A method for controllably releasing venting gases from a manhole space around the perimeter of a manhole cover by allowing the manhole cover to rise up in stages upon development of upward forces in an explosion to vent gases developed by the explosion while being restrained from being completely blown free except under extreme conditions. The total amount of adhesive applied between the label and the paperboard body wall is thereby reduced, so that less moisture is added on to the paperboard relative to conventional can-making processes in which adhesive is flooded onto the entire surface of the label. A second end is connected to the vehicle frame with a spring shackle pivotably connected to a second bracket attached rigidly to the frame at a second position, in order to compensate for length changes of the leaf spring under load conditions. Each cartridge is fired while in its chamber tube by a firing mechanism and each chamber tube and fired cartridge are both ejected by gas pressure exerted on one side of the chamber tube through an ejection port defined by an opening in one side of the firing recess remote from the one side of the chamber tube.