Zombifying this post after my experience today with 10 turkeys (dressed 15-22 pounds).
I was behind on preparations for today and didn't do my homework. Looking at the size of the birds, I wasn't sure my scalpel would do the job humanely and looked for alternate methods. I was also concerned that my cones weren't large enough.
I used an empty feedbag with a hole cut in the bottom (not across the seam) to 'hold' the bird. Once in the bag, they were generally calm. After letting them rest in the bag for 10-15 minutes, I then used .22 target pistol (it's what I had handy) with a contact shot downward through the top of the head (roughly between the ears and eyes). There was some convulsing and flapping but generally it stopped quickly within 20 seconds. With a second pair of hands controlling the bird, I was able to grab the head and cut the carotid with my scalpel (my normal method for chickens) and bleed it out. Most times it worked well enough. A few didn't bleed out sufficiently and it showed in the finished products (bruises, internal clots).
Plan for the next time: Get a high-quality, extremely sharp hatchet and take the head off with the bird in a cone. My alternate for oversized birds that don't sit nicely in a cone is a 5 gallon bucket with a hole in the center.
