A closure wad is placed at the mouth of the launching end of a shotgun cartridge before the mouth is crimped down to seal the launching end of the cartridge. Both the walls of the closure cap and the walls at the mouth of the plastic envelope are crimped down together, thereby forming a substantially impermeable seal at the launching end of the cartridge. In addition, the inventive closure cap substantially reduces the detritus which is sometimes expelled when a launcher cartridge is fired.