Muzzle Velocity Drop in Wear-Limited Army Guns
ARMY BALLISTIC RESEARCH LAB ABERDEEN PROVING GROUND MD
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The authors conclude 1 The ratio of muzzle velocity dropinitial muzzle velocity correlates well with land diameter increaseinitial land diameter for a number of wear-limited Army guns. All exceptions noted to the correlation were for obturated projectiles or for projectiles fired from the M199 cannon. In this cannon, the grooves do not wear until the lands disappear which should reduce gas blowing by the projectile as a function of the land erosion. After complete land erosion, blow-by should increase. 2 Of the two XM185 cannons from which the muzzle velocity drop for the M119 charge was estimated as a function of tube wear, one tube had significantly lower muzzle velocity drop than the drop predicted from the above correlation. Data in the 155mm firing table for M119 muzzle velocity drop vs rounds may underestimate the actual muzzle velocity drop. 3 An obturator placed on the M107 projectiles to reduce muzzle velocity drop did indeed raise the muzzle velocity of the M107 projectile when fired from a worn cannon. The increase was not as great as that for obturators. This suggests the rotating band design is more important than the obturator in reducing the rate of muzzle velocity drop as the gun tube wears and that band diameter must be considered to obtain a universal plot of normalized wear vs velocity loss.
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