Ballistic Coefficient questions

[rangeball]

assuming a similar nose profile, is BC primarily a function of length to width ratio? Does weight figure into it at all?

Assume two 250gr bullets, both .45 but one is twice as long as the second. Which has the higher BC?

[deadeye]

a ballistic coefficient is really a gray area numerical number given to a projectile to resist air or its rated ability to fly,the higher the better,the longer one more likely the higher bc

[edge]

Oct 15, 2010 14:10:20 GMT -5 rangeball said:

assuming a similar nose profile, is BC primarily a function of length to width ratio? Does weight figure into it at all?

Assume two 250gr bullets, both .45 but one is twice as long as the second. Which has the higher BC?

Basically it is Form Factor x Sectional Density.

Two bullets with identical dimensions but of different materials...say a jacketed bullet and pure tungsten.

The tungsten bullet will weigh almost twice as much and the BC will be double too!

A flat based bullet has a BC derived from:

the nose drag ( basically how pointed it is and how abruptly the air has to move past the bullet)

The skin drag, a longer bullet has more skin and this causes friction

Base drag, this is second to the nose drag in importance.

At supersonic velocities the nose drag is the largest component of drag, then base drag, and skin is normally under 10%

Adding mass adds BC fast. A 452 XTP of 250 and 300 grains goes from about 160 to about 200......basically the ratio between their weights.

edge.

[tdillinger]

try Bergers web site for bc stuff I played with it and is accurate to me what i am using and shooting.

www.bergerbullets.com/Ballistics%20Program/index.html

[pposey]

"Two bullets with identical dimensions but of different materials...say a jacketed bullet and pure tungsten.

The tungsten bullet will weigh almost twice as much and the BC will be double too!"

Say what? if the outside dimensions are what forms the BC of a bullet then how will the weight effect the BC if they are the exact same dimensions???

Now a longer bullet with the same diamiter, say .308, will have a higher BC if the base and the point are exactly the same as a lighter weight shorter bullet,,, the 150 grain Accubond has a published BC of .435 while the 200 grain accubond has a published BC of .588,

[edge]

I have a question for you.

Do you think that a 300 grain XTP has a superior BC due to its length or due to its mass?

The answer is MASS!
Length with the same mass only increases skin drag.

A flat based bullet has Nose Drag, Base Drag, and Skin Drag in that order! Skin Drag is generally under 10% on a normal bullet at normal velocities.

Since the shape and mass determine BC, you can have a better shape or add more mass to increase BC

edge.

Oct 17, 2010 9:03:26 GMT -5 pposey said:

"Two bullets with identical dimensions but of different materials...say a jacketed bullet and pure tungsten.

The tungsten bullet will weigh almost twice as much and the BC will be double too!"

Say what? if the outside dimensions are what forms the BC of a bullet then how will the weight effect the BC if they are the exact same dimensions???

Now a longer bullet with the same diamiter, say .308, will have a higher BC if the base and the point are exactly the same as a lighter weight shorter bullet,,, the 150 grain Accubond has a published BC of .435 while the 200 grain accubond has a published BC of .588,

[rossman40]

I have to back up Edge in that mass or SD are a major factor in determining the BC. Basic Newton physics. You also have to remember you have your BC but how you use the BC in your ballistic computation with a drag factor is another important point. The G1 drag factor dates back to the late 1800s with Siacci, Mayevski and James Ingalls ( his books written then are way ahead of their time and still good reading). You come up with a BC and use a drag factor but the data is not right because the bullet doesn't match the drag factor 100%.

But many of us have learned never to trust published BCs and if you want to use computer generated data you have to verify.

[pposey]

Dohhhhh,,, I stand corrected, ;D, for some reason I never connected weight with BC just the longer shape of a heavier bullet being more streamlined, for other not so sure's

en.wikipedia.org/wiki/Ballistic_coefficient

all ya ever needed to know about BC

[rangeball]

Thanks fellas. Another fine idea flushed down the tube due to logic and reason