The average speed of a bullet is 2500 feet per second, anything faster and you are looking at rocket grade. So when Calvin saw the gun fire he knew there was no time, he started running knowing that at best it would hit him and at worst her, he threw himself into the path of the speeding bullet. The bullet hit, his stomach taking most of the force. It was messy he collapsed onto the ground still in shock that he made it in time, people started running over to him and the world started to look strange, his focus swam in and out faces kept appearing and disappearing, he closed his eyes hoping it would slow everything down.
The ambulance arrived and took him to the hospital, they just about managed to catch the bullet before it travelled further into his system and he was set to recover in ICU until he was stronger. His strength grew slowly until he was back on his feet, life was never normal again.
People asked him how he survived, he explained his will to live was more than his desire to die.