The trajectory of a pellet, and the velocity along the trajectory, can be calculated using Newton's Second Law: F=ma Force = mass * acceleration. Two additional pieces of information are needed: the initial velocity leaving the barrel and the drag force equation for the pellet. The derivation and resulting differential equation can be difficult to solve. But there are free software programs on the www that will solve the trajectory for you, and will answer the questions you're asking. I like Hawke Chairgun Pro.