Thank you Bruce, this explanation is excellent , will study it ...if I want to do some long range shooting I need to come to terms with the terminology as well.can't edit the post re b.c. and s.d.
most things in shooting are statistical, rather than empirical.
you need to take a lot of samples and compare them
with s.d., say you are measuring velocities, s.d. is not meaningful for less than 10 shots, and more is better..