  This paper describes experimental methods for free flight measurement of drag
coefficients to an accuracy of approximately 1%. There are two main methods of
determining free flight drag coefficients, or equivalent ballistic
coefficients: 1) measuring near and far velocities over a known distance and 2)
measuring a near velocity and time of flight over a known distance. Atmospheric
conditions must also be known and nearly constant over the flight path. A
number of tradeoffs are important when designing experiments to accurately
determine drag coefficients. The flight distance must be large enough so that
the projectile's loss of velocity is significant compared with its initial
velocity and much larger than the uncertainty in the near and/or far velocity
measurements. On the other hand, since drag coefficients and ballistic
coefficients both depend on velocity, the change in velocity over the flight
path should be small enough that the average drag coefficient over the path
(which is what is really determined) is a reasonable approximation to the value
of drag coefficient at the near and far velocity. This paper considers these
tradeoffs as well as practical considerations for obtaining accurate near and
far velocity measurements and the impact of different sources of error
(velocity, distance, time, atmospheric conditions, etc.) on the resulting
accuracy of drag coefficients and ballistic coefficients. For a given level of
accuracy of various quantities, the method of using near and far velocities
usually produces drag coefficients with about half the uncertainty of the
method using a near velocity and time of flight.
