Originally Posted by flashhole
Please explain this comment. The BC is the figure of performance that bucks wind resistance and keeps the bullet going faster, longer, thus providing better trajectory. The bearing surface of the bullet is what influences stability related to barrel twist. Long bullets in slow twist barrels don't obtain adequate rotational velocity to stabilize. A pointy bullet will have the same stability as a stubby bullet for the same twist barrel but the BC will define down range performance.
looking in the glossary of the Hornaday manual it states:
* "an index of the manner in which a particular projectile decelerates in free flight. Expressed mathematically as c= WID2" ("D" squared)
W= mass weight in pounds
I = form factor or coefficient in form
D= diameter squared
Speer words it a little differently, but is essentially the same thing. The call it out as a ratio of the sectional density to the bullet form. In other words it's ability to overcome air resistance during flight.
The above ratio also helps to fight wind drift. But in the end weight means very little in barrel selection as the ballistic coefficient determines the needed twist rate. A fine example of this is the Speer 52 grain HP is listed at a .225 B/C (#1035), while the 52 grain BTHP Match bullet (#1036) is listed at .253. The difference is purely shape.
rotational speed is old (real old) school technology, and has been proven not to work all that well when figuring what twist rate is needed to stabilize a bullet. Walt Berger once told me that it took 179K rpm to stabilize a VLD. Looks good on paper, but will not always work. Later I changed the way I looked at his idea to 179K at to impact. A little better, but still not right. Then I read a very good article in Precision Shooting on bullet over stabilization. It brought in the center of gravity factor and what an error in C/G can cause. I learned that a .0001 error in C/G (very common and often worse) will cause 3/32" growth in groups at 100 yards, and over spinning them will increase this.