Bullets and BC's... thought experiment

[SAPenguin]

Help me out with a thought experiment...

Lets take 2 bullets that have similar BC, but different weight (maybe a 7mm Berger 184 hybrid vs .30 Berger 215 hybrid). G7 BC's are .356 vs. .354 - nearly identical. Now fire both of those at the same velocity - lets say 2900 fps.

At distance (1000+ yards), does the heavier bullet carry some "inherent" advantage that is not obvious in the ballistics calculators? Maybe under switchy wind conditions?

When i run the ballistics calculators, i get very similar wind drifts (which is expected due to similar BC) But what i am looking for is something more ...intrinsic? Is this why we see more .300 WSM's in the F class / benchrest arena?

Much appreciated!

 

[Ned Ludd]

This is a good question. Over the years I have seen numerous anecdotal comments that go both ways. For example, I shoot the 90 VLDs from a .223 Rem in F-TR. I have seen quite a few comments suggesting that the 90s display anomalous behavior at 1000 yd, a phenomenon I have never personally observed. Solely based on BC and velocity, the 90s should fall somewhere in between typical 185 Juggernaut and 185 Hybrid .308 Win loads in terms of wind deflection. In my hands at 1000 yd, this is exactly what I have observed. The caveat to this observation is that in the smaller .223 Rem case, achievable velocity ES/SD numbers with 90 VLD loads are typically about 1.5 to 2.0X those that can be readily obtained in a good .308 load with 185s. For that reason alone, one would expect slightly greater vertical at 1000 yd. Nonetheless, I have not observed the wind deflection with the 90s to be markedly different from what is predicted by ballistics calculators.

I have had discussions with people that have suggested there may be some aspect of external ballistics that is not fully accounted for by typical ballistics programs that represents some "real-world" difference between bullets having similar velocity and BC, but markedly different weight. When pressed however, they have not been able to specifically identify exactly what those different aspects are, nor have they been able to illustrate exactly how/why current ballistics calculators are supposedly unable to account for ballistic differences in otherwise comparable bullets (i.e. BC and velocity) solely based on bullet weight. Certainly bullet weight is accounted for ballistics programs.

Obviously, there are noticeable differences in cartridge/caliber combinations that include case volume/shape, degree of overbore, bullet design, powder choice, etc., that may contribute substantially to some combinations being better than others. In your example of the .300 WSM, I think you'll find there are far more F-Open shooters using a straight .284 with success than the .300 WSM. One reason for this is that to achieve a small advantage in external ballistics with the .300 WSM requires using a bullet with a sufficient BC and velocity. That generally goes hand in hand with a noticeable increase in recoil, which not every shooter will find desirable. Nonetheless, there are those that have used the .300 WSM with great success. The question as you outlined above is whether that is solely due to a small increase in resistance to wind drift with some specific powder/bullet combination, or is there some undefined characteristic of bullets as a function of mass that is not fully accounted for by current ballistics programs. If there is, I'd love to know more about it and how to better define/use it.

 

[Doug Beach]

Among bullets of truly equal bc, over the velocity range involved, if there is an additional wind advantage to greater weight, it has never been scientifically documented, or explained. Since the study of ballistics is not a new, or immature science, I’m going to say that no advantage exists. There may well be errors in our measurement methods, and our practice of assigning a fixed number to a variable state, but I would be very surprised if the underlying science is in fact incorrect.
If I’m wrong, prove it.

 

[Ballisticboy]

There is a possibility that there could be very small differences in wind response for bullets which have the same BC but different weights when fired at the same muzzle velocity. It will all depend on how the BC has been measured and the shape of the individual drag curves for each of the bullets.

For example some bullets have drag coefficient curves which are high to start with but do not increase very much as the speeds falls. Others start off with a low Cd value but increase rapidly as the speed falls. Thus, if the BC has been derived by measuring the loss in bullet speed over a given range it is possible that the start and end velocities can be the same but the way in which the speed has been lost is different. This would normally be reflected in different times of flight over a given range which will give different wind drift values. Having said that it is possible I would not expect any differences to be large.

If the individual Cd coefficient curves, bullet weights and calibres are used instead of BC then the true wind drifts should be produced for each bullet.

 

[DocUSMCRetired]

BB is correct. Remember that a BC is true at a specific velocity but if you took say 10 different points of that bullets curve then you have a much different story. As you will more than likely have 1 BC point that is the same, and 9 that are different from each bullet.

or

 

[284winner]

LR shooting ( defined IMO as ranges out to 1500 yards) is where I believe the ballistic coefficient numbers have good merit regardless of weigh, caliber or velocity. Meaning a 6mm with the same BC, velocity as a 30 Cal will be about equal in ballistics, wind etc. At 1500 yards and beyond I believe those ballistic numbers do not hold as true. The big 300gr/338 bullets WILL prevail before say a 195gr/7mm bullet moving as fast with equal BCs in high winds. It's not in any print but in theory the bullets should be equal based on any ballistic program. Real world scenarios have proved that wrong many times. If not, you'd certainly see the more manageable 7mms in ELR and not 338/375/416s. It's my opinion that YES the heavier (equal BC/velocity) bullet does hold the advantage. Again I do not see this advantage at ranges to 1200 yards but beyond the 1200 but more noticeably beyond 1500 in wind, the big heavy high BC bullets are certainly an advantage.

