I was puzzled when I read a footnote saying that the B.C. changed with velocity. Is someone pulling my leg?

Nope, nobodies pulling your leg. The B.C. is the ratio of the drag of the bullet in question to a standard bullet. Any difference in shape or weight changes the drag at different velocities. Sierra is the only major manufacturer that accounts for this. Some of their bullets have an almost constant B.C. but others change quite a bit. The 200 gr. .358 RN is the most variable of the ones I use.

.148 @ 2000 fps & above
.203 between 2000 & 1500 fps
.247 between 1500 & 1200 fps
.271 between 1200 & 1000 fps
.230 @ 1000 fps & below

Sierra's Manual explains this quite well. The others give you an average B.C. at the range of velocities they expect you to use the bullet at. Some people's numbers are hopelessly optimistic.

Bye
Jack

OK, educate me here.

I've heard of Ballistic Coefficient. What's it measure, and why do I need to know?

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PowderBurns Black Powder / Muzzle Loading Forum:

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Hi powderBurns,
B.C. can be very important if you want to shoot at long range or with slow bullets. It's very simple, if your bullet have a high BC it "fly" flat and have a little wind sensibility; if BC is low your bullet will brake the fly.
You can see all the right teory at ths Sierra manual, I find it's a good manual.
ciao FAINA

BC is a math drag function- a complex formula that computes a forms ability to slip thru air. No relation to wind bucking ability-- which is SD or sectional density- which computes a slug's ability to resist wind drift. All 200 gr bullets for instance in 30 cal would have the same SD- no matter the ogive shape.

Sectional density has little to do with wind drift. The Ballistic Coefficient and velocity are the important factors. Compare the drift on a round nose vs. pointed bullet of the same calibre, weight and velocity in the charts of the Sierra or Hornady manuals. If you want a real head scratcher, compare the drift of a bullet at MVs of 900, 1200 and 1500 fps. Yup, the slow one drifts the least unless the B.C is very high. Sierra will tell you why.

Bye
Jack