Metallurgical Examination of M61A1 Breech Bolt Assembly Components.

The Naval Air Warfare Center NAWC requested the Army Research Laboratory ARL to perform a metallurgical investigation on 20mm M61A1 Gatling gun system breech bolt assembly components. An unused breech bolt assembly, and an individual locking block a component of the breech bolt assembly were both examined. Each part was taken from inventory at NAWCAD, Patuxent River. The Navy had experienced numerous failures of this system in practice gun mount testing and in service. Failures have coincided with the use of a recently developed more potent replacement for the 20mm M50 Series ammunition. This new ammunition is designated as 20mm PGU Series ammunition. The investigation performed by ARL concentrated on determining the conformance of each component to the governing specifications. The examination included high-voltage testing, visual examination, surface finish measurement, dimensional analysis, magnetic particle inspection, metallography, mechanical testing, chemical analysis, and coating measurements. Findings included a nonconforming surface finish upon the individual locking block, a complete layer of decarburization around the periphery of the individual locking block, and an improper hardness level within the individual locking block. In addition, the firing pin had a nonconforming chemistry, the top and bottom bolt shafts did not contain a nitrided case and had nonconforming chemistry, the assembly locking block had nonconforming chemistry, and the spring pins failed to achieve the minimum double shear load, and had nonconforming chemistry. The author presented these findings to Navy, Air Force and AMCCOM representatives on three separate occasions.