If the bullet hits bone, like a shoulder, the polymer bullets can explode (splash) and not penetrate further. I shot a coyote with a 55 grain VMax in a 223 in the chest at about 75 yards. It spun and took off. I have shot coyotes at aver 200 yards, broadside, and it left a huge exit wound on the far side of the lungs with the same bullet. It did not hit bone on the way in. Several people I know of have switched to hollow point bullets, such as Sierra Gameking or Berger bullets to avoid bone splash.