If you were shooting sabot, that could be your problem. My clock reads the last thing through. At the range of your clock location, the sabot probably have separated from the bullet, and the clock read the speed of the sabot, and not the speed of the bullet. When shooting where i can find sabot, i start finding them on the ground around 8yd out, and don't find any more beyond 20yd. I normally put my clock near 20 yard, and use a ballistic program to calculate muzzle velocity working backward from the 20 yard velocity.