Most of the series we have been reading describe mortar shells as being separately ignited with a linstock and fuse and then being quickly fired from the gun. Otherwise, explosive shells would not have been present on a ship of war in that era. The risks were too great.

I believe the rating of vessels would have actually begun frigates at 20 main deck guns, with sloops, etc. below this number. This would have allowed for a newly minted captain to simply have his ship upgraded by the addition of one gun per side. Remember the rating was based on the number of guns, not the number of gun ports. I would also be surprised if some enterprising commanders had not done the upgrade on their own simply for the sake of the extra, unofficial fire power.