BC values

[jonsidneyb]

It is clear that the highest BC values are not always chosen has other factors are important but out of interest.

starting at 6.5mm and going down what are the highest BC values found being used by accuracy shooters.

I would guess that by downscaling and upscaling the same bullet to various calibers would create the same BC or am I flawed in my thinking?

 

[Cheechako]

jonsi

You are correct. BC is but one factor in considering what bullet is best for your particular shooting discipline. High BC bullets generally find their niche at longer distances,500 yards plus) where retained velocity and wind deflection become important considerations.

To your question specifically, a 6.5mm, 25 cal, or 6mm bullet with a BC of .6 is considered very good. Really high BCs are not as easy to obtain in 22 caliber so a BC of.5 is about the upper limit, and very few of them exist.

Ray

 

[amamnn]

Something I learned a while back:

Contrary to what a lot--maybe most--handloaders think--BC is not a constant and can be changed to a degree by the loader. The BC figure you see published is often an average, sometimes an educated guess. Methods of calculating BC can be misleading. The true BC of your bullet changes over the course of its travel downrange. The velocity you push the bullet at will change BC. BC, though labeled as thus and such by the bullet maker, will not be 100% uniform in a box of bullets. This is one reason long range match shooters cull bullets for base to ogive length as well as weight, and trim meplats.

You might find this to be interesting reading.
http://www.shootingsoftware.com/coefficients.htm

 

[ChrisNam]

"Amamnn:
The velocity you push the bullet at will change BC."

This makes perfect sense. What doesn't make any sense to me is that BC decreases,according to either Bergeror Sierra - can't remember which one) with an increase of velocity once the bullet has reached a specific velocity

 

[dsbw]

Chris

Don't think you need to crack your head around it, with all due respect. A lot of these technical terms are used with a marketing purpose in mind, although there are BC value changes based on velocity differences. More important for you is to keep on practicing, get out on the range sort of thing.

 

[mikecr]

More important for you is to keep on practicing, get out on the range sort of thing.

I have to disagree with this completely.
Thinking, learning, and understanding, are the most important endeavors in your existance. What is gained if not for these?
Why do you shoot?

G1BC goes up with velocity, as drag coefficients decrease above Mach 1. And then your bullet's drag is divided by the G1 standard bullet's drag to get a 'form factor',a bridge). Then BC is calculated by dividing sectional density of your bullet by that form factor.
Your true BC doesn't change at all. You would see this if your drag curve was actually measured from your bullet instead of another standard. We have to accept this bridge/comparison in order to have any standard at all.

 

[amamnn]

understanding BC computation can come in handy when selecting bullets and deciding how much of the bullet maker's claim you are going to believe. It can also help in load development.

 

[jonsidneyb]

I am trying to picture in my mind what effect rifleing marks have on drag.

At first I thought it would be a major factor then realized that the bullet is also spining so as it moves forward the air molecules on the rifling marks will not be at a perpendicular angle to the lines.

I am going to just guess that as the the change in forward velocity becomes different from the rotational velocity the drag of the rifeling increases.

I do realize that doing is important but I am starting to find the science of it facinating. It is nice to think in ways that I am not accustomed to.

I wonder if anyone has done a study on the effects of rifeling marks and bullet flight.

 

[Bryan Litz Ballistics]

JonsidneyB,
I created an account to reply to this thread! I've wondered the same kinds of things as you about scaling effects on BC, rifling marks etc.
As for the scaling effects, I've written an entire 2 part article about it. You can find it on my website:
http://bryanlitz.bravehost.com/
On the bottom left, there's a couple links under "Understanding Long Range Bullets". You want 'Part 1: The nature of scale'.
As for the rifling marks, The answer may be different than you think....
At first thought, you might think the roughness adds drag, right? On second thought, the bullets spinning, so the air flow isn't hitting the grooves perpendicular,like you said). However, the bullets forward motion slows down faster than it's rotational speed, so downrange the speeds don't match up, and there's drag again, right...?
Here's what I think about the effect of riflings on bullet drag...
The grooves are small enough to be burried in the boundary layer where the air isn't moving at full speed, so if there is added skin friction drag, it's negligable. However, there is a bigger effect at work.
A boundary layer can be laminar or turbulent, depending on the length, speed, and surface roughness of the projectile,the projectile's 'Reynolds number'). Now, there's less skin friction drag in a laminar boundary layer than a turbulent boundary layer. So laminar is always better, right? Not always. The case where a turbulent boundary layer is desireable is when you have an adverse pressure gradient, and you want the flow to stay attached to the body, like along a boat-tail, or the back of a racecar, airplane, etc. If the flow stays attached along the tapered boat-tail, the bullet has less base drag. But it's harder for the boundary layer to stay attached if it's laminar. In other words, the extra skin friction drag of the turbulent boundary layer is mroe than made up for by the fact that the turbulent boundary layer is more energetic, and can stay better attached to the tapered afterbody,boat-tail), and yeild less base drag,much more important than difference in skin friction).
It's hard to predict exactly when transition,from turbulent to laminar) will occur, but the things that foster transition are: length, speed, and surface roughness. The riflings are the perfect height of surface roughness to cause boundary layer transition, and help keep the flow attached along the boat-tail. Wind tunnel models will often use artificial roughness strips on the leading edges of subscale aircraft and missile models to trigger early transition in an attempt to match the flow charachteristics of larger scale airframes.,getting off topic, sorry...)
Back to riflings and boat-tails...
I'm not saying that boat-tails would be completely ineffective with a laminar boundary layer, but by working with a turbulent boundary layer,thanks in part to the 'surface roughness' riflings), the boat-tail can perhaps opperate at a steeper angle. For example, laminar flow might separate from a 4 degree boat-tail. However, a turbulent boundary layer might stay attached to a 8 or 9 degree boat-tail, yielding less base drag than the 4 degree boat-tail. The trick is finding the steepest boat-tail angle you can get away with, while maintaining attached flow. If the flow separates, the bullet might as well be a flat-base from a base drag point of view.
If all this sounds like I'm talking 'smack', did you ever wonder why golf balls have dimples? It's for the EXACT reason I'm talking about. The dimples cause boundary layer transition which enables the flow to stay attached a little farther around the back of the ball, and have a smaller area of detached flow, ie, less base drag.
In conclusion, I believe that if riflings have any effect at all on the overall drag of a bullet, it's a net decrease in drag. The riflings help to transition the boundary layer to turbulent, thus enableing the flow to negotiate steeper boat-tail angles, and decrease the area of separated flow behind the bullet, which decreases base drag.
-Bryan

Reference:
1. Modern Exterior Ballistics By Robert McCoy page 77
2. Basic Fluid Mechanics By David Wilcox
3. Also see: This link
and
this other link

 

[jonsidneyb]

It doesn't sound like smack at all. It makes sence to me, I just wonder what the difference in theory and reality is on this.

Why I kept thinking about it is it made me wonder if different rifling types will have a difference in BC. Like the number of groves, how deep they are, how wide, or if it is polygonal.