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. Naval authorities decided that the new cartridge should be adaptable to both
rifles and
machine guns. 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 . 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. As adopted by the Navy after tests at the Naval Torpedo Station, 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. The new 6 mm cartridge achieved a
muzzle velocity of 2,550 fps (777 m/s) 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 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 , 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. The 6 mm Lee Navy cartridge proved expensive to produce, with a per round cost approximately double that of the previous .45-70 service cartridge. Colonel-Commandant
Charles Heywood of the Marine Corps reportedly refused initial deliveries of small numbers of Lee rifles to Marine battalions until he received assurances that an appropriation would be made to the Corps for at least 3,000 Lee rifles, improved target ranges, and enough ammunition to continue existing marksmanship training programs. Despite this threat, the report of the Marine Corps quartermaster as of September 1897 begged the secretary for an additional $10,000 appropriation for 6 mm ammunition to allow the Marines to conduct live fire and target practice with the Lee rifle. The report went on to warn that, except for drill practice, enlisted Marines were "entirely unfamiliar with the use of this arm", since all target practice had to be conducted using the old single-shot Springfield and .45-70 ammunition. The round was also issued to small formations of attached Cuban rebels participating in the
Guantánamo Bay campaign. The extra cartridges proved useful when early ammunition resupply from Navy ships was disrupted at the outset of the Guantanamo operation, allowing Marines to continue their assault even while individually resupplying Cuban rebels who had run short of ammunition. However, the high velocity 6 mm round was ahead of its time in terms of powder technology and barrel steel metallurgy. In addition to complaints of bore fouling with corrosive nitro-compounds M1895 rifles in service began to display signs of early barrel wear and throat erosion caused by inconsistent chamber pressures and powder burn rates, despite the Navy's requirement of nickel-steel barrels. Several factors led to the discontinuance of the 6mm Lee as an official U.S. military cartridge. The most important was the decision of the war department, based upon a recommendation by a board composed of Army, Navy, and Marine Corps officers, to standardize on the .30 Army cartridge for all services. Additionally, the 6 mm's special—and expensive—semi-rimless beveled-rim case was designed to work optimally in the charger clips of the M1895 rifle. This proved an unnecessary complication when adapting the cartridge to other, more modern machine guns and small arms. A further complication was that issues of barrel wear from the
Rifleite smokeless powder and corrosive primer used in the 6 mm cartridge continued to plague Navy ordnance authorities. Finally, due in part to its long, thick-gauged semi-rimless rim case and beveled rim, the 6mm Lee was one of the most expensive service cartridges to produce in terms of cost per round, yet it was already becoming obsolescent in comparison to ammunition that used more efficient powders and true rimless cases. In 1907, the Navy adopted the
.30-06 Springfield cartridge for both rifles and
machine guns. In 1917, the rest of 6mm Lee ammunition still in military stores auctioned off, where it was sold to F.W. Bannerman & Co, a private
military surplus dealer. Unfortunately for Bannerman, the powder used in the these cartridges had deteriorated so much they were determined to be unfit for sale, and the company destroyed the entire lot rather than reselling them to the public. ==Commercial and hunting use==