 

[dcali]

The short answer is "no, there is no inherent advantage to the heavier bullet". BC already accounts for weight and caliber differences.

The real answer has already been given above. The difference is due to variation from the standard drag function and will be very small. There's also no reason the heavier bullet would be the better one. It could be worse. Practically speaking, they'll be the same. You can trust the ballistic calculator at least into the transonic range, where things get weird.

 

[Milo 2.0]

I think what you are asking is, would the heavy bullet withstand a deflection better

LR shooting ( defined IMO as ranges out to 1500 yards) is where I believe the ballistic coefficient numbers have good merit regardless of weigh, caliber or velocity. Meaning a 6mm with the same BC, velocity as a 30 Cal will be about equal in ballistics, wind etc. At 1500 yards and beyond I believe those ballistic numbers do not hold as true. The big 300gr/338 bullets WILL prevail before say a 195gr/7mm bullet moving as fast with equal BCs in high winds. It's not in any print but in theory the bullets should be equal based on any ballistic program. Real world scenarios have proved that wrong many times. If not, you'd certainly see the more manageable 7mms in ELR and not 338/375/416s. It's my opinion that YES the heavier (equal BC/velocity) bullet does hold the advantage. Again I do not see this advantage at ranges to 1200 yards but beyond the 1200 but more noticeably beyond 1500 in wind, the big heavy high BC bullets are certainly an advantage.

I agree with you, I have seen higher bc 6mm bullets traveling 200 fps faster at the muzzle get stomped by lesser 30 cal bc bullet in wind, even from 900-1200 yards. If a up or downdraft is present in the last 33% of flight, it is even more lopsided in favor of the heavy bullet, they handle unexpected deflection better.
Even though not a remotely close comparison, but if you left Cheyenne WY headed west on I 80 and high wind warnings were in effect, would you want to be in a 6K lb Cadillac Eldorado or a 3K lb Kia?

 

[284winner]

I think what you are asking is, would the heavy bullet withstand a deflection better

I agree with you, I have seen higher bc 6mm bullets traveling 200 fps faster at the muzzle get stomped by lesser 30 cal bc bullet in wind, even from 900-1200 yards. If a up or downdraft is present in the last 33% of flight, it is even more lopsided in favor of the heavy bullet, they handle unexpected deflection better.
Even though not a remotely close comparison, but if you left Cheyenne WY headed west on I 80 and high wind warnings were in effect, would you want to be in a 6K lb Cadillac Eldorado or a 3K lb Kia?

Exactly. The ballistic programs do not account for that extra payload. Only equal BC/velocity.

 

[284winner]

The short answer is "no, there is no inherent advantage to the heavier bullet". BC already accounts for weight and caliber differences.

The real answer has already been given above. The difference is due to variation from the standard drag function and will be very small. There's also no reason the heavier bullet would be the better one. It could be worse. Practically speaking, they'll be the same. You can trust the ballistic calculator at least into the transonic range, where things get weird.

For me the answer is YES. Heavier does help in higher winds( beyond 1500 yards) even tho BC and velocity are equal. I think theoretically speaking rather than practically speaking is the phrase. I can't speak for others experiences but mine have been that the heavier bullets do better in higher winds beyond that 1500 yard range. In theory according to the ballistic programs, all things are equal. In reality, not necessarily so.

 

[Milo 2.0]

Exactly. The ballistic programs do not account for that extra payload. Only equal BC/velocity.

I think they do account for the weight, but weight never shows up as a gain because just off the top of my head, conditions need to be stable or the same to even test a bullet. The guys that do ballistic testing are 5 times, maybe more intelligent than I am.
I just shoot in differing terrains, with wind, we are all good in constant conditions, I have yet to see anyone forecast the coming gust of wind that hoses us just before, during, or right after a shot is broken, The heavier bullet stays on coarse better when this happens. We can't set wind flags every 100 yards to 2K, not feasible.

 

[284winner]

I think they do account for the weight, but weight never shows up as a gain because just off the top of my head, conditions need to be stable or the same to even test a bullet. The guys that do ballistic testing are 5 times, maybe more intelligent than I am.
I just shoot in differing terrains, with wind, we are all good in constant conditions, I have yet to see anyone forecast the coming gust of wind that hoses us just before, during, or right after a shot is broken, The heavier bullet stays on coarse better when this happens. We can't set wind flags every 100 yards to 2K, not feasible.

