I see what the op is saying ,
if you hold a mortar up , the end that the firing pin strikes must be sealed to stop the propellant falling out yes ?
so if its a sealed unit , how does the propellant erm .... propell , without leaving either a sealing cap or something behind ?
Probably not a good idea to start googling this subject either, unless you want your i,p to be flagged for security checks .