Home Forums Hunting & Shooting Hunter's Campfire Ballistic Coefficient Question

BC is simply SD multiplied by a form constant.

You can think of BC as penetrating air. The big difference between air and tissue is that the bullet holds its form in air, but deforms in tissue.

I think you’re confusing drag coefficient with ballistic coefficient. I understand BC to be a calculation of both drag and sectional density along with velocity.

BC=Mass over cross section x drag coefficient
Not sure if I have the formula correct but it’s the basic idea.

Think of it this way.

Okay thanks.

My mistake on this one...

Well, yes and no.

The number we call BC is a comparison between the aerodynamic of a given bullet and a reference projectile, typically the 1" Krupps shell from WWI. At different velocities projectiles will have different relative performance to the reference projectile and hence a variable BC. Some bullet manufactures such as Sierra will give you a Bullets BC at a specified velocity, and this is why.

You're presuming they have same profile. The .243 bullet probably has more of a VLD profile.

Some of you fellas are overlooking a salient point. Cd is a variable in relation to velocity and bullet form can have surprising influence. For example, Cd diminishes as velocity increases above the realm of 3,000 FPS. Flat base bullet fare better at subsonic velocity than boat tails.

It’s enough to give a fella a headache.

There is a really good technical discussion of this on the Berger Bullets website. What is interesting is that the BC can be estimated by calculations, but only empirically determined by shooting the bullet and measuring speed differential along the flight path. Berger says they now use doppler radar to gauge the speed at all points along the path, which they say is more accurate than the old two point measuring system because the BC varies with velocity. All this loses me, and I go back to the basics--find a bullet that works and stick with it. I discount the manufacturers BC's as estimates only. You guys who are really into this stuff can have a heyday with it. Have fun!

Lima has a good point and it is complicated. The Berger manual has a pretty good writeup by Bryan Litz. it's probably the same thing on the site. I think the G1 drag coefficient was formulated for cannon projectiles and the G7 profile is more in line with what we have now.

Jack O'Connor had some good information on ballistics years ago. That's why he gravitated to the .270. It worked. We've come a long way since then and really for hunting purpose at sensible ranges G1 works ok and slower twist rates for hunting guns works fine. I will say that as you go up in speed you go down on barrel life. The 26 Nosler is fine but you are burning a lot of powder for not much gain. Same for most all the Weatherby line, the 300Wby being the exception. Those two mile rifles are pushing long, heavy bullets fast but barrel life isn't so much a consideration. I think Carol Shelby said, "it isn't how fast you can go, it's how fast can you afford to go".

Hahaha great quote.

That makes sense, for the drag coefficient at least. I still am not seeing how mass doesn't have an affect on the actual B/C, as mass gives momentum. That is of course assuming the posters saying that mass or weight has no bearing on B/C are correct (which is a big assumption). Most every link I see on Google Fu says mass is a function of B/C.

I get that speed is the biggie in determining wind drift, for the most part. That makes perfect sense, but in theory wouldn't a longer bullet (assuming the same b/c and diameter) have more surface area for wind to act upon and thus drift more?

Mass, velocity,, point of aim, and BC are all major actors in determining trajectory. But BC is separate from mass and velocity. You can think about it this way: Momentum is the "fuel" that makes a projectile go. As drag acts on the projectile, the projectile sheds momentum. The hollow aluminum shell and the solid tungsten bullet with the same shape will have the same drag. But the aluminum shell does not have much momentum, so it "gives up" in a hurry.

Wind deflection does not mostly come from the wind blowing against the side of the bullet. It happens because the bullet will very slightly "nose into" a crosswind, because that is the orientation of least resistance. So you have the drag vector pointing out of the base of the bullet, and it mostly points toward the muzzle, but slightly points toward the side. It's that component pointing toward the side that moves the bullet sideways. So bullets with higher BC are less deflected by the wind.

If you wanna be a scientist, go be a scientist.

For the purposes of an end user BC means how slippery the bullet is and nothing else.

It should also be noted that bullet makers lie for the sake of selling schit and proof is always in the pudding.

This!

The simplest way to view BC is how a bullet compares to the "standard projectile". Standard Projectiles are 1" in diameter and weigh 1 lb. The G1 standard projectile (the Krupp shell as Antelope Sniper calls it) is a pretty pokey looking design with a 3 caliber ogive IIRC...but by definition it has a G1 BC of 1.000 So all these zoot slippery bullets we're shooting mostly fall short of the ballistic performance of that one pound chunk of projectile (mass matters). G1 BC greater than 1.000, then your bullet is outperforming the G1 Standard Projectile. The G1 BC is good for 500 yards or so, then it's predictive value diminishes. The G7 Standard (again...one inch diameter, one pound but with a more VLD shape) is a better predictor because it's shape is a closer match to the bullets we shoot today.

BC is a measure of how well a bullet retains velocity over distance.

BC can be easily observed on the range. A doppler chronograph will graph velocity loss over range and determine BC.

Note that Sierra documents (as a rule) three bc values for their bullets according to velocity, recognizing that BC will change as velocity changes downrange.

If a computer is calculating BC from observed velocity loss, it will give differing values for bullets constructed of different materials. Ex: Tungsten, cup & core, Copper, Tin, or Aluminum.

Even if the Tungsten bullet, and the Aluminum bullet have identical profiles, and each has a 3000 fps mv, the Aluminum bullet will be traveling much slower at 500 yds down range. Thus it has a lower BC.

Here’s a high bc .257 bullet you need a 1-8 minimum twist. Looks like .664 G1 BC .332 G7

https://blackjackbullets.com/product/blackjack-131-grain-ace-match-bullet-100-ct/?v=ed5e635ddcfb

The other variable to consider is that the longer the bullet, the more twist it takes to stabilize it. More twist keeps it more steady on it's axis and helps counter act effects of wind. As an example, think of a football that is spun and thrown in a spiral vs a non spiral lame ass throw. The one thrown in a spiral deflects less off target in the wind.

Well my stab at it. Ballistic coefficient is a product of length, diameter and shape. A .224 bullet and a .308 bullet can have the same BC but the 30 caliber bullet will have to be heavier and longer to overcome the bigger frontal area.