Well I can offer an example to illustrate what's been said. Before that I did kind of wonder about using an 18 inch circle as your reference. I believe a 10 inch circle is a more appropriate pattern to look at. A good amount of pellets in this area will generally give you enough in the kill zone of an average turkey. 18 inches is a pretty big area. You could fit a lot of pellets in that area and still not necessarily be hitting a lot of a turkey's vitals. Look at the patterning target below. Any pellets outside of this circle are not really gonna play into the equation and all I am really counting is the flyers that you see dotting the rest of the page.
Back to the choke question. I have just finished patterning my 1300 NWTF short. I shot the Winchester X-tended 3 inch #6 using the Win X-full, X-long turkey choke and a Briley Hevi Shot Turkey choke.
At 30 yds with the factory turkey choke I got this
218 hits in a 10 inch circle. Not bad BUT.........
when i moved out to 40 yds that pattern just was gone. I dropped to 54 pellets in a 10 inch circle. Tried the Federal Flight Control 3 inch #6 and I had 47 hits.
I was a little shocked but I had the hevi shot Briley and so I put that choke in and my pellet count went to 130 pellets in a 10 inch circle with the Winchester load. I added 10 yds of distance to the effective killing range by doing nothing more than changing the choke
Spent about $50-$75 in shells playing around with the combinations however[&:]
I would like to try the Undertaker (All loads) choke and also throw in the Federal high Density load and Rem Hevi shot as well. There very well could be some combination in there that gets my range out to 45 and 50 yds. That is for another day and more for fun since I have the preference to bring 'em inside 40 anyway.
My buddy shot the Federals but he tried my Winchester load in his Rem 870 26 inch with his rem factory full choke. He got 153 hits in a 10 inch circle at 40 yds. Even better than my set up!
You juts have to play around.