MK 38 25-mm machine gun



by Andrew Kristovich, NAVSEA T&E Office


COMOPTEVFOR released his report on the Operational Evaluation of the MK 38 Machine Gun System (MGS) and found it operationally effective, but not operationally suitable. He recommended that the MK 38 MGS not be approved for fleet introduction at this time. The suitability deficiencies for which OPTEVFOR withheld a favorable recommendation were the inability of the gun to meet its mean-round-between-failure (MRBF) and mean-time-to-repair (MTTR) thresholds. The following is SEA 91T's analysis of the events which occurred from the time of TECHEVAL through OPEVAL and lessons learned.

The MK 38 MGS is an unstabilized, manually pointed, deck mounted cannon with semi-automatic and automatic modes of fire. It is designed to provide close range defense against patrol boats, swimmers, floating mines, and various targets ashore including; enemy personnel, lightly armored vehicles and terrorist threats. One crewman is required for operation and two for maintenance.

TECHEVAL was conducted on a Navy Patrol Boat. There were no major reliability or maintainability problems noted during TECHEVAL. Upwards of 5000 rounds were fired by the PB #759 crew.

The Operational Test Readiness Review Board (OTRRB) met to assess the readiness of the MK 38 MGS for OPEVAL. All TEMP thresholds were discussed and the test results looked good. The TEMP had not yet been approved by CNO, but had been signed by NAVSEA and COMOPTEVFOR. It was decided to ask OPNAV in the certification message for permission to begin OPEVAL in advance of CNO approval of the TEMP. CNO granted the TEMP waiver the next day.

OPEVAL commenced aboard PB #759. Ten days later, COMOPTEVFOR placed the system in deficiency status -- water had intruded the pistol grip firing key causing the gun to short out and stop firing.

A new hermetically sealed trigger was installed, and testing with the pistol grip immersed in water confirmed the problem was corrected.

Water intrusion into the aimpoint sight and power supply compartments were also discovered. To correct this, new gasket material was installed. Normal gun vibration induced loosening of components, which was corrected by installing lockwashers, lockwires, double wall shrink tubing, locktite and new connectors where applicable.

The OT Readiness Review Board (OTRRB) met to assess readiness to resume OPEVAL. The Board recommended certification, and certified the MK 38 MGS ready to restart OPEVAL.


During the second phase of OPEVAL, the gun system was fired against shore targets, Seaborne Powered Targets (SEPTARs), floating surface targets, and towed aerial targets. Based on these firings, OPTEVFOR withheld a favorable recommendation for production due to suitability deficiencies. The water intrusion problems for which OPTEVFOR had placed the system in deficiency status originally had been corrected and did not reappear. The problems that did occur during the second phase of OPEVAL were:

1. The gun failed to return to the sear position, thereby not completing a full load/fire cycle. After 4 hours of trying to isolate the fault, the crew turned the gun over to NWSC Crane personnel for repair as directed by the technical manual.

2. The receiver latch assembly was degraded in freezing conditions and salt spray. This failure required 10 hours for the crew to relubricate, tighten and align parts.

3. Water intrusion into the electrical connector receptacle of the receiver motor. It took 7 1/2 hours to correct this.

COMOPTEVFOR categorized the first two as critical (mission preventing) reliability failures and the last as major (mission degrading) failure. He failed the system because it did not meet the TEMP thresholds for mean-rounds-between-failure (MRBF = 706 vice threshold of 1000) and mean-time-to-repair (MTTR = 7.2 hours vice 20 minutes) and because he felt the training course did not provide the knowledge required for adequate maintenance.

The gun did not return to the sear position because the sear solenoid plunger was too long. The Army had encountered this tolerance problem in early production and corrected it in the later cannons. The cannon that caused the reliability failure during the second phase of OPEVAL was from early production and did not have the correction. The Navy had not been advised by the contractor or the Army that the problem existed (OPTEVFOR mentioned this in his report.) There were no formal Navy configuration control procedures for the Army cannon in effect at the time of TECHEVAL and OPEVAL. An ILS audit was conducted after the first OPEVAL readiness review, but prior to the second. The lack of a configuration management (CM) plan for the overall system was identified as a finding. Such a plan was prepared and the finding was closed. However, the CM plan did not include specific configuration control procedures for the Army cannon. In other words, had the plan been published and been in effect prior to the initial TECHEVAL, there still would have been no Army notification to the Navy of fixes. Navy representation at the Army Configuration Management Board was lacking, as was the technical liaison between the gun manufacturer and the Navy In-Service Engineering Agent.

The problems with the receiver latch assembly, were difficult to determine. When the gun was returned to Crane after the second phase of OPEVAL, the adjustment screws that had loosened did not have their required lock-wires.

Regarding water intrusion into the electrical connector receptacle of the receiver motor, it was found that the receptacle was left exposed to freezing salt spray during boat transit. For security reasons, the cannon was removed each evening and replaced the next morning. This particular morning, the crew did not reinsert the connector in the receptacle, thereby disregarding established procedures. During boat transit to the operational area, water accumulated in the receptacle and froze.

OPTEVFOR said the training course was inadequate to provide sailors with the knowledge required to troubleshoot and correct system casualties. The cause, however, was more likely a manning deficiency. As OPTEVFOR said in their report "only two of the seven personnel provided by Special Boat Unit (SBU) 20 had any gunner's mate experience, and only two of the crew individuals attended both courses". SBU 20 was undermanned, which is typical of East Coast SBUs. With an allowance of 17 gunners mates (GMs), only 7 billets were filled. When the reliability problems occurred, the only two people on board the boat who had received any gun training were an engineman and a seaman -- the GM who had received the most recent (two week) course was not available. While it is doubtful that even the trained GM could have repaired the first "critical" reliability problem, it is equally doubtful that the engineman's and seamen's futile attempts to repair the gun are a meaningful indicator of the adequacy of the training course.

TECHEVAL should ordinarily be a good indicator of readiness for OPEVAL. So, why three reliability failures during OPEVAL and none during TECHEVAL? The primary reason seems to be that the cannon used through the second phase of TECHEVAL, was not the one used during the second phase of OPEVAL. After the system was placed in deficiency status, that cannon was returned to Crane for correction of the deficiencies. Many parts were changed and/or upgraded. That cannon, along with the spare that did not have the benefit of all the "fixes", was then shipped to the SBU for OPEVAL. The SBU, not knowing that there was a difference, arbitrarily picked the one without the fixes.

The OTRRB proceedings went smoothly, and according to standard practice. All documentation requested by the Board was provided and all questions were satisfactorily answered. When the gun was placed in deficiency status, the Divsion Director emphasized to the program manager that the corrections were to be thorough, verified and reported in an engineering, disciplined manner. The Board, of course, was unaware of the design problem with the sear solenoid plunger (since it did not appear in development testing). The Board was also unaware that the cannon might be switched or that there was a difference between the primary and spare after the second phase of TECHEVAL.


Revised: March 16, 1998