A guy asked us to cull his massive turtle population when I was a youth. We had an air rifle and an old .22 rifle.

If you are shooting in the same flat plane as the turtle, any round will go through the wide opening in the front of their shells and probably be fatal. (Be careful, shooting flat to the water GREATLY increases your chances of bullets skipping and carrying far beyond your target.)

We did shoot a huge snapping turtle in his shell with a .22 long. It just made a big chip in his shell (but I think we were using soft lead hollow points).

If you are shooting turtles from "above" I would think almost any centerfire caliber with a FMJ bullet would go right through a shell.