I've never hunted Turkey with a "Rifle" muzzleloader.
The few I have taken with a muzzleloader was all with my .69 cal. smoothbore, and two of those were pretty close...probably less than 50 - 60ft.
Now, having said that, I "think" that if I were faced with this decision, I would choose the .45 cal....not just because I own one, and really like it, but rather because I have owned, at one time or another, all of the calibers listed, and I base this decision on the extremely flat trajectory you can get with a .45.
Like many others, I am terrible at telling you if something is fifty, or seventy five yards away.
If it's a Deer, or an Elk, there is a good chance I can tell you if it is more than 100yds, but anything up to that 100 +/- a few yds. is up for grabs as far as I'm concerned.....and a Turkey, at 50, 60, 80 yds, I don't think I could come anywhere close, therefore I would like to have the flattest trajectory, with the least wind disturbance possible.
I have no doubt that the lowly "three-two" can take a Turkey out to 75 yds, or even more, as the .32 is, in my opinion, one of the most underrated little muzzleloaders I have ever owned and shot.
The problem with the .32, and the .36, is the "hitting", not so much the killing. The .40 may make up for that difference, but for sure fire longer range shootin, I would definitely choose the .45.
I envy you guys using your "Riflegun" to hunt Turkey, that has gotta be top sport.
Good luck, either way you go. I just know you're going to have a blast and I only wish I could be there with you.
Uncle Russ...