When you launch a firework shell from a mortar, you should, unlike when you fire any kind of gun, have a small "glitch" between the pipe and the shell. This is due to the low density of firework shells; the atmospherical pressure equals 11 m of mercury(!), so quite some gas from the lift should be allowed to pass around the shell to decrease the local air pressure. The recently passed away Takeo Shimizu, the Grand Master of fireworks all times and categories made a table for this.