Home Forums Hunting & Shooting Hunting Rifles Ballistic Coefficient question

So, I've seen several references to a bullet's ballistic coefficient changing if it is moving above X velocity, below Y velocity or between X and Y velocity, etc. The Sierra website is a prime example of this but I have seen it elsewhere as well.

I thought ballistic coefficient was simply how aerodynamic a bullet is...how well it moves air out of the way as it speeds through. It is a function of weight, shape, ogive, meplat dimensions, boattail design, etc. right? I don't see how all those factors change at different speeds.

A given bullet's flight path certainly will change with speed...as in a faster bullet flies flatter and a blunt bullet sheds velocity faster than a VLD type bullet, but how does velocity alone change a ballistic coefficient?

Can anyone explain this for a relative newbie?

T - I'm waiting for a good answer myself.

I could understand IF, IF somehow more/less speed actually affected the bullet's shape
'sufficiently' ? ?
To date I've been doubtful about the BC changing.

Jerry

A ballistic coefficient is just a mathematical model of a given bullet compared to a theoretical bullet of similar shape. This theoretical bullet is called a G1, G2...G7, etc. At different speeds the model either matches closer to the actual bullet, or further away. The variation is compensated for by changing the coefficient. The closer/more perfectly the model matches the actual bullet, the less variation there is in BC over a wide range of velocities. This is seen by comparing the G1 BC of a VLD-type bullet over a wide range of velocities, with the G7 BC figures. Form factor is a bit better way of modeling a bullet's aerodynamic properties compared with other bullets, but it still relies on modeling the bullet based on a theoretical bullet shape.

Ballistic Coefficient
ELD® Match and ELD-X® Ballistic Coefficient Values
Hornady originally published “800 yard”* average Ballistic Coefficient (BC) values for ELD-X and ELD Match bullets. This was done to provide the most usable BC from a trajectory prediction standpoint. Unfortunately many shooters did not understand that Hornady was listing a more useable “800 yard” BC while other manufacturers list “200 yard”* BCs. When doing pre-purchase research either on the web or at the store, the increased performance of the ELD Match and ELD-X bullets wasn’t always realized in a head-to-head BC comparison. Hornady will now publish “200 yard” (Mach 2.25 / 2,512 fps @ ICAO Standard Atmosphere**) G1 and G7 Ballistic Coefficient (BC) numbers for all ELD-X and ELD Match bullets. Mach 2.0 (2,233 fps @ ICAO Standard Atmosphere) and Mach 1.75 (1,954 fps @ ICAO Standard Atmosphere) BC values will also be available on the Hornady website (www.hornady.com/BC). As Hornady moves the industry to drag coefficient based trajectory calculations for these types of projectiles (www.hornady.com/4dof) the Ballistic Coefficient is becoming somewhat irrelevant except for those still using BC based trajectory calculators or when using BC as a rating criteria for bullet performance.

Ballistic Coefficient (BC) values at Mach 2.25 (2512 fps @ ICAO Standard Atmosphere) should be used when comparing to other published BC values within the industry. Ballistic Coefficient values at Mach 1.75 (1954 fps @ ICAO Standard Atmosphere), should be used when calculating trajectories for shooting beyond 600 yds. Ballistic Coefficient values at Mach 2.0 and 1.75 offer comparisons to other manufacturers that publish multiple BC’s based on velocity.

In addition to the change listed above, a select few ELD-X and ELD Match bullets initially had BC values that were determined from prototype bullets. As Hornady moved into production of these bullets, the BC value increased (a positive gain) due to the improvements in the manufacturing process from R&D to production machinery.

*The Science Behind It
Ballistic Coefficient (BC) values can, and usually do change in value with changes in velocity. Most bullets exhibit a lowering BC as velocity slows. The extent of how much a BC will change depends on each unique bullet shape. When comparing BC’s of different bullets, it is important to use an apples to apples approach. To do this, bullets should be compared at a given Mach number (Mach 1 = 1116.48 fps @ ICAO Standard Atmosphere). Mach number is the velocity of the bullet divided by the speed of sound. If a Mach number is unavailable, velocity can be used if the BC’s are corrected to Standard Atmosphere which is fairly typical practice within the industry. The majority of industry published BC’s are measured over relatively short ranges of 100 -300 yards which corresponds to velocities around 2500 fps depending on muzzle velocities. Some manufacturers offer BC values at different velocity ranges.

** Hornady BC values have been corrected back to International Civil Aviation Organization (ICAO) Standard Atmosphere (Sea Level, 59 degrees F, 29.92 inHg, 0% Humidity).

Expanded BC Values for ELD Match Bullets
Bullet
Sectional Density
Use this for velocities above 2,500 fps and distances out to 300 yards.

Mach 2.25
2512 fps* and above
Use this for mid-range shooting where the bullet spends time in flight after slowing down.

Mach 2.0
2232 fps*
Use this for long-range shooting where much of the bullet flight occurs after velocity loss.

