BC to bullet weight ratio

[Rocketvapor]

Thanks for the math check
My little 22 Nosler weighs 17 (20 pounds with a 3 pound bipod or a lead filled bag rider).

Thanks for the links in your NEXT post.

 

[Beiruty]

Using data in Litz book over a wide range of bullet calibers, designs, and weights there is a perfect correlation between G1 and G7 meaning the characterization is not different for the two methods. Of course the flight models are different with a
resulting impact on ballistics.

A fixed numerical relationship between G1 BC and G7 BC is not accurate, G1 BC varies a lot with respect to the bullet velocity as it flies down the range, the G7 BC is almost stable after the mid-range,

Using G7 BC drag model would predict the trajectory more accurately than the G1 BC model all the way down to 1.2 Mach

Bryan Litz

G1 vs. G7 Ballistic Coefficients — What You Need to Know « Daily Bulletin

The better, up-to-date ballistics programs let you select either G1 or G7 Ballistic Coefficient (BC) values when calculating a trajectory. The ballistic coefficient (BC) of a body is a measure of its ability to overcome air resistance in flight. You've probably seen that G7 values are...

bulletin.accurateshooter.com

More science and data

G1 BC vs G7 BC vs Bullet-Specific Drag Models

“In the past 4-5 years we’ve made quantum leaps when it comes to predicting bullet trajectories.” –...

precisionrifleblog.com

 

[CharlieNC]

A fixed numerical relationship between G1 BC and G7 BC is not accurate, G1 BC varies a lot with respect to the bullet velocity as it flies down the range, the G7 BC is almost stable after the mid-range,

Using G7 BC drag model would predict the trajectory more accurately than the G1 BC model all the way down to 1.2 Mach

Bryan Litz

G1 vs. G7 Ballistic Coefficients — What You Need to Know « Daily Bulletin

The better, up-to-date ballistics programs let you select either G1 or G7 Ballistic Coefficient (BC) values when calculating a trajectory. The ballistic coefficient (BC) of a body is a measure of its ability to overcome air resistance in flight. You've probably seen that G7 values are...

bulletin.accurateshooter.com

More science and data

G1 BC vs G7 BC vs Bullet-Specific Drag Models

“In the past 4-5 years we’ve made quantum leaps when it comes to predicting bullet trajectories.” –...

precisionrifleblog.com

I agree completely that the models provide different trajectories. But given a perfect correlation, how would G7 tell one anything different than G1? In other words if I know G1 I can calculate G7 so the two characterizations do not include any unique information vs each other.

 

[Beiruty]

I agree completely that the models provide different trajectories. But given a perfect correlation, how would G7 tell one anything different than G1? In other words if I know G1 I can calculate G7 so the two characterizations do not include any unique information vs each other.

A fixed numerical relationship between G1 BC and G7 BC is not accurate, G1 BC varies a lot with respect to the bullet velocity as it flies down the range, the G7 BC is almost stable after the mid-range,

 

[Beiruty]

My 308" 22" thin barrel

Microsoft OneDrive

1drv.ms

 

[Beiruty]

My 6.5 PRC 30" long, 1.2" thick barrel

Microsoft OneDrive

1drv.ms

 

[Rocketvapor]

Nice looking rig.
I shoot against quite a few nice looking rigs.
Don't win but still have fun. Still have room to improve/

You are right about barrel life. I've got close to 2000 rounds down this X-Caliber 28" bull and this year have to load longer by about 0.050".
My 88s are out to 2.510" @ 30 off.
Probably replace this winter.
Using 6MM HAGAR resized to 22 NOSGAR I start losing pockets by 3rd fired (12 out of 120).
Only a fool would shoot a 22N in F-Open, but that's sort of the point

 

[Smokedpole]

ballistically speaking, one should consider the ‘bullet impact angle’. The steeper the impact angle, the smaller the hittable surface area for the xring, 10 ring or 7 ring. The axis of the circle is changed. You can say the same for wider bullet drifts. Hypothetically speaking, a target leaning forward 3 degrees from perpendicular will score lower than a target leaning backwards 3 degrees at 1000 yards. Shooting high drifting high altitude bullets is always a disadvantage.

