The effect of the broadening of the gas reservoir on the exit velocity of the projectile during acceleration in the barrel by compressed gas is investigated. The simulation was carried out in the framework of quasi-one-dimensional and two-dimensional axisymmetric gas-dynamic models. Models are validated on experimental results. It is shown that the use of a wider reservoir with the same volume and initial pressure can increase efficiency by 25%. The significant effect of air compression in front of a light projectile under supersonic conditions on its speed is demonstrated. Accounting for air compression in front of the projectile was carried out using an approximate formula, which showed good results compared to the two-dimensional model. The effect of accelerating a light projectile due to the expulsion of an air column through the muzzle has been revealed.