Taurus Judge choke threads...?

i have a Taurus Judge (.45 LC/.410 shotshell) and i wanted to try to modify it (most likely hire a licensed gunsmith for legality's sake) to accept a choke. because of the rifled barrel the shotshell pattern expands way to fast. i am not familiar with shotguns, but i do know that a choke tightens up the pattern. are the threads for a choke on the inside of the barrel or the outside? if so, does that mean that i would always need a choke when firing shotshells (so the threads are protected from the buckshot)???