger and heavier .30 caliber M-14 rifle for short range jungle combat in Vietnam does not mean that it is the optimum service rifle for all wars and theaters. In the intervening years, the advantages of the 5.56mm cartridge have become obvious, and so have its shortcomings. Perhaps it is time to initiate a dialogue about its future replacement.
On the 5.56mm's plus side are its low recoil, flat trajectory, and relatively small size and weight (compared to the 7.62mm NATO and most other service rounds). It is also a relatively inexpensive round to mass produce. The light weight of the rifles that chamber it, and its suitability for selective fire rifles that offer single, 3 round burst, or full automatic fire are also pluses.
Compared to its main competition in the infantry rifle cartridge sweepstakes, the 7.62x39 Soviet, the 5.56mm NATO cartridge has much higher velocity (for flatter trajectory), and slightly more energy downrange. Neither actually has much punch at medium to long range: at 200 yards they have 860-875 ft. lbs. of energy, and at 300 yards they are down to only 655-710 ft. lbs. (The velocity, energy, trajectory, and wind drift figures quoted in this article are taken from the 1998 Federal, Remington, and Winchester ammunition catalogs.) When you consider that 900 ft. lbs. of remaining bullet energy is generally considered the minimum for reliably killing an inoffensive deer, these numbers are not impressive. For what it's worth, at each range the slightly higher figure belongs to the 5.56mm.
On the 5.56mm NATO's debit side are its low energy, small caliber wound channel, poor ballistic coefficient, and poor sectional density. The first two factors are responsible for its poor killing power; the last two factors contribute to excessive wind drift and poor penetration, especially at longer ranges. Except for its larger diameter bullet (.30"), the 7.62x39 Soviet cartridge has the same drawbacks as the 5.56mm, plus considerably lower velocity, which makes it very difficult to hit long range targets.
Although many experts consider the 7.62x39 slightly superior it is, at best, a step sideways from the 5.56mm. Any proposed replacement should preserve, as much as possible, the virtues of the 5.56mm NATO (primarily light recoil and flat trajectory) while correcting its deficiencies in penetration, wind drift, and killing power.
Which brings us to the consideration of a replacement for the .223 service cartridge. It has been nearly 40 years since the U.S. military's decision in favor of the .223 Remington cartridge, and NATO's (reluctant) acceptance of the cartridge as the 5.56mm. Ever since the adoption of the miniature round, I have wondered why the military went for such a small caliber. The disadvantages are obvious to anyone with a rudimentary knowledge of exterior ballistics. In view of the controversy still surrounding this choice, it seems reasonable to speculate about a new service cartridge for the U.S. and NATO.
After careful consideration, I would like to propose that the .243 Winchester cartridge be considered as the successor to the 5.56mm NATO. The military would undoubtedly call the cartridge the "6mm NATO."
I chose the .243 Win. partly because of the old 6mm Lee Navy rifle of 1895, which was once service standard for the U.S. Navy. Thus, the U.S. military has some history with 6mm cartridges. I admit that the 6mm Remington, .257 Roberts, and other similar cartridges would serve about as well as the .243. I favor the latter because it is the best known and most popular of all the .24-.25 caliber cartridges, and because it was created by simply necking down the 7.62mm NATO case. I thought that case commonality would appeal to the military, and simplify mass production of both cartridges in wartime. (The 7.62mm remains the standard NATO machine gun cartridge.)
I also took a long look at the .25-08 wildcat and .260 Rem., both also based on a necked down .308 case. I determined that the .243 offered better penetration than the .25-08 with the same weight bullet at the same velocity, due to superior sectional density (SD), at no increase in recoil. The situation is similar when comparing the .243 to the .260, only more so. With the same weight bullet at the same velocity, the .260 is inferior to both the .25-08 and the .243 in penetration. The .260 requires approximately a 15% increase in bullet weigh to equal the .243 in sectional density (and thus penetration). I suspect that the military would find the resultant increase in recoil and decrease in velocity unacceptable.
Remember, the purpose of this exercise it to retain, as much as possible, the low recoil and flat trajectory of the 5.56mm NATO while addressing its shortcomings. The .243 preserves these benefits better than any of the other contenders. The .243 would have several significant advantages over the current 5.56mm NATO, which I will discuss in the following paragraphs.
Clearly, the .243 offers a modest but worthwhile increase in bullet diameter and frontal area. This increases lethality by enlarging the wound channel.
The sectional density of an 85-100 grain .24 caliber (6mm) bullet is far superior to that of any .22 caliber bullet. It also compares favorably to .30 caliber bullets. Sectional density is the ratio of a bullet's weight in pounds to the square of its diameter in inches. Other things being equal, sectional density is the primary factor in determining penetration. Thus, if we are comparing two similar non-expanding boat tail spitzer bullets (like typical military full metal jacket ball ammunition), fired at the same velocity, the one with the greater sectional density will penetrate deepest.
