The 6mm Lee Navy (6×60mmSR), also known as the 6mm U.S.N.Hanson, Jim, The 6mm U.S.N. - Ahead Of Its Time, Rifle Magazine, Vol. 9, No. 1 (January–February 1977), pp. 38-41 or .236 Navy, is an obsolete American rifle cartridge.Barnes, Frank C., ed. by John T. Amber, "6mm Lee Navy" in Cartridges of the World (Northfield, IL: DBI Books Inc., 6th ed., 1984), p. 102 It was the service cartridge of the United States Navy and Marine Corps from 1895, replacing the .45-70 Government cartridge, and until 1899 when it was succeeded by the .30-40 Krag.
History and development By 1894, the U.S. Navy desired to adopt a modern small-bore, smokeless powder service cartridge and rifle in keeping with other first-line naval powers for both naval and marine forces.Sampson, W.T., Annual Reports of the Navy Department: Report of Chief of Bureau of Ordnance, Washington, D.C.: U.S. Government Printing Office (1895), pp. 215–218 Naval authorities decided that the new cartridge should be adaptable to both rifles and machine guns.The New York World, The World Almanac and Encyclopedia: Rifles Used by the Principal Powers of the World, Vol. 1 No. 4, New York: Press Publishing Co. (January 1894), p. 309: The Naval Small Arms Board reported that in adopting the 6 mm government cartridge specification, "due consideration has been given to the desirability of using the same cartridge for machine guns as for the small arm, and the Board deems that no difficulty in the manufacture or manipulation of machine guns will be caused by their use of 6 mm ammunition." Noting that the world's military forces were adopting smaller and smaller caliber rifles with higher velocity cartridges, U.S. naval authorities decided to leapfrog the trend by adopting a cartridge in 6 mm (0.236 inch) caliber, with a semi-rimmed case capable of holding up to of 'Rifleite' smokeless powder and proof chamber pressures of up to . While the government cartridge was being developed, the Navy tested rifle barrels in various alloys and rifling twists, eventually settling on a barrel steel made of 4.5 percent nickel steel, with a rifling twist of one turn in .
As the Chief of the Bureau of Naval Ordnance stated in his 1897 Small Arms report to the Secretary of the Navy, "In making what may appear a radical departure in its selection of a caliber smaller than as yet adopted elsewhere [...] the Bureau is convinced it is looking to the future [...] it is surely wise to attempt to advance with one stride as far as existing conditions allow toward the goal to which the world is moving with slow steps."Sampson, W.T.,Annual Reports of the Navy Department: Report of Chief of Bureau of Ordnance, Notice to Inventors and Others, Washington, D.C.: U.S. Government Printing Office (1894), pp. 385–387 The report went on to list the advantages of the smaller 6 mm caliber: greatly increased velocity, the flatness of bullet trajectory, reduced recoil, a 100% increase in penetration compared to the former .45-70 Government cartridge, and the ability to carry twice the number of cartridges per individual sailor or Marine. The report also acknowledged two disadvantages of the 6 mm round: its small caliber would not sufficiently wound an enemy to put them out of action; the "shock" or stopping power of the smaller bullet would not "stop the onset of excited men at short range". In answer to these objections, the report gave three responses: first, "the battle of the future will be fought at long range, and men will not live to come to close quarters with an enemy who stands his ground"; second, "99 percent of wounded enemy soldiers were unlikely to investigate the severity of their wound, but simply retire to the rear", and last, "the explosive effect of a small-caliber, high-velocity bullet against the human body—the bullet tumbles or fragments to produce devastating wounds against bone or fluid-filled organs—would be more incapacitating at all ranges than wounds made by a slow-moving bullet of large caliber".
On August 1, 1894, a naval test board was convened at the Naval Torpedo Station in Newport, Rhode Island, to test submitted magazine rifles in the new 6mm Navy caliber developed by naval ordnance authorities. The government notice of bid requirements said no other calibers or cartridges would be considered. Winchester made the ammunition for the government, which in turn provided each competitor with 6 mm USN cartridges and 0.236-inch caliber rifle barrels, the latter supplied unchambered and with receiver threads uncut. The rifle action was required to withstand the firing of five overpressure (proof) cartridges with a chamber pressure of 60,000 psi. Several inventors and rifle manufacturers submitted models for testing, among them James Paris Lee, who offered two designs, a turnbolt and a cam-action or locking bolt type, the Lee straight-pull. The Lee straight-pull won in repeated trials and the U.S. Navy adopted the round in 1895 as the Lee Rifle, Model of 1895, caliber 6mm.Twining, N.C. (Ens.), Dept. of the Navy, The Annual Reports of the Navy Department: Report of the Secretary of the Navy – Annual Report of the Inspector of Ordnance, pp. 319–323The Annual Reports of the Navy Department: Report of the Secretary of the Navy – Report of Naval Small Arms Board, November 22, 1894, Washington, D.C.: United States Navy Dept. (1894), pp. 306–310The Annual Reports of the Navy Department: Report of the Secretary of the Navy, Washington, D.C.: United States Navy Dept. (1895), p. 308Dieffenbach, A.C., (Lt.) Dept. of the Navy, The Annual Reports of the Navy Department: Report of the Secretary of the Navy – Report on the Manufacture and Inspection of Ordnance Material, Washington, D.C.: United States Navy Dept. (1897), pp. 321–323
As adopted by the Navy after tests at the Naval Torpedo Station, the ball cartridge, 6mm or 6 mm USN cartridge, featured a long, bottle-necked semi-rimmed brass case and a round-nosed, cupro-nickel coated, steel-jacketed, lead core bullet.Ouston, Alex, The United States Navy Rifle and the Injuries It Produces, British Medical Association: British Medical Journal, Vol. 1, (1899), p. 756Beyer, Henry G., Observations On The Effects Produced By The 6-mm Rifle and Projectile, Vol. 3, Boston Medical Journal, Boston Society of Medical Sciences (1899), pp. 117–136 The initial version utilized a 135-grain steel-jacketed lead core bullet with a 2.35-inch-long case and 0.517-inch rim, with a bullet diameter of 0.244 inches (the bore diameter before rifling was 0.236 inches). The rifle powder for the new cartridge was Rifleite, a flake nitrocellulose smokeless powder originally developed for use in the British .303 cartridge.Reports of Companies, The Chemical Trade Journal and Oil, Paint, and Colour Review, Vol. 18, June 20, 1896, p. 401: The Smokeless Powder Co., Ltd. originally developed Rifleite for the British .303 cartridge.Walke, Willoughby (Lt.), Lectures on Explosives: A Course of Lectures Prepared Especially as a Manual and Guide in the Laboratory of the U.S. Artillery School, J. Wiley & Sons (1897) p. 343: Rifleite was a flake smokeless powder composed of soluble and insoluble nitrocellulose, phenyl amidazobense, and volatiles similar to French smokeless powders; unlike cordite, Rifleite contained no nitroglycerine.