Yes they do account for the weight. My meaning was when the winds gas up and down, I just believe that helps my heavier bullet make it to steel easier than the lighter bullet does it. I've shot my 6.5-284 with 140 ELD-Ms(.64 bc)at 1000 yards in high wind. Next to it was my LA 284 win with 180 VLDs( .65 bc). Almost identical BCs but the 6.5 was moving at 150 fps faster and yet the 180s did better when the winds were switching and acting goofy. Was actually around 1075 yards and I did better with the 180s. It's more of an environmental condition that's unpredictable and having the edge in bullet weight I think helps.

 

[dcali]

Exactly. The ballistic programs do not account for that extra payload. Only equal BC/velocity.

Ballistic programs do account for the extra weight. That's why you can't factor it in again - you'd be double counting.

 

[Milo 2.0]

Ballistic programs do account for the extra weight. That's why you can't factor it in again - you'd be double counting.

You are right, I have yet to see where rapidly changing atmospheric conditions give a rats ass as to what predictions we make though, especially a wind gust.

 

[Ledd Slinger]

Weight, surface area/shape, and time in flight are what dictate wind effect.

A 7mm bullet may be lighter, but might be more 'aerodynamic' so to speak than a larger heavier 30 cal bullet with identical BC.

As a base, everyone knows that given identical shape, diamteter, velocity, flight time, and wind forces on a given path, the wind is going to push a lighter object more than it will a heavier object. Simple physics there.

Now take a heavier bullet vs. a lighter bullet with identical BC, flight speed, shape, and time. The extra weight of the heavier bullet will help it resist wind effects better, but that advantage may be somewhat negated due to larger surface area on which the wind can apply force. Thus making the two bullets experience similar wind drift.

Now if the weight difference between the two bullets in the example above exceeds the difference in surface area difference between the two, the heavier bullet will most likely experience less wind drift.

This could only truly be tested in a fully controlled environment with a wind force that is continuously identical in speed and direction at all times. Testing bullets from different rifles in 'switchy' wind conditions at the range and drawing conclusions by seeing tighter groups on paper cannot be viewed as concrete evidence of the true effects. Theres just way too many unknown variables involving the different rifles and their build components, ammunition, optics, varying wind forces, and of course human error.

 

[davidjoe]

There probably is an intrinsic difference but it’s not weight, it’s about proportions. The heavier bullet, to have the same BC as a lighter one, will be more compact (shorter).

So the question could be phrased, does a shorter bullet of the same BC resist lateral deflection better? By the principle of leverage, it takes less pressure on the front or rear of a long object to redirect it, than on a short one. I hesitate to use logical analogy (lol) but picture car jack handles, grist mills, or long canoes.

 

[fyrewall]

What is it ? - two bullets, each have equal and desired low form factors (FF) but the lighter of the two can be driven faster - what would be selected? FF = SD (sectional density)/G7 BC.

 

[270WinDude]

You are right, I have yet to see where rapidly changing atmospheric conditions give a rats ass as to what predictions we make though, especially a wind gust.

From shooting air guns my experience is the chaotic wind is what COULD give the heavy advantages. For me bc is a tool to use when evaluating the application in whole. I’m thinking twist rate and gyroscopic stability are also variables that make it not as simply as comparing bc to bc.

 

[Ledd Slinger]

What is it - two bullets, each have equal and desired low form factors (FF) but the lighter of the two can be driven faster - what would be selected? FF = SD (sectional density)/G7 BC.

That's where some people can get a little carried away with BC's. The extra speed capability of a lighter bullet with lesser BC will not be overtaken for elevation drop by a heavier higher BC bullet until you get way out there...'IF' elevation drop is all you are concerned with.

For competition, a person will most likely be considering wind drift as a major factor in which case the heavier bullet might be a better choice. For hunting a person has to consider wind drift as well as retained energy where again the heavier bullet might be a better choice. It all depends on how far you plan to shoot and your intentions for the ammunition.

 

[dcali]

You are right, I have yet to see where rapidly changing atmospheric conditions give a rats ass as to what predictions we make though, especially a wind gust.

Right. I agree there. I have only once or twice seen the conditions that are assumed by a ballistic calculator- constant wind velocity from shooter to target. Every time I’ve seen it the wind value has been zero. Every other time I go shoot, the wind is way more complicated than that.

So what we really have is an inability to measure or even estimate the actual wind conditions. If we could, and we could input them into a calculator, we’d see the wind numbers are pretty accurate. Just because the calculator isn’t useful in predicting a single shot under real conditions doesn’t mean it’s not going to tell you which bullet is better.