A device for sealing the cartridge chamber of a small arms weapon that fires a caseless cartridge consisting of a propellant charge and a bullet that is connected to the propellant charge. The device includes a pot-shaped seal having a base and a side. The seal is disposed on the weapon's butt front edge so that at least a portion of the side's outer surface presses on an adjacent inner surface of the cartridge chamber when the butt is closed. The outer surface further sealingly engages the chamber adjacent inner surface in response to the gas pressure from the propellant charge when the cartridge is fired. The side's outer surface is defined by a first generatrix that is parallel to the chamber axis and the adjacent chamber inner surface is defined by a second generatrix, where the first and second generatrices converge. The seal further includes a replaceable insert that is secured to the butt's front edge and is disposed proximate the seal's base so the insert's top front side is adjacent the propellant charge's base. The insert further includes an extractor claw, extending parallel to the chamber axis, that engages a peripheral groove around the propellant charge so an unfired caseless cartridge can be withdrawn from the cartridge chamber.