A study of the Browning M2 Quick Change Barrel and problems experienced in the Norwegian Armed Forces

Norway has used the Browning M2 heavy machine gun for decades. The first guns were US weapons given to us through the Mutual Defence Assistance Program. They were M2 Heavy Barrel (HB) versions. All weapons were converted to a so-called Quick Change Barrel (QCB) in the early 1990s. Around year 2000 some weapons were rebuilt from a closed-bolt to an open-bolt action. The openbolt weapons were named NM218 and the closed-bolt weapons were named NM214. Severe weapon failures have occurred in recent years with the QCB-weapons, and a lot of weapons show cracks in the threaded area on both barrel and barrel extension. Much work has previously been done to investigate these failures, but no clear answers have been found. The Norwegian Defence Research Establishment (FFI) have been tasked to do a kinematic analysis of the weapon system together with chamber pressure measurements, and an analysis of non destructive crack detection methods. From the kinematic analysis it was found that the barrel, barrel extension and bolt do not behave as one piece in the milliseconds after shot initiation; this is due to backlash in the system and gives rise to high forces on the parts. It was also found a very good correlation between maximum chamber pressure (Pmax) and forces acting in the coupling between the barrel and barrel extension. The crack analysis shows almost all cracks are situated in only two of the three threaded segments. This indicates that the forces between barrel and barrel extension are mainly transferred through two of the segments. This results in an highly asymmetric and increased load on the barrel and barrel extension compared to if the load was evenly distributed over all segments. The best and easiest way to reduce the forces acting on the barrel and barrel extension would be to use ammunition with temperature-insensitive powder, like the NM241F2 when available. Reduced backlash/smaller tolerances in the threaded area of the barrel and barrel extension and small design and material improvements should also greatly reduce the risk of fatigue failures in the barrel and barrel extension. Crack analysis shows that neither Magnetic Particle nor Dye Penetrant testing detects all cracks that are shown from microscopic examination of cross section cut. Due to the higher fail rate of the Penetrant testing, it is recommended to continue to use the Magnetic Particle testing for discarding barrels, but other test methods for non-destructive testing should be assessed and tested. Ultrasound is one possible alternative.