When you fire a shotgun shell the shell body and the shell heads expand, to seal the chamber, then both shrink slightly.
Although they shrink, they're still larger then unfired shells and think makes them tighter in the chamber.
Without opening up the chamber to unsafe size you won't get shells to just fall free except possibly from a very clean chamber.
If you'd like to smooth the chamber, you can wrap some 0000 steel wool around a used bore brush on a cleaning rod and chuck the rod in an electric drill.
Run the steel wool and brush in the chamber at medium speeds, keeping the brush constantly moving.
You can also do this to polish the bore and remove any really stubborn fouling.
After polishing, clean the bore and chambers thoroughly to remove the residue.
NEVER, EVER do this in a rifled bore, it damages the rifling.