Necked UP?
The .308 bore diameter is 7.62mm. It's necked DOWN to 7mm, not up, unless 7mm-08's actual bore diameter is like 7.7mm, which would be silly, but possible.
Anyway, I think a 6mm round would be great, but one with more compact case dimensions, to allow greater magazine capacity without excessive weight.
Here's what Chuck Hawks
http://www.teleport.com/~chalu/index2.htm has to say about the .243 as a military cartridge.
NEW .243 SERVICE RIFLE CARTRIDGE, A PROPOSAL
(part one, as the whole thing is too long for one post)
By Chuck Hawks
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 assault 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 U.S.A.F. (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 (compared to the 7.62mm NATO service round). 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 (I favor bullets in the 90-95 grain range for the .243). 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.
