If you're not shooting at extreme distances and body shooting your turkey, use a smaller shot size because you are shooting for the head and neck area. And with those shots all it takes to drop a turkey is one shot in the brain or spinal column; and the smaller the shot size, the greater the probability you'll hit those areas. Many posters here have had personal experience or know of LC Smith gunsmith Buck Hamlin who is an avid turkey hunter. I've been to his shop, and when I say avid turkey hunter, I mean avid. Buck has a number of turkey vertebrae and skulls from his kills cleaned and wired together with each shot hole marked; and believe me, these bones clearly show one or two 7 1/2 shot pellets will do the trick. Buck has experimented with all sorts of loads and choke alterations to get the patterns he wanted and showed me a 40 yard pattern of #7 1/2 shot fired from an A-5 barrel (which he'd "Buck choked") that could be covered by a ball cap. Obviously any gun shooting a pattern that tight could also be a recipe for clean misses, so Buck had also fabricated a special sight so that he could aim the gun. Obviously none of us will be going to that extreme so the point to remember is that small shot is all that is required to kill a turkey when taking head/neck shots at sporting ranges; and because that is true, a smaller shot size will provide a denser pattern and increase the odds of hitting this vital area. Personally, I'd not use a shot size smaller than #6; but turkey season will soon be here, so good luck.