Mach 1.75
1953 fps*
22 Cal 73 gr. ELD Match 0.208 0.398 G1
0.200 G7 0.389 G1
0.195 G7 0.382 G1
0.192 G7
22 Cal 75 gr. ELD Match 0.214 0.467 G1
0.235 G7 0.459 G1
0.230 G7 0.441 G1
0.222 G7
22 Cal 80 gr. ELD Match 0.228 0.485 G1
0.244 G7 0.477 G1
0.239 G7 0.462 G1
0.232 G7
6mm 108 gr. ELD Match 0.261 0.536 G1
0.270 G7 0.526 G1
0.264 G7 0.520 G1
0.261 G7
6.5mm 100 gr. ELD Match 0.205 0.385 G1
0.194 G7 0.377 G1
0.189 G7 0.369 G1
0.186 G7
6.5mm 120 gr. ELD Match 0.246 0.486 G1
0.245 G7 0.465 G1
0.233 G7 0.450 G1
0.227 G7
6.5mm 123 gr. ELD Match 0.252 0.506 G1
0.255 G7 0.493 G1
0.247 G7 0.481 G1
0.242 G7
6.5mm 130 gr. ELD Match 0.266 0.554 G1
0.279 G7 0.542 G1
0.272 G7 0.535 G1
0.269 G7
6.5mm 140 gr. ELD Match 0.287 0.646 G1
0.326 G7 0.637 G1
0.320 G7 0.616 G1
0.310 G7
6.5mm 147 gr. ELD Match 0.301 0.697 G1
0.351 G7 0.662 G1
0.332 G7 0.637 G1
0.321 G7
7mm 162 gr. ELD Match 0.287 0.670 G1
0.338 G7 0.655 G1
0.329 G7 0.637 G1
0.320 G7
7mm 180 gr. ELD Match
1 in 8.75" Twist 0.319 0.777 G1
0.391 G7 0.748 G1
0.376 G7 0.731 G1
0.368 G7
7mm 180 gr. ELD Match
1 in 7.5" Twist 0.319 0.816 G1
0.411 G7 0.812 G1
0.408 G7 0.782 G1
0.394 G7
30 Cal 155 gr. ELD Match 0.233 0.461 G1
0.232 G7 0.451 G1
0.227 G7 0.439 G1
0.221 G7
30 Cal 168 gr. ELD Match 0.253 0.523 G1
0.263 G7 0.516 G1
0.259 G7 0.498 G1
0.251 G7
30 Cal 178 gr. ELD Match 0.268 0.547 G1
0.275 G7 0.527 G1
0.265 G7 0.506 G1
0.254 G7
30 Cal 208 gr. ELD Match 0.313 0.690 G1
0.348 G7 0.683 G1
0.343 G7 0.669 G1
0.337 G7
30 Cal 225 gr. ELD Match
1 in 10" Twist 0.339 0.777 G1
0.391 G7 0.752 G1
0.378 G7 0.719 G1
0.362 G7
30 Cal 225 gr. ELD Match
1 in 7" Twist 0.339 0.798 G1
0.402 G7 0.782 G1
0.393 G7 0.749 G1
0.377 G7
338 Cal 285 gr. ELD Match 0.356 0.829 G1
0.417 G7 0.814 G1
0.409 G7 0.796 G1
0.400 G7
*Velocity at International Civil Aviation Organization (ICAO) Standard Atmosphere (Sea Level, 59 degrees F, 29.92 inHg, 0% Humidity).



Expanded BC Values for ELD-X Bullets
Bullet
Sectional Density
Use this for velocities above 2,500 fps and distances out to 300 yards.

Mach 2.25
2512 fps* and above
Use this for mid-range shooting where the bullet spends time in flight after slowing down.

Mach 2.0
2232 fps*
Use this for long-range shooting where much of the bullet flight occurs after velocity loss.

Mach 1.75
1953 fps*
6mm 103 gr. ELD-X 0.249 0.512 G1
0.258 G7 0.505 G1
0.253 G7 0.498 G1
0.251 G7
6.5mm 143 gr. ELD-X 0.293 0.623 G1
0.314 G7 0.604 G1
0.303 G7 0.584 G1
0.294 G7
270 Cal 145 gr. ELD-X 0.270 0.536 G1
0.270 G7 0.521 G1
0.262 G7 0.512 G1
0.257 G7
7mm 150 gr. ELD-X 0.266 0.574 G1
0.289 G7 0.563 G1
0.283 G7 0.558 G1
0.281 G7
7mm 162 gr. ELD-X 0.287 0.631 G1
0.318 G7 0.626 G1
0.314 G7 0.615 G1
0.308 G7
7mm 175 gr. ELD-X 0.310 0.689 G1
0.347 G7 0.683 G1
0.343 G7 0.578 G1
0.341 G7
30 Cal 178 gr. ELD-X 0.268 0.552 G1
0.278 G7 0.543 G1
0.273 G7 0.538 G1
0.271 G7
30 Cal 200 gr. ELD-X 0.301 0.597 G1
0.301 G7 0.588 G1
0.295 G7 0.578 G1
0.291 G7
30 Cal 212 gr. ELD-X
1 in 10" Twist 0.319 0.663 G1
0.334 G7 0.649 G1
0.326 G7 0.643 G1
0.324 G7
30 Cal 212 gr. ELD-X
1 in 7" Twist 0.319 0.702 G1
0.354 G7 0.686 G1
0.345 G7 0.677 G1
0.341 G7
30 Cal 220 gr. ELD-X 0.331 0.654 G1
0.329 G7 0.643 G1
0.323 G7 0.643 G1
0.323 G7
*Velocity at International Civil Aviation Organization (ICAO) Standard Atmosphere (Sea Level, 59 degrees F, 29.92 inHg, 0% Humidity).

