The "trick" to making running shots is to have a good idea of what the time of flight is foryour bullet to the target. This isn't something you consciously calculate, but rather know instinctively from lots of shooting at targets that "react" to the bullet hit. Once your brain knows the time of flight, it is just a matter of figuring out how far the running deer is going to travel, relative to your line of sight, before the bullet gets there - andkeep the gun swinging as you anticipate that spot.