The .38/200 (or 9×20mmR) was a British military revolver cartridge identical to Smith & Wesson's .38 S&W cartridge but with specific loadings for military service.
The .38 S&W was modified for use by the British military and called the .38/200 (also known as 380/200 Revolver Mk I) in 1922 for .38 caliber pistols and revolvers (such as the Enfield No. 2 Mk I and Webley Mk IV) which replaced the larger .455 and .476 inch handguns. The .38/200 cartridge was initially issued to British military forces as the .380/200 Revolver Mk I round. British authorities later issued a different .38 S&W military cartridge with a lighter, 178–180-grain (12 g) jacketed bullet, known as the .380 Revolver Mk IIz.
After the First World War, there was a move away from the larger .455 calibre. The professional core of the pre-war British Army had been decimated and replaced by a larger and mostly conscripted force. It was recognized that the short training period available to the new recruits did not give them time to become proficient with the large-bore .455 revolvers, and that a smaller caliber would be easier for new recruits to develop competence with in pistol shooting.
British Army initial specifications submitted to ammunition manufacturers for the .38/200 military loading were as follows:
Cartridge: .38 S&W (.38-200)
Bullet: (Mk I) 200 grain, lead alloy, lubricated
Muzzle velocity: 625 ft/s (191 m/s) (+ or – 25 ft/s (7.6 m/s)) from 4-inch (102 mm) barrel.
Chamber pressure: 13,000 PSI (Max.)
Webley demonstrated a lighter version of their Mk III revolver with modified .38 S&W ammunition, firing a heavy 200-grain (13 g) bullet, later known as the 380/200. It received favorable reports from the Army and the revolver was accepted in principle.
As Webley had used the .38 S&W cartridge dimensions for their revolver, and the cartridge length was fixed by the size of the cylinder of the revolver (the same as for the wider .455), Kynoch produced a cartridge with the same dimensions as the .38 S&W but with 2.8 grains (0.18 g) of "Neonite" nitrocellulose powder and a 200 grain (13.0 g) bullet. This combination gave a velocity of 630 ft/s (189 m/s) at the muzzle, and over 570 feet (170 m) per second at 50 yards (46 m). In tests performed on cadavers and live animals, it was found that the lead bullet, being overly long and heavy for its calibre, become unstable after penetrating the target, somewhat increasing target effect. The relatively low velocity allowed all of the energy of the cartridge to be spent inside the human target, rather than penetrating completely. This was deemed satisfactory and the design for the .38/200 cartridge was accepted as the "380/200 Cartridge, Revolver Mk I".
After a period of service, it was realized that the 200 gr (13 g) soft lead bullet could arguably contravene the Hague Convention, which outlawed the use of bullets designed so as to "expand or flatten easily in the human body". A new cartridge was therefore adopted into Commonwealth Service as "Cartridge, Pistol, .380" Mk II" or ".380 Mk IIz", firing a 180 gr (11.7 g) full metal jacket bullet. The 380/200 Mk I loading was retained in service for marksmanship and training purposes. However, after the outbreak of war, supply exigencies and the need to order readily-available and compatible ammunition, such as the .38 S&W Super Police, from U.S. sources forced British authorities to issue both the 380/200 Mk I and MkII/IIz cartridge interchangeably to forces deploying for combat.
The Cartridge S.A. Ball Revolver .380 inch Mark II and Cartridge S.A. Ball Revolver .380 inch Mark IIz cartridge was theoretically phased out of British service in 1963, when the 9×19mm semi-automatic Browning Hi-Power pistol was finally issued to most British and Commonwealth forces.
The Cartridge, Pistol, .380" Mk IIz is still produced by the Ordnance Factory Board in India, for use in revolvers of the Indian Army and some African countries.
Revolvers chambered for .38/200 may also fire .38 S&W (AKA .38/145), .38 Colt New Police, and .38 Banker's Special cartridges, along with the .380" Mk IIz round, though caution should be exercised as always when using ammunition designed for more modern guns to different specifications.