Try out several different loads and shot sizes with that gun and let the pattern board tell you which load to go with for head shots. In my experience nothing can be assumed about the effectiveness of a load, especially when you have to make one shot count.
The very core of the pattern, possibly only 12" diameter is the working part of a head shot load. Many loads that look good at 30 yards have large gaps at 35. Also, with the heavier lead loads (1 1/2 oz.+) most of the bottom layers are deformed and add nothing downrange. This is why carefully constructed handloads of 1 1/8 oz of nickle plated 6s can produce 90% patterns at 40 yards through standard fixed (full)chokes. The inner cores of these patterns at shorter ranges look like a salt shaker.
I know some traditionalists will scoff and I don't recommend shooting hard (tungten)shot through just any old double, but I find Hevi-13 (# 6 size pellets) turkey loads are among the best, especially at ranges beyond 35 yards. You may get good results with that load through the modified choke. I've used the 3" (1 5/8 oz) in one of my heavy barreled doubles, it outshoots anything else for high-energy, focused patterns out to 45 yards.
Incredible to me, a lot of turkey hunters never pattern their guns. If you don't, with the exception of very short ranges you can never be confident of how close he needs to get before it's a sure thing.
