Abstract: In order to ensure simultaneously the reliability of the ejection and the first round loading of the cased telescoped ammunition machine gun, method of processing the blocking slope on the detention support was proposed in the design of the cased telescoped ammunition machine gun. It was required that the slope could block the first loaded cased telescoped ammunition, but would not the cartridge case from being pushed out of the chamber in the subsequent firing. In view of the situation that there were gaps between the ammunition belt and the cased telescoped ammunition, as well as between the cased telescoped ammunition and the chamber, a contact collision model with gaps was established, and it was applied into the multi-body dynamic equation. Through the comparison and analysis of multiple sets of values, the height of the inner bottom of the detention support meetings the requirements of the first round loading and subsequent cartridge case ejection was calculated, which was 0.3 mm. By adjusting the physical prototype of the cased telescoped ammunition machine gun according to the parameters obtained from the simulation solution, the velocity curve of the bolt carrier with the height of the inner bottom of the detention support being 0 and 0.3 mm respectively was measured. The test shows that the calculated inner bottom height of detention support is reliable in practical application.