The clearances in pitch mechanism of gun, such as barrel-guide, have important influence on firing accuracy. Since the sliding pair is the typical characteristics of slide plate of gun barrel and guide’s contact, the clearance can be simplified to the cantilever beam with a small gap. Dynamics models which influence the factors on cantilever clearance are established by cantilever clearance sets. The influence of clearance and the flexibility of clearance are simulated and vilified by numerical analysis. The results of the experiments are that the response signals in time domain reflect clearance size: in the same intensity under sinusoidal excitation, with enlargement of the clearance, the peak value of gun barrel time-domain signal is smaller. The current work can be referred by the gun firing accuracy design.