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That's not a exact answer but good info.

T you would enjoy Brian's books. Start with Applyed ballistics for the long range shooter.

Brian is quite simply a genius and his work bringing lay terms to ballistics is stand alone.

Jordan, I appreciate the answer...I see what you're getting at, but it still isn't totally clear. I (somewhat) understand the difference between G1 and G7 and realize one is more accurate for flat point bullets and one is better for VLD type bullets...this alone gets me confused. A blunt bullet obviously sheds velocity faster than a VLD and thus has a lower ballistic coefficient value...no need for different models...


I thought ballisticians measured ballistic coefficient by firing a given bullet at a given muzzle velocity at several different ranges, measuring the difference in drop and then crunching the numbers to come up with the ballistic coefficient. Only a change in bullet mass or shape would change the determined value of how well it pushes air out of the way...especially since the bullets are fired at ICA Standard atmosphere conditions (sea level, 59 degrees F, 29.92 inHg, 0% Humidity) as depicted in Fred's post above.

Am I wrong about that?

I think you and I need to just stick to the 223AI thread

Thanks Fred. That kind of hurts my head ...especially the different distance (200 yard vs 600 yard vs 800 yard, etc.) ballistic coefficients but do see what you and Jordan are getting at.

I'll have to look into Brian's books.

Thanks!

It's not that hard. Picture airflow over a Porsche at different speeds. Not hard to picture how it could vary, right? Same-same.

Jeffrey,

why are you still concerned with things about firearms, bullets and ammo
when after publicly admitting to growing and using dope?

According to the Federal Government, someone who habitually uses Federally illegal drugs,
much growing them to boot, are not considered to legally able to own firearms.

Wonder when that BATF knock on the front door is coming?

Keep us posted when that happens...

We'll all have a pity party for ya, with you as the guest of honor!

BC is pretty important. Then it isn't.

BC is only important to those who are unethical hunters and shoot at game too far away....

It's been decades since college, but I think the short answer is: It's the air that changes WRT velocity, not the shape of the bullet. For example, below Mach 1, air is a compressible fluid, above Mach 1 air is incompressible. Because the medium (in this case air) is variable, the drag descriptor (in this case BC), will vary as well.

T I have been entering the data hornady provide for the ELD line of bullets. Using g7. And then the different BC's at the different Mach's. And the lead lands just near perfect every time I do my part.

The type of answer given by someone clueless about BC. The fact is that lightweight bullets with average or even poor BC can be shot faster than heavier high BC bullets and at even long range hunting situations the lower BC bullet will shoot flatter. You don't see the heavier bullets start to have less drop until well past the ranges any hunter would shoot. We're talking over a mile.

To a hunter high BC bullets mean more speed and energy at impact at even moderate ranges. A 30-06 shooting a 180 gr bullet with just decent BC's will hit with more speed and energy at 100 yards than a 180 gr RN bullet fired from a 300 WM.

If I compare my 308 with high BC 180 gr bullets to 30-06 with average BC 150 gr bullets the 30-06 bullets leave the muzzle 350 fps faster. But by the time both bullets reach 250 yards the 180's from my 308 are exactly the same speed with over 300 ft lbs of energy advantage. The 30-06 still shoots flatter at 250 yards, by 1". Most people wouldn't call 250 yards unethical, but by carefully choosing your bullets you can get more performance with less recoil with high BC bullets. Push the range to 500 yards and the 308/180 is still at almost 1900 fps and over 1400 ft lbs energy. The 30-06/150 is down to under 1700 fps and less than 1000 ft lbs. It still has less drop though, about 1".

The difference is even more dramatic when you compare different calibers.

Good grief, JMR....Greggy forgot his sarcasm font....

I was only repeating what the .270 shooters have said numerous times...

Yup. All that bullschit sounds good on paper....

BC over .475 launched around 2,950 will take care of business. Diameter won't matter. But it's only been talked about a few thousand times.

Basically I sum it up as , when zeroed 2.5" high round nose bullets at reasonable speeds shoot flat enough to 200 yds. Pointy bullets add about another 125 yds. At that point you need to get closer.

Unless one has practiced and is proficient at longer ranges. That's what you meant, isn't it?