The new 6 mm cartridge achieved a muzzle velocity of 2,550 fps (777 m/s) and was initially manufactured by Winchester (WRA) for the Navy. Because of production delays and the desire to have a secondary ammunition source in the event of war or another emergency, the Union Metallic Cartridge (UMC) Company was also awarded a contract to produce 6 mm ammunition. After the adoption of the M1895 Lee, the case rim of the government cartridge was redesigned with a slight bevel (rebated rim) for optimum performance in the en bloc charger-loaded magazine of the M1895 rifle. The revised cartridge was designated the 6 mm U.S.N., but continued to be referred to in naval stores reports as the ball cartridge, 6 mm. Owing to difficulties in obtaining acceptable lots of Rifleite smokeless powder for the cartridge, as well as cartridge clips, Winchester and UMC did not fill the initial April 1896 Navy order of 1,000,000 cartridges for more than a year.
In March 1897, after reports of excessive chamber pressures and short barrel life (3,000 rounds) despite the use of 4.5 percent nickel-steel barrels, the specifications for the 6 mm round were changed to incorporate a round-nosed lead core bullet with a copper jacket, which was then tin-plated to prevent corrosion from salt air.Newton, Charles, Outing, Ballistics of Cartridges: Military Rifles III, Outing Publishing Co. Vol. 62 (1913), pp. 370–372 Winchester and UMC had difficulty in achieving the pressure limit specification (43,000 psi) for the new loading with Rifleite, and chamber pressures varied widely from lot to lot of powder, ranging up to 60,000 psi. The new M1895 Colt–Browning machine gun was also modified to accept the new loading the same year, and a further 500,000 cartridges were ordered to accommodate machine-gun usage.
Firing a bullet at from a barrel,Sharpe, Philip B., The Rifle In America (2nd ed.), New York: Funk & Wagnalls Co. (1947), pp. 235–236 the 6 mm U.S.N. was the highest-velocity cartridge used by any military force at the time of its adoption. Designed to achieve better penetration than the Army's .30 Army cartridge used in the Model 1892/98 (Krag) rifle, the round was intended to perforate the hulls of small enemy craft such as patrol and torpedo boats, and could penetrate of softwood at , of low-carbon steel at 10 feet (3 m); 0.375-inch (9.5 mm) of ordinary boiler plate at , or a -thick suspended chrome steel alloy boiler plate at . The round-nosed bullet had a flatter trajectory than its predecessor at all ranges up to 500 yards; above this distance, the heavier bullet began to show an advantage. The new loading had an effective range (individual rifle fire) of 600-700 yards, at which point the blunt-tipped bullet rapidly lost velocity and energy. The bullet, like its predecessor, demonstrated an inclination to instability unless rapidly rotated. The M1895 Lee rifle therefore received a rapid barrel rifling twist of one turn in 7.5 inches (19 cm) (with right-hand twist) to keep the bullet from wobbling or tumbling at extended ranges. At some point, this rifling was changed in later production rifles to one turn in 10 inches RH (25 cm).Mann, Franklin W. (Dr.), The Bullet's Flight From Powder to Target: The Internal and External ballistics of Small Arms, New York:Munn & Co. Publishers (1909), p. 303 Contemporary medical reports of the day noted the bullet produced noticeably greater damage to tissue and bone than other contemporary military cartridges when fired at full velocity (2,560 fps), and the bullet's copper jacket frequently fragmented or detached entirely from its lead core after penetrating a hard substance such as bone or metal. However, when the 6 mm bullet struck muscle or air-filled cavities such as the lungs, the bullet tended to make a perforating wound of small diameter that caused little damage.Plummer, Ralph W., Annual Report of the Surgeon General, U.S. Navy, to the Secretary of the Navy: Report of Ralph W. Plummer, Assistant Surgeon, U.S.N., U.S.S. Princeton, U.S. Navy Dept., Bureau of Medicine and Surgery, Washington, D.C.: U.S. Government Printing Office (1900), pp. 322–323