Since your gun does not have screw-in chokes, you'll just need to pattern it and see how it does. That will probably make the selection of shotshells a bit more more important.
Over the years, very many turkeys have been killed with guns that were not specifically "turkey guns" and did not have screw-in chokes. All that means is that you'll need to know how your gun shoots and then take shots that are within that range.
One final recommendation is that prior to patterning your gun, you really clean that barrel very well. I usually refer to that as "deep cleaning". Many barrels have never had a very thorough cleaning and may have built-up plastic, powder and even residue that's left when the barrel was manufactured. I've seen some very dramatic changes in performance after a barrel has been "deep cleaned". I wrote an article outlining the procedure that has helped many shooters and you can read it at:
I'll be interested to hear how your gun patterns and again, if I can be of any help, just let me know.