This study examines the effect of channel leak geometry on blast overpressure attenuation in the rear of a muzzle-loaded large caliber cannon. Effects of three primary geometric parameters including leak volume as well as number and length of channels are studied. Reduction in blast overpressure, and thus peak overpressure, is most influenced by the leak volume; however, leak volume needs to be selected carefully to limit the loss in the projectile exit velocity. Modification of the channel height in the current range has a minimal effect on peak overpressure, but the number of channels can have a significant effect due to the constriction experienced by the leaking flow, thereby limiting the attenuation. Two channel lengths are considered where the longer channel length, is found to be more effective. The best configuration showed over 50% reduction in peak overpressure at all monitored locations with about 4.8% loss in the projectile exit velocity.