Let's argue about BC's

[Michael Eichele]

Bold below:

Hey Michael, I thought you were in waiting for your rig to get back from the smith I guess it's my turn to be put through the washing machine

You gotta admit, I held my fire on this thread for quite a while! Sorry, that bone was too tempting to not chew on........I actually am looking VERY much forward to getting her back. Regardless, one theory or the other will have some better validation or confirmation. At this point, I really dont care who is right or wrong. In all honesty, it would be far easier for me to just say I was wrong even if there is some validity to the next double chrony test. In any event, I will take the high road and share the results for what they are.

Intersting stuff... I don't know much about "coning" but would like to learn. If you any article refferences, I would like to read them before drawing any conclusions. and yeah, i think Sierra is credible... but I would like to actually see their data and conclusions.

Sierra Reloading Manual 4th Edition. Also, I think they have much of those references on their web sight. It may take a bit of digging but I think they are there.

Next time you're in Momtana, give me a buzz and we'll test BC theories on some PD's

Likewise. Only the varmints up here are much bigger. Maybe bring your 300?

They use the term 'coning' when I think the real term is 'gyroscopic precession'. where the axis is in line with the trajectory.

 

[Michael Courtney]

I apologize that I don't have time to speak to every question that has been raised, but I hope to summarize some of the science in an orderly way to promote a more scientific and less emotional discussion. There are several hypotheses related to why observations with different rifles might yield different estimates of ballistic coefficient. I tend to group these in two categories:

Rifle-centered differences:

Yaw/gyroscopic precession increases drag during the first part of the flight until it damps out and the bullet "goes to sleep." Rifles that tend to shoot bullets with greater peak yaw early in the trajectory will produce more drag, on average. These rifles probably also produce more shot-to-shot variations in drag. Two ideas mentioned as sources of larger yaw are imperfections in the crown and thin barrels. More data is necessary to determine how common this phenomena is and how much drag increase it creates, although Bryan Litz's computations are compelling that if the initial yaw angle is 11 degrees then the drag increase over the first 100 yards is near 7%.

Deformation of the bullet in the barrel results in increased drag. Bullets are swaged down to the barrel diameter. This swaging leads to a smaller cross sectional area and higher sectional density in tighter bores, but it might also lead to deformations in the ogive and boat tail if too much metal needs somewhere to go. Different rifles also vary greatly in the depth and roughness of the rifling markes imprinted on the bullet. Finally, the extreme accelerations of some cartridges cause the lead in the nose to obturate and change the nose shape. Relatively small dimensional changes can produce significant differences in aerodynamic drag. This hypothesis is probably best tested by comparing bullets shot from sabots (thus no rifling marks) with bullets shot in the usual way. A careful experimental design will be needed to minimize possible confounding factors.

Bullet-centered differences:

Tests have shown 1-4% variations in bullet BCs even within the same box of bullets. Most of the bullet-to-bullet variations within the same box are thought to be related to inconsistencies in the bullet tip, as these can be reduced in plastic tipped bullets and by using post-manufacture dies to ensure greater tip uniformity. Some useful references are:

Whidden Bullet Pointing Die System

http://www.appliedballisticsllc.com/index_files/Berger155grainVLD.pdf

Bullets can show more pronounced dimensional variations between lots, and this can produce significant BC variations. In a few cases the manufacturer will announce the switch to a new die along with a difference in their published BC. In other cases, these dimensional variations are not announced by the manufacturer. We've measured average BC variations as large as 13% between different lots. (Our BC numbers in the earlier post showing rifle-to-rifle variations are from the same lot of 125 Nosler Ballistic Tips, so we attribute the measured differences to the rifles).

Michael Courtney

 

[groper]

I apologize that I don't have time to speak to every question that has been raised, but I hope to summarize some of the science in an orderly way to promote a more scientific and less emotional discussion. There are several hypotheses related to why observations with different rifles might yield different estimates of ballistic coefficient. I tend to group these in two categories:

Rifle-centered differences:

Yaw/gyroscopic precession increases drag during the first part of the flight until it damps out and the bullet "goes to sleep." Rifles that tend to shoot bullets with greater peak yaw early in the trajectory will produce more drag, on average. These rifles probably also produce more shot-to-shot variations in drag. Two ideas mentioned as sources of larger yaw are imperfections in the crown and thin barrels. More data is necessary to determine how common this phenomena is and how much drag increase it creates, although Bryan Litz's computations are compelling that if the initial yaw angle is 11 degrees then the drag increase over the first 100 yards is near 7%.

Deformation of the bullet in the barrel results in increased drag. Bullets are swaged down to the barrel diameter. This swaging leads to a smaller cross sectional area and higher sectional density in tighter bores, but it might also lead to deformations in the ogive and boat tail if too much metal needs somewhere to go. Different rifles also vary greatly in the depth and roughness of the rifling markes imprinted on the bullet. Finally, the extreme accelerations of some cartridges cause the lead in the nose to obturate and change the nose shape. Relatively small dimensional changes can produce significant differences in aerodynamic drag. This hypothesis is probably best tested by comparing bullets shot from sabots (thus no rifling marks) with bullets shot in the usual way. A careful experimental design will be needed to minimize possible confounding factors.

