tests. General Electric fired over 3000 rounds on the four guns and sufficient data was accumulated to assure that the weapon would be more reliable than the well proven M61. The XM188 guns have been fired at rates of from 300 spm to nearly 2000 spm. The limited amount of ammunition fired was due to a lack of ammunition not to gun problems. All design goals were reached.
The design of the 30mm XM188 weapon was undertaken to meet the need for a large-calibre, air-to-ground, close-support weapon for use on armed helicopters. The gun was designed to fire at intermediate rates. Nominal firing rate was 2000 rounds per minute and the gun was also capable of firing at lower rates. Selection of2000 rpm was based upon the tactical requirements of helicopters and fixed-wing aircraft in attacking ground targets. Power for driving the gun can be supplied from external sources through AC or DC electric drives or a hydraulic motor.
The XM188 was a candidate for Bell Helicopters YAH-63. The XM188s significant performance advantages, coupled with incorporated weight reductions, provided the YAH-63 with greater survivability.
The cycle of operation is identical to the standard M61A1 Vulcan. Each of the gun's three barrels fires only once during each revolution of the barrel cluster. Barrels are attached to the gun rotor by interrupted threads. No head space adjustment is required. The gun rotor is journalled within a stationary outer housing and contains the three gun bolts. The bolts slide fore and aft on tracks and provide the ram, lock, fire, unlock, and extracting functions. The stationary outer housing includes a main cam which drives the bolts through their respective functions. The three rotating barrels contribute to a long weapon life by minimizing barrel erosion and heat generation. This method of operation also eliminates erratic recoil associated with multiple gun installations.
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