Abstract: A rock-breaking cartridge (10) includes a casing (16) of deformable material that forms an internal cavity (22) and a tapered part (82). A seal (90) extends around the outside of the tapered part (82) and a piston (92) inside the cavity (22) butts against the inside of the tapered part (82) to seal against the inside of the casing (16). A charge (46) is provided inside the cavity (22) on the side of the piston (92) that is opposite from a narrow end of the tapered part (82) and when the charge (46) is ignited, it causes the piston (92) to slide towards the narrow end and to force the tapered part (82) and seal (90) to expand. The expansion causes the seal (90) to seal against the inside of a hole in which the cartridge (10) has been placed and the seal (90), tapered part (82) and piston (92) form a sturdy plug to contain gasses generated by detonation of the charge (46).