Bullet-centered differences:

Tests have shown 1-4% variations in bullet BCs even within the same box of bullets. Most of the bullet-to-bullet variations within the same box are thought to be related to inconsistencies in the bullet tip, as these can be reduced in plastic tipped bullets and by using post-manufacture dies to ensure greater tip uniformity. Some useful references are:

Whidden Bullet Pointing Die System

http://www.appliedballisticsllc.com/index_files/Berger155grainVLD.pdf

Bullets can show more pronounced dimensional variations between lots, and this can produce significant BC variations. In a few cases the manufacturer will announce the switch to a new die along with a difference in their published BC. In other cases, these dimensional variations are not announced by the manufacturer. We've measured average BC variations as large as 13% between different lots. (Our BC numbers in the earlier post showing rifle-to-rifle variations are from the same lot of 125 Nosler Ballistic Tips, so we attribute the measured differences to the rifles).

Michael Courtney

Michael, just quickly, i can understand a LOWER than normal BC from extreme/uncommon circumstances that you outlined above, in particular teh nose slumping in very high velocity cartidges, but can you fathom or hypthesize an explanation for an extreme INCREASE of 20% or more from the normal measured BC that you or Bryan Litz derived, as opposed to a decrease? Or would you simply say that it defies logic?

 

[Broz]

Holy Crap boys!! What a thread. After reading all this I may never use the term BC again. Wheww! I need a Cold Beer..... Oops that is B C backwards I better have a whisky.

Jeff

 

[ericpetritz]

Bullet 125 NBT

Rifle/Cartridge Near V (fps) G7BC/uncertainty G1BC/uncertainty

TC Encore/30-30 2245 0.153/0.003 0.306/0.005
Rem 700/.308 2794 0.153/0.005 0.308/0.010
Rem 700/.30-06 3010 0.158/0.005 0.319/0.011
Rem 700/.300 WM 3008 0.141/0.002 0.283/0.004

The table above shows our measured BCs (two chronograph method) for the Nosler 125 grain ballistic tip from four different rifles. The measurements from the first three rifles are very close and the differences are well within the uncertainties and expectations from the muzzle velocity differences. In contrast, the BC measurement from the .300 Win Mag is 11% lower than the .30-06 in spite of being fired with very nearly the same muzzle velocity. The 11% difference is significant since the uncertainties in BC for the different rifles are 1-3%. We've measured lower BCs in this rifle for every bullet we've tested in it.

Michael

Have you been able to figure out why this one particular barrel tends to lower the BC's?

Another question:
This particular case shows an example where you measured a BC that was significantly lower than the norm. Have you ever found a scenario where a BC was significantly higher than the norm?

Eric

 

[Buffalobob]

bullets just don't get deformed to any SIGNIFICANT difference by a rifle bore.

In the world of hunting at long range one can consider the antelope I shot last year and explore the limits of the term "significant".

http://www.longrangehunting.com/forums/f85/7-allem-mag-smokes-some-fur-62836/index3.html

The antelope is hit at the bottom of the spine severing the thoracic aorta at 1080 yards -/+; however, I can increase the G7 BC used in the calculations by 13% and not significantly affect the outcome. I will still hit the bottom of the heart and get a one shot bang flop kill. So one could if one wished to use the word significant say that a 13% change in BC is not significant. However, if we move back about another thousand yards and shoot at the same antelope now at 2K yards we will find that we can not even tolerate a 1% change in BC before it becomes significant and we do not kill the animal. Interestingly enough this is inside the margin of error of measuring BCs. And that less than one percent error has to include other errors such as human, mirage, wind and ranging.

 

[MontanaRifleman]

Holy Crap boys!! What a thread. After reading all this I may never use the term BC again. Wheww! I need a Cold Beer..... Oops that is B C backwards I better have a whisky.

Jeff

You just read the whole thing now??? Must be really cold and blowing up in Townsend

 

[LR3]

Thanks Michael C for your post about possible barrel effects. Makes sense. I did not expect my question about how different rifles produce different BC values for the same bullet would stir so much discussion but it sure has been interesting. Clearly an accurate scope is critical. So one more question. For the commonly used G1 and G7 ballistic programs do they produce different Bc values from measured trajectories
Assuming one is not do testing like Bryan but matching performance.

 

[MontanaRifleman]

Thanks Michael C for your post about possible barrel effects. Makes sense. I did not expect my question about how different rifles produce different BC values for the same bullet would stir so much discussion but it sure has been interesting. Clearly an accurate scope is critical. So one more question. For the commonly used G1 and G7 ballistic programs do they produce different Bc values from measured trajectories
Assuming one is not do testing like Bryan but matching performance.

No dig intended here, but Bryan doesn't use performance to derive BC's??? That's news to me. I think if you look at his methods, he uses "actual" performnce to a much greater degree than anyone else here.

 

[Michael Eichele]

Deleted.

 

[MontanaRifleman]

Bold below:

They use the term 'coning' when I think the real term is 'gyroscopic precession'. where the axis is in line with the trajectory.

