An internal pressure relieving device for anti-armour ammunition. The device is mainly adapted for use with barrel-loaded ammunition, that is a naked round including a cartridge case, which normally would be located in its packaging. A plurality of slot formed openings have been cut out through a wall of the cartridge case and the openings have been sealed by inserts of polymeric plastic introduced with a tight fit into each opening. Hence, by the characteristics of the plastic material, the inserts can be configured to open up at a predetermined level of temperature. Alternatively, the inserts open up at a predetermined level of pressure in the cartridge case.