Abstract: A cased telescoped ammunition machine gun adopts the structure of separated chamber and barrel, and its chamber has two working positions in the whole automatic cycle, namely the ammunition supply position and the firing position. The interaction between the guide pillar on the bolt carrier and the spiral groove on the chamber outside drives the chamber to swing to and fro, so as to realize the mutual conversion between the two working positions of the chamber. Preliminarily, the regular spiral line was selected when the spiral groove in the chamber was designed. Later, in the actual test, it was found that the impact force on the spiral groove from the guide pillar of the bolt carrier when the guide pillar entered the spiral groove in the chamber outside was very large. The impact not only caused significant kinetic energy loss of the bolt carrier, but also reduced the life of the guide pillar. The maximum impact force between the guide pillar of bolt carrier and the spiral groove in the chamber outside was taken as the objective function, and the quintic function curve was selected as the theoretical contour curve of the spiral groove in the chamber outside, and the genetic algorithm was used to optimize the theoretical contour curve of the spiral groove in the chamber outside, so that the maximum impact force between the bolt carrier guide pillar and the spiral groove in the chamber was reduced to 42.27% of the original value before optimization. It is concluded by firing test that the velocity loss due to the interaction between the bolt carrier guide pillar and the chamber spiral groove was reduced from 30.3% to 25.6%. The research results provide a reliable theoretical basis for the improvement of the gun.