You need a dense enough pattern to hit the brain or neck vertebrae, with enough penetration to break bone. 1-1/8 of 5's sounds about perfect as a first try at the patterning board. I shoot and load for 5 different 16 ga. guns, and have owned several more. Few 16's will pattern 4's or bigger shot with any consistency. Yours might, but I'd bet good hard #5 shot will give you deadly patterns. I do not hunt turkeys, but have done a fair bit of penetration tests of various shot sizes. I don't think that you should hunt turkeys with anything smaller than 6's because the difference in penetration between 6's and 7-1/2 is amazing. For example, at 30 yds, 6's will shoot right through a 3/8's spruce plywood, 7-1/2's will stick in the near ply. 5's are better penetrators than 6's of course, and 4's better yet, but pattern density is the main limiting factor.
If you want to try big shot for body shots, I have been very disappointed with the patterning of 2's and BB's in the 16 ga, but both will kill big stuff well if you are close, say within 30 yds or so. Just 10 yds more and the big shot does very poorly out of a 16, patterns have too many holes.