 

[Ned Ludd]

A bullet's weight has already been figured into its BC (i.e. BC = (weight/7000)/(caliber*2 x i) where i= the form factor of the bullet). If you divide bullet BC by the weight of the bullet, which has already been factored into the BC calculation, you are simply looking at a multiple of 1/i for a specific caliber, because the caliber and 7000 values are both constants (i.e. BC/W = (1/7000)/(caliber*2 x i). I don't see how this is really any more useful for the average shooter than the original BC value. If you just want to look at the form factor, Berger provides this value for their bullets, or you can buy a copy of "Ballistic Performance of Rifle Bullets" by Bryan Litz, which contains form factor values for a fairly wide range of bullets from other manufacturers. Otherwise, how does dividing the BC by bullet weight, which is a fixed value and already part of the BC, really tell you anything more than the BC itself?

Regarding the topic of "fixed value" relationship between G1 and G7 BCs, again, it all boils down to the form factor. If one divides the average G7 BC for a given bullet by its average G1 BC using actual measured BC values such as are found in Bryan Litz's book, what they will find is that a value of fairly close to 0.51 will be obtained for a surprisingly wide range of bullets. I believe that turns out to be the relationship between the G1 and G7 form factors, which is not surprising because in the formula listed above to calculate BC, all the other variables will be identical for a single bullet except the two different form factors, and thus will drop out of the equation, leaving only the ratio of the G1 and G7 form factors. So yes, there is a "fixed" relationship between the G1 and G7 BC values for a given bullet, as described in the BC equation. The difficulty in directly comparing the two lies in that BCs derived using the G1 method need to be "velocity banded" to better fit the actual drag curve across the typical velocity range. This is because the G1 form factor is a relatively poor representation of the actual shape of the bullets most of us are using, so the BC needs to be velocity banded in order for the actual data to fit. The G7 BC is much less sensitive to velocity than the G1 BC because the G7 form factor is a much better representation of the shape of the bullets we typically use in precision shooting.

 

[Rocketvapor]

You mean I did all that math for nothing?

 

[davidjoe]

A bullet's weight has already been figured into its BC (i.e. BC = (weight/7000)/(caliber*2 x i) where i= the form factor of the bullet). If you divide bullet BC by the weight of the bullet, which has already been factored into the BC calculation, you are simply looking at a multiple of 1/i for a specific caliber, because the caliber and 7000 values are both constants (i.e. BC/W = (1/7000)/(caliber*2 x i). I don't see how this is really any more useful for the average shooter than the original BC value. If you just want to look at the form factor, Berger provides this value for their bullets, or you can buy a copy of "Ballistic Performance of Rifle Bullets" by Bryan Litz, which contains form factor values for a fairly wide range of bullets from other manufacturers. Otherwise, how does dividing the BC by bullet weight, which is a fixed value and already part of the BC, really tell you anything more than the BC itself?

I agree it’s purely derivative, and form factor is looking towards the same efficiency principle, but form factor cannot be easily ascertained. You have to know the sectional density of the bullet (and g7 BC).

Whereas bullet weight is accurately published and verifiable, and BC is usually close nowadays, if optimistic in the past, sectional density and G7 BC is just not a specification the manufacturers list in publicly available information, at least not widely.

Added: It’s kind of an ease to maximize, and efficiency quotient, helpful in flagging contending bullet choices when BC alone might not. A 200-20X has a BC of .640 while a DTAC 115 is .595, if even, (3.2 versus 5.2). The disparity is backed up, before even knowing use specifics, in that through the same short action and .308/.243 parent case, the 115 is predicted to readily shoot inside the 200 even at LR, and it does even though the TR standard is at the very high end of .308 pressure.

 

[Beiruty]

@davidjoe

As I understand it, the cross-sectional density is defined at bullet weight / cross sectional area of the bullet. The values should be readily available.

 

[davidjoe]

@davidjoe

As I understand it, the cross-sectional density is defined at bullet weight / cross sectional area of the bullet. The values should be readily available.

The bulletin here last June defined it:

The formula for calculating sectional density is pretty simple and straight forward. Take the bullet weight and divide by 7000. This number is then divided by the bullet diameter squared. Two bullets of equal weight and the same diameter will have equal sectional sectional density. No regard is given to the bullet construction. This is where the fly hits the soup in considering sectional density as far as penetration is concerned.

Section Density Formula: (Bullet Weight divided by 7000) divided by Bullet Diameter squared.

