Based on the operation principle of rarefaction wave gun,the selection and calculation methods for venting opportune moment are invastigated. Considering property of the rarefaction wave,taking the center of muzzle section as initial calculation point,supposing that at the moment projectile arrives to the muzzle,the rarefaction wave arrives to the base of projectile,the rarefaction wave velocity along the barrel can be obtained by fitting calculation of the interior ballistic data of the same closed gun and reverse deduction. And then,the optimal venting time can be found out correctly based on the rarefaction wave velocity.
