Basically, BB ammos were shot from an airsoft gun into a water filled tank. The experiment was recorded using a video camera. I can calculate the approximate instantaneous velocity of the bullet under water at a given time using Logger Pro. 1. Relevant equations Drag force = 0.5 ρAC0v2 2. The problem statement, all variables and given/known data I have kept the density of water, ρ and the reference area, A constant. ρ of water = 997kg/m3 A = ∏ rsq. ( r = 0.003 m) I want to calculate Coefficient of drag of a spherical ammo to find a relationship between velocity and drag coefficient. Secondly, the velocity of the BB ammos under water that I calculated using Logger Pro indicates a very low velocity of about 0.7-3m/s (0.2 seconds after under water). The Reynolds number comes to about 0.03 3. The attempt at a solution I have searched online and found that the theoretical drag coefficient of a spherical object should be 0.47. The low Reyonlds Number indicates that the drag coefficient is not constant. I am facing difficulty because I have two unknown variables, Drag force and Cd. Another question: Can the drag force be calculated by Mass * deceleration. I know the mass to be 0.12g and can calculate the deceleration of the bullet... I am also not sure if Strokes Law may apply to this experiment because the Reynolds number is really low.