I have a ballistics chart for virtual every common rifle round, in various bullet weights. There's initials for a lot of things that I understand, (I.e MRT = mid range trajectory, MPBR = max. point bullet range). But the first line is BC, which I understand from the text, must mean "Ballistic Coefficient". A 55gr. .223 is .235 @ 3,000fps., a .308 averages .410 in three common weights at about 2,600fps. What is Ballistic Coefficient? What is it measuring?