A cylindrical plug made of elastomeric material is introduced into a cylindrical cartridge. The rim of the cartridge is swagged-in to reduce the diameter of its opening to one-third of the cartridge internal diameter and of the closely matching plug external diameter. The cartridge is held in a chamber. A tube slidingly passing through the wall of the chamber comes in contact with the plug, and holds it within the container while the chamber is filled with a highly pressurized fluid. As the cartridge is filled with the pressurized fluid the plug is tightly held against the internal end of the tube. The tube is partially withdrawn drawing the plug against the swagged opening of the cartridge. As the tube continues to be withdrawn its internal open end is separated from the plug, and pressurized fluid begins escaping through the tube indicating that the cartridge is filled. The pressurized fluid can now be evacuated from the chamber. A central part of the plug bulging through the swagged aperture provides a positive indication that the cartridge is fulled and tightly loaded.