In order to understand the irreproducibility of the auxiliary pump technique, an interior ballistic solver taking into account reservoir collapse has been used to simulate the performance of launchers. Launchers with different detonation velocities, explosive lengths, and timing delays (the difference between the initiation time of the pump tube explosives and auxiliary pump explosives) of the auxiliary pump have been calculated. The effective timing delay region, which could achieve a velocity gain larger than 1.0 km/s, has been discussed. And its influence factors, such as the detonation velocity of auxiliary pump explosives and the inner-wall velocity of the reservoir, have been analyzed. Results show that the velocity gain decreases with an increase in the timing delay and increases with the increasing length of explosives. The effective timing delay region is about 2μs and depends weakly on the detonation velocity and the length of explosives when using the same explosives for the pump tube and the reservoir. Nevertheless, low detonation velocity of the reservoir explosives and high inner-wall velocity could improve the effective timing delay region, but the maximum effective timing delay region cannot exceed 10μs, which is not easily accomplished experimentally. Therefore, the auxiliary pump technique should not be a very reproducible technique.