I have a CVA Side By Side DBL 12, and use 2 3/4 drams( 75 grains) of 2Fg powder, and 1 1/4 oz. of #5 shot to shoot pheasants that break too soon. ( long range shooting- 25-35 yds.) I am confident that the #5 shot carried enough pellet energy to take Turkey out to 30-35 yds.
If you use the oiled Mattress ticking shot cups, the pattern should be tight enough, even from a cylinder bore barrel, to take Turkey at 35-40 yds. That is a long shot at turkey, even with modern Shotguns and loads.