Abstract: The wear of key portion and performance degradation of automatic mechanism have an impact on the mechanism function and action reliability in firing life-span, which leads to an influence on the performance and operational effectiveness of the large-caliber machine gun. A large-caliber machine gun is used as the experimental object. The wear of key portion and performance degradation of automatic mechanism during firing life-span are measured in detail, and the failure statistics of automatic mechanism are completed. Experimental results show that the wear losses of bolt locking supporting surface and locking curve groove during firing are low, but the wear loss of bolt carrier guiderail is high. With the increase in fired shot quantity, the locking force decreases, the length of recoil spring is shortened, the standard deviation of recoil velocity increases, and the moving fault of automatic mechanism increases. There is a positive correlation among the moving fault and standard deviation of recoil velocity and the wear of key portion of automatic mechanism. The empirical models of locking force attenuation and the free length attenuation of recoil spring, and a wear model of bolt carrier guiderail and impact surface when bolt carrier recoils to position and returns to original position are established, respectively. The empirical models are good agreement with the experimental data. These models can be used to estimate the wear and performance degradation of automatic mechanisms.

Key words: ordnance science and technology, large-caliber machine gun, automatic mechanism, longevity experiment, wear, performance degradation