I always find that if you dont look directly at them (eg; look on the ground so they cant see your eyes), helps you get much closer to them.
And also if you aim on top of thair head, if the pellet drops a few centimeters its either going to hit him in the head, neck or the heart and lungs.
If you think your hitting them when you see some feathers come off it, your most likely wrong as about 30-40% of a magpie is feathers, thair body is quite small in consideration so you could just be skimming feathers off them , you'll know when youve hit them because you'll hear that unmistakeable "