What Ed said, they have very strong skulls, and like armor on a tank, the sides also slope. They are very strong and tough for their body size, which can become quite large.
Ex-brother-in-law had problems with them at his sheep farm in east Conn. He gave up trying to hunt them with his 22 rimfire and called in a trapper. Only place he could see them during the day was out in the water and with other houses in the area, he was afraid on a rickochet.
The girl friend and I watched a large one from the window table of a riverside resturant that used to be a mill. We were told the DEC estimated that one was in the 50 - 60 lb range. When it slapped it's tail and dove under in the mill pond, the splash was like a gun shot, and about the size you'd see if you threw a cement block in.
Since they spend so much time in the water, you'll be lucky if you get to see more than just the top half of their head. Plus, you have to beware of what's beyond them if shooting over water. They have good eye sight and hearing, it's very tough to get close to one. They are not easy to "hunt" - that's one of the main reasons they use traps to hunt them.
Have you tied Predator pellets in your XL yet? Good energy and much better penitration than a domed pellet.
Paul.