 

[Longrange57]

Ned Ludd observes and describes what we have all come to understand from the ballisticians. I have raised the OP's observations over the past number of years. I hope damoncali weighs in on this conversation. He always has great insight! It does not bother me so much for folks to tell us that bullet weight is calculated into the BC. I'm ok with that and the majority of the time at 5-600 yards light weight versus heavy weight bullets appear to come rather close on our calculations. However, I witnessed it on many occasions where a 6mm versus a 7 or 30 cal. in high wind conditions out to 600-1500 yards, the light bullet in a direct crosswind moves much more, when the ballistics showed them to be the same. Many very good competition shooters will tell you the same. A group of us literally tested these theories with different calibers out to 1400 yards a few years ago. I won't bore you with details.....heavies seem to rule in the wind. I'm not smart enough to know how to calculate or somehow incorporate this into our programs. I simply go shoot them and see what they do. For me the ballistics I run on paper give me a good idea.....thats it!

 

[Beiruty]

Ned Ludd observes and describes what we have all come to understand from the ballisticians. I have raised the OP's observations over the past number of years. I hope damoncali weighs in on this conversation. He always has great insight! It does not bother me so much for folks to tell us that bullet weight is calculated into the BC. I'm ok with that and the majority of the time at 5-600 yards light weight versus heavy weight bullets appear to come rather close on our calculations. However, I witnessed it on many occasions where a 6mm versus a 7 or 30 cal. in high wind conditions out to 600-1500 yards, the light bullet in a direct crosswind moves much more, when the ballistics showed them to be the same. Many very good competition shooters will tell you the same. A group of us literally tested these theories with different calibers out to 1400 yards a few years ago. I won't bore you with details.....heavies seem to rule in the wind. I'm not smart enough to know how to calculate or somehow incorporate this into our programs. I simply go shoot them and see what they do. For me the ballistics I run on paper give me a good idea.....thats it!

Per Bryan Litz, a higher BC would trump marginal MV gain.
The OP was about not just higher BC, rather more efficent bullet (lowest G7FF) that can be launched at the highest possible MV per case volume/caliber of the rifle.

G7FF alone is a very good tool to compare Bullets in the same caliber regarless of the bullets weight.
Once we are comparing lowest G7FF from different calibers, one has to consider what is Max MV possible from a given case caliber for the two bullets. Moreover, the rifle weight is a factor if it is a limitation

 

[Ned Ludd]

Ned Ludd observes and describes what we have all come to understand from the ballisticians. I have raised the OP's observations over the past number of years. I hope damoncali weighs in on this conversation. He always has great insight! It does not bother me so much for folks to tell us that bullet weight is calculated into the BC. I'm ok with that and the majority of the time at 5-600 yards light weight versus heavy weight bullets appear to come rather close on our calculations. However, I witnessed it on many occasions where a 6mm versus a 7 or 30 cal. in high wind conditions out to 600-1500 yards, the light bullet in a direct crosswind moves much more, when the ballistics showed them to be the same. Many very good competition shooters will tell you the same. A group of us literally tested these theories with different calibers out to 1400 yards a few years ago. I won't bore you with details.....heavies seem to rule in the wind. I'm not smart enough to know how to calculate or somehow incorporate this into our programs. I simply go shoot them and see what they do. For me the ballistics I run on paper give me a good idea.....thats it!

This is not a new debate, as you noted. I have also conducted tests using .223 Rem and .308 Win F-TR loads that were predicted [on the surface] to have similar external ballistics. In contrast to your observations, I have found that the [much] lighter .224" bullets behaved exactly as the external ballistics indicted they should based on their BC and velocity. So, there are clearly two different schools of thought on this, both largely based on anecdotal evidence. Unfortunately, the methodology required to draw a definitive conclusion is not as simple as having two shooters side-by-side with two different bullet weights fire at approximately the same time and see which bullet moves farther offline. There are a number of caveats involved that must be addressed, such as:

Were the external ballistics of the loads tested really predicted to be the same? This is not as simple as it seems because it involves using an accurate BC for the Lot # bullets fired, not just the box value. Other issues such as the actual gyroscopic stability coefficient for each bullet (i.e. the relative barrel twist rate used with each load) and the actual intrinsic precision of each separate load are also considerations.

