Improved Military Rifle (IMR)

Improved military rifle propellants are tubular nitrocellulose propellants evolved from World War I through World War II for loading military and commercial ammunition and sold to civilians for reloading rifle ammunition for hunting and target shooting. These propellants were DuPont modifications of United States artillery propellants. DuPont miniaturized the large artillery grains to form military rifle propellants suitable for use in small arms. These were improved during the first world war to be more efficient in rimless military cartridges replacing earlier rimmed rifle cartridges. Four-digit numbers identified experimental propellants, and a few successful varieties warranted extensive production by several manufacturers. Some were used almost exclusively for military contracts, or commercial ammunition production, but a few have been distributed for civilian use in handloading. Improved military rifle propellants 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.

John Bernadou patented a single-base propellant while working at the Naval Torpedo Station in 1897. Bernadou's colloid of nitrocellulose with ether and alcohol was formulated for the reaction pressures generated within naval artillery. The colloid was extruded in dense cylinders with longitudinal perforations to decompose in accordance with Piobert's law. If all external surfaces of the grain are ignited simultaneously, the grain reacts inward from the outside of the cylinder (creating a reaction area of decreasing size), and outward from each perforation (creating a reaction area of increasing size.) Propellant decomposition is initiated by heat causing the colloid to melt and form bubbles of reactive gas which decompose in a luminous exothermic reaction after the bubbles burst. Rate of reaction is controlled by heat transfer through the temperature gradient from the luminous reacting gas through the bubbles to the intact colloid. Heat transfer (and rate of reaction) is faster if the bubbles are under pressure, because heat transfer is more efficient through smaller bubbles. These propellants may not react satisfactorily at low pressures within the oxygen-deficient atmosphere of a gun barrel.

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