Improved Military Rifle (IMR) describes a series of tubular nitrocellulose smokeless powders evolved from World War I through World War II for loading military and commercial ammunition and sold to private citizens for reloading rifle ammunition for hunting and target shooting. IMR powders are coated with dinitrotoluene (DNT) to slow initial burning and graphite to minimize static electricity during blending and loading. They contain 0.6% diphenylamine as a stabilizer and 1% potassium sulfate to reduce muzzle flash.[1]
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IMR # 16 appeared in 1916 with grains 0.03-inch diameter and 0.085 inches long. Through World War I this powder, and the similar IMR # 17, were used to load military rifle cartridges for France, Russia, and the United Kingdom. In 1923 IMR # 17½ added 2% tin to IMR # 16 to reduce metal fouling from cupronickel-jacketed bullets.[2] Grains of 0.03-inch diameter were 0.084 inches long with 8% DNT coating.[1] IMR # 3031 replaced IMR # 17½ in 1934. IMR # 3031 is intended for mid-range loads and medium sporting and military cartridges like the .257 Roberts, .30-30 and .348 Winchester.[3] Grains of 0.029-inch diameter are 0.083 inches long with 7% DNT coating.[1]
The first IMR powder, IMR # 15, was designed in 1914 as a replacement for cordite for the .276 Enfield cartridge.[2] Grains of 0.032-inch diameter were 0.084 inches long with 9% DNT coating. In 1919 IMR # 15½ added 2% tin to IMR # 15 to reduce metal fouling from cupronickel-jacketed bullets.[1] IMR # 4064 replaced both powders in 1935. IMR # 4064 is intended for magnum capacity cartridges like the .250-3000 Savage, .35 Whelen and .375 H&H Magnum.[3] Grains of 0.031-inch diameter are 0.083 inches long with 6.5% DNT coating.[1]
Military Rifle (MR) powder MR # 21 appeared in 1913 for loading traditional sporting cartridges including the self-loading .25, .30 and .32 Remington and the .22 Savage Hi-Power, .25-35 and .30-30 for lever action rifles. Grains of 0.03-inch diameter were 0.04 inches long. Over the next decade MR # 21 was replaced by IMR # 18 with 0.03-inch diameter grains 0.05 inches long. IMR # 4198 appeared in 1935 as a replacement for IMR # 25½. Comparatively little information has been released concerning IMR # 25½, since DuPont never sold it to handloaders.[2] IMR # 4198 is intended for short range loads and medium capacity cartridges like the .300 Savage, .32 Remington, and .32 Winchester Special.[3] Grains of 0.026-inch diameter are 0.083 inches long with 4.5% DNT coating.[1]
IMR # 1204 powder appeared in 1925 for short lever action cartridges like the .25-20, .32-20, .38-40, and .44-40.[2] Grains of 0.025-inch diameter were 0.021 inches long with 8% DNT coating.[1] IMR # 4227 replaced IMR # 1204 in 1935 for small capacity cartridges like the .22 Hornet, .25-20 and .32-20.[3] Grains of 0.024-inch diameter are 0.023 inches long with 6.5% DNT coating.[1]
IMR # 1147 appeared in 1923 for military cartridges like the .30-06 Springfield and the 7.92x57mm Mauser.[2] Grains of 0.034-inch diameter were 0.042 inches long with 8% DNT coating.[1] IMR # 4320 replaced IMR # 1147 in 1935.[2] Grains of 0.034-inch diameter are 0.042 inches long with 6.5% DNT coating.[1] IMR # 4320 is intended for large capacity sporting and military cartridges like the .220 Swift, .270 Winchester and .30-06[3]
In 1910 MR # 10 was designed for the .280 Ross. Large grains 0.033-inch diameter and 0.12 inches long were intended to slow the burning rate. MR # 10 was replaced by IMR # 13 in 1917 for high-capacity bottleneck cases. Grains of 0.035-inch diameter were 0.08 inches long.[2] IMR # 4350 replaced IMR # 13 in 1940. Grains of 0.038-inch diameter are 0.083 inches long with 5.5% DNT coating.[1]
IMR # 4831 was used to load Oerlikon 20 mm cannon cartridges through World War II. Salvaged powder became available to civilians when the Oerlikon cannon was declared obsolete following the war.[4] Grains of 0.038-inch diameter are 0.083 inches long with 7% DNT coating. IMR # 4831 contains 1.1% diphenylamine (0.5% more than other IMR powders.)[1]
Originally known as 1909 Military or Pyro DG (for Diphenylamine Graphited), 500 tons per day of MR # 20 was manufactured for military loading of the original 150-grain M1906 bullet in the .30-06 through World War I.[2] Grains of 0.03-inch diameter were 0.084 inches long with 9% DNT coating.[1] IMR # 1185 was used when military .30-06 loadings shifted to the 173-grain M1 bullet from 1926 to 1938.[2] Grains of 0.032-inch diameter were 0.084 inches long with 7% DNT coating. IMR #4895 was used for loading the 152-grain M2 bullets in the .30-06 through World War II. Grains of 0.032-inch diameter are 0.056 inches long with 5.5% DNT coating.[1]
IMR Trail Boss is a bulky powder created in 2005 for use in older cartridges.[5] Old cartridges designed for blackpowder have a lot of empty space when using modern smokeless powder. IMR created Trail Boss to be bulky enough that handloaders are not able to accidentally double-charge a cartridge. The unique donut-shaped grains are part of what make the powder so bulky.[6]