My point is simply this - is there something missing from ballistic prediction algorithms that would factor in bullet weight in a way that is important, but not addressed in most current ballistic programs? Maybe. I am not sufficiently expert in the matter to say one way or another. However, the only evidence to support that notion seems to be largely anecdotal. Until such time as those more expert than I am provide definitive proof and update their ballistic algorithms, I will continue to have faith that the numbers produced by current versions are pretty reasonable, because that's what my experience has been. Anyone that has had a different experience will probably believe otherwise, and rightly so. Regardless of which camp someone falls in, I think the notion of using external ballistic predictions as a rough guide, then modifying one's conclusions based on personal experience is really the only way to go.

Edited to add: I left Damon a message asking him to chime in on this. I also find his insight to be valuable and useful.

 

[Longrange57]

This is not a new debate, as you noted. I have also conducted tests using .223 Rem and .308 Win F-TR loads that were predicted [on the surface] to have similar external ballistics. In contrast to your observations, I have found that the [much] lighter .224" bullets behaved exactly as the external ballistics indicted they should based on their BC and velocity. So, there are clearly two different schools of thought on this, both largely based on anecdotal evidence. Unfortunately, the methodology required to draw a definitive conclusion is not as simple as having two shooters side-by-side with two different bullet weights fire at approximately the same time and see which bullet moves farther offline. There are a number of caveats involved that must be addressed, such as:

Were the external ballistics of the loads tested really predicted to be the same? This is not as simple as it seems because it involves using an accurate BC for the Lot # bullets fired, not just the box value. Other issues such as the actual gyroscopic stability coefficient for each bullet (i.e. the relative barrel twist rate used with each load) and the actual intrinsic precision of each separate load are also considerations.

My point is simply this - is there something missing from ballistic prediction algorithms that would factor in bullet weight in a way that is important, but not addressed in most current ballistic programs? Maybe. I am not sufficiently expert in the matter to say one way or another. However, the only evidence to support that notion seems to be largely anecdotal. Until such time as those more expert than I am provide definitive proof and update their ballistic algorithms, I will continue to have faith that the numbers produced by current versions are pretty reasonable, because that's what my experience has been. Anyone that has had a different experience will probably believe otherwise, and rightly so. Regardless of which camp someone falls in, I think the notion of using external ballistic predictions as a rough guide, then modifying one's conclusions based on personal experience is really the only way to go.

Agree! You nailed it! However, at 71 yrs of age and shooting for some 57 years and conducting these tests with not 2 shooters, but 6-8 at a time with local F-class shooters under basically the same conditions at two different ranges should be enough to get the point across. This is not a single opinion.
Last paragraph....."is there something missing"....is not a question that just a few of us have...it is precisely the question many have asked for years. I think this is all good dialogue. Maybe someone will get this figured out before I'm too old to shoot! Some think that time has come! LOL....

 

[Longrange57]

By the way Ned....I've always liked your thoughtful responses! That goes for many others on here as well.

 

[CharlieNC]

Regarding accurate predictability of the ballistics program, given that BC is the comparison to the ideal trajectory of the standard bullet, it always seemed strange to me that we expect typical BC values around 0.3 would behave similar to the standard with BC = 1. Seem like a standard closer to typical bullets would be much more similar in performance and not require as many band aids like the velocity banded BC in an effort to force fit the real world to the ideal.

 

[243Mendoza]

Ned Ludd observes and describes what we have all come to understand from the ballisticians. I have raised the OP's observations over the past number of years. I hope damoncali weighs in on this conversation. He always has great insight! It does not bother me so much for folks to tell us that bullet weight is calculated into the BC. I'm ok with that and the majority of the time at 5-600 yards light weight versus heavy weight bullets appear to come rather close on our calculations. However, I witnessed it on many occasions where a 6mm versus a 7 or 30 cal. in high wind conditions out to 600-1500 yards, the light bullet in a direct crosswind moves much more, when the ballistics showed them to be the same. Many very good competition shooters will tell you the same. A group of us literally tested these theories with different calibers out to 1400 yards a few years ago. I won't bore you with details.....heavies seem to rule in the wind. I'm not smart enough to know how to calculate or somehow incorporate this into our programs. I simply go shoot them and see what they do. For me the ballistics I run on paper give me a good idea.....thats it!

This may be due to the higher inertia of heavier bullets to movement by the wind ( effect on bullet acceleration by force of wind).