I'll reserve judgement until I learn a little more about this. But the basic physical laws of the universe that we live in tell me that BC is determined by the shape and mass of the bullet.... period. Bringing in outside factors such as coning, I would call external factors that might need to be compensated for. These "external" factors may affect the drop, TOF and windage of the bullet.

If different rifle bores have this affect, that's certainly is news to me for consideration.

This is what I'm looking for in this thread. Some "good" data and conclusions. When someone claims... "Hey, I shot these bullets and my BC was .123", that really doesn't mean much to me. What were the environmental conditions. What equipment were you using and and how was it performing? I mean like hey dude when a cloud passes over my chrony it spits weird junk and blah, blah, blah You get what I'm saying... I hope.

 

[eddybo]

I like you, you get it, when it comes to the internet. I am sorry if I let my attitude spill over to you. Understand while I have at times let people get under my skin in the past, I am a different person sans the tumor. I have at times been frustrated probably by my own inability to explain a situation, or by others reading comprehension. Either way I hold no ill will to anyone, even if I am a smartie pants sometimes. I use my phone most of the time to access this site because I am usually too busy living to sit around and type all day. Unfortunately on my phone smileys are a pain. Since I just got in from the shop and am logged in on a real computer and am in a particularly good mood...........

Good luck.

Eddybo, first off, you haven't bothered me at all, you really haven't.... why should I have been bothered? I don't take any of this personally and I hope you don't either. Internet debates can sometimes come across negatively on a personal level and sometimes folks do get personal, but believe me, i have not taken or given anything personal in this thread. Ant that is why i use smileys... and I'm still smiling When you posted that thread on the long range elk shots with you and your buds, and got some heat for it, I was one of many who stuck up for you. Dude, this is the kitchen and sometimes it gets warm in the kitchen. If you wanna mess in the kitchen, you gotta take the heat. You and everyone else in this thread are welcome to my campfire anytime. We all love this sport so why get hung up on some of our different ideas and approaches?

On the extreme example, sure, we can bend a thin jacketed bullet all out of shape if we pus it too hard... but c'mon man... that's not why the OP started this thread. Bullets fired within their operational parameters do not significantly change shape or mass from one rifle to another. It just doesn't happen, and if you think they do, prove it to me.

Til then, happy shooting and take care

 

[eddybo]

I can agree with that, especially when referring to G1 BCs.

MontanaRifleman,

I'm not trying to tell anyone how it is. I'm just trying to understand what you and others have learned through research. And, I respect your wisdom and experience which far exceeds mine.

As you acknowledged, the barrel engraves the exterior of the bullet. Not so much the head/ogive/meplat/boat tail which I assume has a larger influence on BC as compared to the bearing surface that gets etched. But, I think it has some affect which you and others say is very little. Perhaps even a tight barrel due to manufacturing tolerances or fouling may squeeze the OD enough to have an influence on BC?

So, while that little bit may be infintessimal at shorter ranges, I'm seeing credible sources saying that it's noteworthy at extreme ranges.

What I'm gathering from this thread is that BCs are imperfect to begin with. And, they are simply a mechanism to model the trajectory of a bullet giving us some predictability of where the bullet will intersect its target. Afterall, nobody practices at every 1 yard increment from 100-2000 yds.

As such, it seems reasonable to tweak the BC slightly for your rifle. But, any major fudging is likely due to either a misrepresented published BC or a wrong assumption on the part of the shooter.

In the case of the published BC, it's public forums like this which will likely bring that to light.

Whereas tweaking the BC substantially due to invalid assumptions about scope height/tracking, MV, etc.. is only going to set you up for disapointment under some set of conditions down the road.

Thanks,
Richard

 

[eddybo]

Keep it up I may figure out my "problem." This has me thinking a LOT. I know a few guys using .84 as a working BC for the 300gr SMK. Sierra claims .768, but the guys using .84 are using stepped BCs to make things work, just as I am using .89 as my starting BC with another bullet. Hmmm high velocities plus high BCs masking BC errors, much like shooting short ranges masking BC errors, interesting, but still over my head.

I guess I gotta keep shooting my rifles and making them match my trajectories by masking all my errors on one particular rifle. That is okay I guess, since I like to shoot that rifle anyway.

In the world of hunting at long range one can consider the antelope I shot last year and explore the limits of the term "significant".

http://www.longrangehunting.com/forums/f85/7-allem-mag-smokes-some-fur-62836/index3.html

The antelope is hit at the bottom of the spine severing the thoracic aorta at 1080 yards -/+; however, I can increase the G7 BC used in the calculations by 13% and not significantly affect the outcome. I will still hit the bottom of the heart and get a one shot bang flop kill. So one could if one wished to use the word significant say that a 13% change in BC is not significant. However, if we move back about another thousand yards and shoot at the same antelope now at 2K yards we will find that we can not even tolerate a 1% change in BC before it becomes significant and we do not kill the animal. Interestingly enough this is inside the margin of error of measuring BCs. And that less than one percent error has to include other errors such as human, mirage, wind and ranging.