Does velocity affect ballistic coefficient?

CONatureBoy

Well-Known Member
I loaded up a load for my 7 rum using 145 grain hammers. Average velocity of this round is 3635 FPS over 12-15 rounds fired over a magnetospeed. I zeroed the load at 200 yards and entered all the data into my ballistic calculator and went out to shoot steel and verify drops. I used a bc of .230 as indicated on hammers website, which they said was verified by drops, not estimated.

At 415 yards, I was about 1 1/4” high. Not too bad, so I backed off to 525 yards and was 3” high. When I got back home, I started making adjustments to the bc in the calculator. When I entered it as .320, it’s dead on with the results I saw on the steel plate.

I know my velocity numbers are good, as the SD of my load was single digits and the ES was around 13. I also measured the sight height, so that’s not giving me any issues either. The only thing “unknown” is the bc. Is it common to have to adjust the bc that far from the advertised bc? I know this is a pretty fast load, so maybe that has something to do with it? What are y’all’s thoughts?
If you want a fairly definitive, not-too-technical answer to the question how BC varies with velocity, consult "Chapter 2: The Ballistic Coefficient" of Bryan Litz, Applied Ballistics for Long-Range Shooting, 3rd Ed., pp. 15-39. "The ballistic coefficient is a measure of how well a bullet penetrates the air" (italics in the original). Form factor is one of the quantities in the formula for BC. It compares a bullet's drag to that of a standard (theoretical) bullet. Form factor in turn depends on the bullet's drag coefficient. The drag coefficient, hence form factor and BC, varies with speed. "As the bullet slows down, the drag coefficient increases until it slows to subsonic speed (below 1,120 fps in standard conditions); then it drops abruptly" (italics in the original). Here's an example of a published drag curve:

Hunt_4life

Well-Known Member
it seems like your speed and bc are both a bit off. So maybe up your speed 50 FPS and then play with the bc. But if the .32 bc makes all of your drops correct then it is just because the high speed.

ButterBean

Well-Known Member
LRH Team Member
If you want a fairly definitive, not-too-technical answer to the question how BC varies with velocity, consult "Chapter 2: The Ballistic Coefficient" of Bryan Litz, Applied Ballistics for Long-Range Shooting, 3rd Ed., pp. 15-39. "The ballistic coefficient is a measure of how well a bullet penetrates the air" (italics in the original). Form factor is one of the quantities in the formula for BC. It compares a bullet's drag to that of a standard (theoretical) bullet. Form factor in turn depends on the bullet's drag coefficient. The drag coefficient, hence form factor and BC, varies with speed. "As the bullet slows down, the drag coefficient increases until it slows to subsonic speed (below 1,120 fps in standard conditions); then it drops abruptly" (italics in the original). Here's an example of a published drag curve:

View attachment 294906
That's some great information but isn't that exactly what I said ?

CONatureBoy

Well-Known Member
That's some great information but isn't that exactly what I said ?
1. Uh, no.
2. It's different when a published expert says it with engineering precision.
3. I gave the audience a pointer to the technical literature, so they can learn more from a real expert if they want to.
Freedom of speech is a fine thing.

jraulsten

Well-Known Member
If you are using Applied Ballistics, try the Custom Curve. I've used it my last two guns and not had to true the BC...........yet

ButterBean

Well-Known Member
LRH Team Member
1. Uh, no.
2. It's different when a published expert says it with engineering precision.
3. I gave the audience a pointer to the technical literature, so they can learn more from a real expert if they want to.
Freedom of speech is a fine thing.
Yes it is, ..................But you have lot to learn Neighbor

ButterBean

Well-Known Member
LRH Team Member
If you are using Applied Ballistics, try the Custom Curve. I've used it my last two guns and not had to true the BC...........yet
With Hammers?

ButterBean

Well-Known Member
LRH Team Member
Nope.......One with Bergers and one with ELDs
Copy that, It wont work with Hammers

Well-Known Member
I loaded up a load for my 7 rum using 145 grain hammers. Average velocity of this round is 3635 FPS over 12-15 rounds fired over a magnetospeed. I zeroed the load at 200 yards and entered all the data into my ballistic calculator and went out to shoot steel and verify drops. I used a bc of .230 as indicated on hammers website, which they said was verified by drops, not estimated.

At 415 yards, I was about 1 1/4” high. Not too bad, so I backed off to 525 yards and was 3” high. When I got back home, I started making adjustments to the bc in the calculator. When I entered it as .320, it’s dead on with the results I saw on the steel plate.

I know my velocity numbers are good, as the SD of my load was single digits and the ES was around 13. I also measured the sight height, so that’s not giving me any issues either. The only thing “unknown” is the bc. Is it common to have to adjust the bc that far from the advertised bc? I know this is a pretty fast load, so maybe that has something to do with it? What are y’all’s thoughts?

ButterBean

Well-Known Member
LRH Team Member
Didn't upset me a bit, I've been saying the same for a long time , Shoot to verify and write it down

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Blackdirt Cowboy

Well-Known Member
What scope are you using and ballistics calculator?

BC is velocity dependent and all ballistic calculators need to be trued
Nightforce nx8 and Ballistic AE.

mulie

Well-Known Member
What is the twist rate on the barrel? Also where is the bullet stability cal. at your barrel's twist rate you maybe just on the edge. Also you need to insure your zero is really your zero as this does change over time and changes in temperature conditions can put ya that much off. And yes BC's can very alot from on the package. BC's change as your rifle ages (shot count) typically velocity will increase over time.

ButterBean

Well-Known Member
LRH Team Member
What is the twist rate on the barrel? Also where is the bullet stability cal. at your barrel's twist rate you maybe just on the edge. Also you need to insure your zero is really your zero as this does change over time and changes in temperature conditions can put ya that much off. And yes BC's can very alot from on the package. BC's change as your rifle ages (shot count) typically velocity will increase over time.
He has it figured out and is good to go