A 55 grain bullet for the 5.56mm NATO has a SD of only .157. A 150 grain bullet for the 7.62mm NATO has a SD of .226. This explains why, when the military changed from the 7.62mm (.30 cal.) to the 5.56mm (.22 cal.), they found that it didn't penetrate nearly as well. The poor penetration of the 55 grain .22 bullet led to the eventual adoption of the heavier 62 grain bullet for the 5.56mm NATO cartridge. Muzzle velocity fell to around 3,000 fps. Sectional density was increased to .177. Penetration improved, but is still considerably inferior to that of the 150 grain .30 caliber bullet. If we adopted the .243 Win. with a 95 grain bullet, whose SD is .230 (slightly better than the 150 grain .30 bullet), penetration would easily exceed that of the 62 grain .22 bullet. This is a valid comparison, as the .243 can drive a 95 grain bullet at 3,100 fps. I think it is clear that the .243 is a winner compared to the 5.56mm or the 7.62mm in terms of sectional density and penetration.
.243 bullets are also winners in terms of ballistic coefficient (BC). Without getting too technical, ballistic coefficient indicates a bullet's ability to overcome air drag. This is important for flat trajectory, and for minimizing wind drift. (The higher the BC, the slower a bullet sheds velocity, and consequently the less it drifts in the wind.) Ballistic coefficient is influenced by many factors, and changes with velocity, so all BC figures should be taken as approximate.
For example, using Nosler Ballistic Tip (boat tail spitzer) bullets for comparison, the 150 grain .30 caliber bullet has a BC of .435. The 95 grain .243 bullet has a BC of .379. The 55 grain bullet for the 5.56mm NATO has a BC of only .267, despite its streamlined appearance.
At typical 5.56mm velocities, this bullet's lateral drift at 300 yards in a light 10 MPH crosswind is 14.2 inches. This is enough to blow a perfectly aimed bullet completely off a man-size target! The 5.56mm 62 grain FMJ-boat tail spitzer has a BC of .307. This is still very inferior to the BC of the .243's 95 grain bullet. At typical .243 velocities, the 95 grain bullet's lateral drift at 300 yards in a 10 MPH crosswind is about 6.3 inches.
The 5.56mm NATO is a flat-shooting cartridge, much better than the 7.62x39 and somewhat superior to the 7.62mm NATO. From a rifle zeroed at 200 yards, a 55 grain Ballistic Tip bullet at a MV of 3,240 fps. hits 20.8 inches below the point of aim at 400 yards.
For comparison, a 150 grain 7.62mm NATO Ballistic Tip bullet at a MV of 2,820 fps hits 22.7 inches low at 400 yards. Zero a 7.62x39 Soviet rifle at 200 yards, and the typical FMJ bullet hits 43.5 inches low at 400 yards. (That's over 3 feet below the point of aim!)
But the .243 shoots even flatter than the 5.56mm. From a rifle zeroed at 200 yards, the 95 grain Ballistic Tip bullet at a MV of 3100 fps hits only 18.9 inches below the point of aim at 400 yards.
Clearly, when it comes to slipping through the air, the .24 caliber bullets are among the best. As civilian varmint shooters have known for years, the .243 is an excellent long range cartridge that combines a very flat trajectory with minimum wind drift.
Civilian deer, sheep, goat, and antelope hunters know that the .243 Win. is a much better killer on animals in the 100-350 pound class than the .223 Rem. A 95 grain .243 boat tail spitzer bullet at a muzzle velocity of 3,100 fps. retains 1,455 ft. lbs. of energy at 200 yards, 1,225 ft. lbs. at 300 yards, 1,024 ft. lbs. at 400 yards, and 890 ft. lbs. at 500 yards (Winchester figures). The .243 is more lethal at 500 yards than the 5.56mm NATO or 7.62x39 are at 200 yards!
Civilian shooters have also learned that the recoil of the .243, even in a lightweight rifle, is quite tolerable for extended shooting sessions. Light recoil is very desirable, not only to avoid flinching and promote accurate shooting, but because modern military rifles must be capable of delivering rapid aimed fire. (Rapid unaimed fire is pointless--you can't miss fast enough to win a gunfight.) To shoot both quickly and accurately, recovery time from full recoil must be rapid. While the .243 Win. kicks more than the 5.56mm NATO, it kicks much less than the 7.62mm NATO, and does in fact allow quick recovery. A 7.5 pound .243 rifle shooting a 95 grain bullet generates around 10 ft. lbs. of recoil energy. This is about half of what an experienced shooter can tolerate. Even inexperienced shooters will not find this bothersome. Light recoil (plus flat trajectory and proven effectiveness) is why the .243 is such a popular hunting cartridge, and so widely recommended for youth, women, and anyone sensitive to recoil.
A modern selective fire military rifle with single fire and three-shot burst capability should be easy to develop for the .243 Win. cartridge. After all, the .243 case is based on the 7.62mm NATO case, which was developed specifically for use in automatic rifles. Sustained fully automatic fire is also quite possible for a 6mm service rifle, although I question its value. Even with a cartridge as under powered as the 5.56mm NATO, full auto fire has proven to be a waste of ammunition. Bullets must be aimed if they are to hit the target, and experience has shown that if a shooter can't hit the target with his first three shots, he probably won't hit the target at all.
On the other hand, a bipod mounted light machine gun (a successor to the old BAR) chambered for the long range, hard hitting .243 Win. cartridge might be a very effective weapon. However, that is a subject for another article.
Copyright 2001, 2012 by Chuck Hawks. All rights reserved.