A low energy mechanical operating cartridge has an inner regulator core (13) supporting a projectile (21) slidably engaged in a primer casing (19). The regulator core acts to control the velocity at which the projectile is propelled from a firearm (61), and the primer casing carries the explosive propellant (25) necessary to generate the required energy to launch the projectile. To improve the operation of the cartridge and ejection of the cartridge from the firearm, the primer casing is configured to efficiently slide on the regulator core in a rearward recoil action, while robust gas seals are maintained between the casing and regulator core.