Good advise above. There are so many variables now days with gun/chock/load options. Best advise... Shoot your gun at a 10 inch cicle at 40 yards with as many variations as you can afford. This is the same method that Nitro Company (Cheack out their website) uses to test their loads. I have shot Nitro Company loads for the past 5 years. Amazing results, but I have found that it is extremely hard to beat hevi-shot #6's in a 2oz load for the money. I shoot a Mossberg 835 Grandslam with a cheap non-ported hastings chock at .675, and I get an average of 225 hits in a 10 inch cicle at 40 yds with Hevi-shot 3 inch 6 shot in 2 oz. When using Nitro Company (tri-plex loads, I get 250 averge hits in 10 inches. Nitro company is 2x as much per shotshell.
Another point to consider... Although my gun will shoot 3.5" shells, I shoot 3". A 3" shell will shoot flatter at 40-60 yards than a 3.5" due to the added pellet count to powder charge ratio.
Or we could all just call that turkey into 25 yards and kill him with a factory choke and El cheapo shot shells.