The 6mm Optimum
March 17, 2004
By Stan Crist. Reprinted by permission.
During World War II, the Wehrmacht developed the first assault rifle to be issued on a large scale. The StG44 was chambered for the 7.92x33mm round, a cartridge of lesser power and more compact size than that fired by the standard infantry rifle. Because studies had shown that most infantry combat occurred at relatively short engagement distances, it was deemed needlessly wasteful to continue using the heavier, bulkier ammunition.
In contrast, when Germany’s paratroopers undertook the development of the FG42 — the world’s first practical, select-fire, battle rifle — they chose to use the full-power, 7.92x57mm service cartridge. Because they had been outranged by British riflemen and machine gunners during the early stages of the 1941 airborne assault on Crete, the paratroopers reasoned that it was decidedly better to have long-range capability and not need it, than to desperately need it and not have it!
After WWII, these opposing philosophies appeared again, during the effort by NATO countries to adopt a standard rifle cartridge. One faction advocated the .280 British assault rifle round, while the U.S. Army wanted to retain the range and power of the .30 caliber cartridge then in use. As the most influential member of NATO, the United States got its way, and the 7.62x51mm round became NATO-standard shortly after the end of the Korean War.
Another reason the U.S. preferred full-power ammunition was to simplify logistics. The Army had fought WWII and the Korean conflict with a wide variety of small arms, chambered for two different calibers. The standard .30 caliber cartridge was used in the M1 infantry rifle, M1D sniper rifle, M1918A2 Browning automatic rifle, M1917A1, M1919A4 and M1919A6 machine guns, while the .30 Carbine round was fired in the M1, M1A1, M2 and M3 carbines. The Army wanted to replace this menagerie with only two basic weapons — a rifle and a general purpose machine gun — and one caliber of ammunition.
Sadly, this very worthwhile objective was doomed to failure. The substantial recoil of the 7.62 NATO round caused the M14 rifle to have poor controllability in full-auto fire, even in the heavier, M14A1 automatic rifle version. And, by insisting on a full-power cartridge, the Army ensured that the M14 — which was originally intended to be a “light rifle” — would be nearly as heavy as its predecessor!
The weight factor, perhaps more than any other, sealed the M14’s fate. In the early 1960s, the Air Force — which had previously refused to adopt the M14 — purchased the AR15, a rifle that was almost as delightfully lightweight as the obsolescent carbines then in its inventory. Not long after that, the Secretary of Defense directed the Army to also buy the AR15 (subsequently dubbed the M16A1), and discontinue acquisition of M14 rifles, thereby forcing the Army back into a two-caliber system.
Could the Army have taken a course that would have avoided a two-caliber system? Clearly, not by sticking to the demand for a full-power 7.62mm round. As the British had correctly noted, the power of the ammunition determines the size and weight of both the cartridge and the weapon. Nor could it have been avoided by adopting the .280 British caliber. Every nation that has issued assault rifles to its armed forces has found it necessary to also retain a full-power cartridge in the inventory for use in machine guns and sniper rifles.
Considering the difficulties involved, is a one-caliber family of small arms any longer an achievable goal? To answer that question requires a look at the desired characteristics of the infantry rifle, the machine gun, and the sniper rifle:
Infantry rifle: The weapon and ammunition should weigh as little as possible, consistent with a maximum effective range of at least 500 meters.
Machine gun: Same weight considerations as for the infantry rifle, but with maximum effective range of 1100-1200 meters. Ball ammunition should be able to defeat “hard” targets at least as well as 7.62 NATO rounds. Tracers should be visible to over 800 meters during daylight.
Sniper rifle: Weight of weapon and ammunition is not as important as accuracy and effective range, which should be greater than 800 meters, with a flat trajectory.
Of the above characteristics, the sniper’s need for a flat trajectory and short time-of-flight to the target would seem to be the most critical, so that is the logical start point in designing the optimum small arms cartridge. In order to achieve a flat trajectory, the bullet must have a very streamlined shape and be propelled at fairly high velocity.
A bullet’s shape can be judged by its “ballistic coefficient” (BC) — the larger the BC, the more streamlined the projectile. For instance, the Navy .300 Winchester Magnum load has a 190-grain bullet with a BC of 0.54, whereas the projectile of the 7.62mm M80 Ball round is only about 0.42. The .300 Win Mag’s high BC, combined with a muzzle velocity of 3000 fps, produces a very flat trajectory and short flight time. To incorporate these characteristics into the optimum cartridge will mandate the use of a bullet with a similarly high BC.
In order to achieve the penetration capability needed by the machine gun, projectile weight will have to be substantially heavier than the 62 grains of 5.56mm M855 Ball. At the same time, it must also weigh less than the 147 grains of 7.62mm M80 Ball in order to minimize the infantryman’s load. A bullet weight of about 100 grains seems like a reasonable, if intuitive, compromise.
Without resorting to exotic, expensive materials like tungsten or depleted-uranium, a 100-grain bullet with a BC of 0.54 will have to be made in 6mm caliber. 6.5mm and 7mm projectiles of this weight have inadequate BCs, while technical factors prohibit the use of such heavy, lead-core bullets in 5.56mm. A 100-grain, 6mm projectile that is launched at close to 3000 fps would have not only the flat trajectory of the .300 Win Mag, but it should also have penetration capability on a par with 7.62 NATO. The photo below shows the 6mm Optimum projectile flanked by 5.56mm NATO and 7.62mm NATO, then the three with cartridges.
Another benefit of 6mm ammunition is improved tracer performance. The 6mm XM734 round, developed in the 1970s at Frankford Arsenal, reportedly produced a trace that was visible in daylight to 1000 meters. That is better than either 5.56mm or 7.62mm tracer performance!
The cartridge case for the 6mm Optimum would naturally have to be larger than that of 5.56 NATO, but it would also be significantly smaller than 7.62 NATO. To minimize cartridge case volume, muzzle velocity might have to be limited to perhaps 2900-2950 fps, but this should still produce exceptional performance. In essence, it would be a magnum version of the 6mm XM732 Ball round made for the squad automatic weapon program in the 1970s.
The Army had a logistically sound idea in trying to create a one-caliber, two-weapon, small arms system. Unfortunately, the wrong caliber was chosen, and a golden opportunity was lost. By insisting on developing the best 7.62mm cartridge, rather than the best possible cartridge, the logistical situation of today is at least as complex as it was in the 1940s, with the 5.56x45mm cartridge for use in the M16A2 rifle, M4 and M4A1 carbines, and M249 light machine gun, while the 7.62x51mm round is fired in the M21 and M24 sniper rifles, and the M60, M240B, and M240G medium machine guns.
One caliber can do it all. By sending a 100-grain, very low drag bullet downrange at over 2900 feet per second, the 6mm Optimum would provide snipers with the flat trajectory of the .300 Winchester Magnum. It would give machine gunners the penetration potential and tracer capability of 7.62 NATO, thereby permitting the development of an infantry machine gun that is light enough to replace both the 7.62mm medium machine gun and the 5.56mm squad automatic weapon. And, by virtue of being a compact, lightweight cartridge, with low recoil impulse, it should also allow the creation of a combat rifle that is little or no heavier than the M16A2.
The 6mm Optimum combines the best features of several existing cartridges into a compact, lightweight round that should be capable of all around performance unequaled by any other caliber. If the future brings about another quest for a single caliber common to the infantry rifle and machine gun, let the 6mm Optimum be the one for all!
), so I really couldn't tell you that. However, it wasn't I who suggested it, so I think the person that did (I wanna say it was PercyShelley) knew what they were doing.
