Trueing muzzle velocity vs BC

[dfanonymous]

Here is a potential curve ball. Some years ago, probably 3-4, we were testing a prototype .338 bullet weighing 260 gr with an average G1BC of 0.813 over 1250 yds and measured with an Oehler 88 system. We then took it out to 2900 yds. Altitude was about 1400 ft. Using the Oehler we measured the average G1BC of 0.90! It remained stable through the transition through to subsonic which happened at about 2400 yds out, so the bullet traveled subsonic for 500 yds before impacting the target creating a perfect round holes. The cartridge was a 338 Lapua Mag AI and initial MV was 3020 fps or so as I recall. We had several impacts using the G1 BC 0.813. How do you explain the increase in BC with subsonic transition?

Without bringing in atmospherics, there isn't a increase in BC, normally. However, there is plenty of opportunity for mistakes, especially if taking data off something like a chrono vs an actual Doppler radar. Then further processed in 4dof, 6dof or whatever is being used now days.

It's like I said before…BC's are models. The thing about the G1 model is that, for starters, it doesn't even represent high BC bullet characteristics. Especially at subsonic. Transonic and subsonic is where you usually see any significant difference between the 2 models.

Either way, to have a good data, one needs an appropriate BC for a series of different ranges depending on your desire for accuracy, in the case of splitting hairs.

Many who compete, and who insist on still using regular drag models like G1/G7 will often enough have different BC numbers in their dope for varying ranges, or more precisely, match a BC to particular mach ranges they expect the bullet to be running at.

 

[BrentM]

Here is a potential curve ball. Some years ago, probably 3-4, we were testing a prototype .338 bullet weighing 260 gr with an average G1BC of 0.813 over 1250 yds and measured with an Oehler 88 system. We then took it out to 2900 yds. Altitude was about 1400 ft. Using the Oehler we measured the average G1BC of 0.90! It remained stable through the transition through to subsonic which happened at about 2400 yds out, so the bullet traveled subsonic for 500 yds before impacting the target creating a perfect round holes. The cartridge was a 338 Lapua Mag AI and initial MV was 3020 fps or so as I recall. We had several impacts using the G1 BC 0.813. How do you explain the increase in BC with subsonic transition?

Parasitic drag. Litz actually addresses drag in one of his articles that discusses high velocity and drag. It's a neat article but very egg headed and uses terms I had not heard before. Basic premise is that with higher velocity you have increased drag. Lower velocity you have decreased drag. In my head it's more or less resistance...... the faster you have push a bullet or anything the more it has to overcome in the way of friction.

 

[waveslayer]

Here is a potential curve ball. Some years ago, probably 3-4, we were testing a prototype .338 bullet weighing 260 gr with an average G1BC of 0.813 over 1250 yds and measured with an Oehler 88 system. We then took it out to 2900 yds. Altitude was about 1400 ft. Using the Oehler we measured the average G1BC of 0.90! It remained stable through the transition through to subsonic which happened at about 2400 yds out, so the bullet traveled subsonic for 500 yds before impacting the target creating a perfect round holes. The cartridge was a 338 Lapua Mag AI and initial MV was 3020 fps or so as I recall. We had several impacts using the G1 BC 0.813. How do you explain the increase in BC with subsonic transition?

Could be margin of error in addition to using G1. G1 or any G7 (g7 is more realistic) are still estimates. Hence why you have to true the data. If G1 or G7 were spot on you wouldn't need to true with today's technology. G1 and G7 are a guesstimate or a standard we start with, it's not the end all be all. Then we'll introduce CDM's which are the actual drag models that are proprietary unless you guysbwant to chip in for doplar radar and we can sell the data like OTHERS do. Which I agree with (selling it, that is).

So not really a curve ball .

May I ask, what was the wind doing on both occasions?... that will change your BC. Most guys using G1 will have a trued calculator out to 1200. Then you have another set of data trued out 2000, etc... because the BC is changing you need to true and have several trued up profiles for the same bullet and load at several distances.

 

[nralifer]

Without bringing in atmospherics, there isn't a increase in BC, normally. However, there is plenty of opportunity for mistakes, especially if taking data off something like a chrono vs an actual Doppler radar. Then further processed in 4dof, 6dof or whatever is being used now days.

It's like I said before…BC's are models. The thing about the G1 model is that, for starters, it doesn't even represent high BC bullet characteristics. Especially at subsonic. Transonic and subsonic is where you usually see any significant difference between the 2 models.

Either way, to have a good data, one needs an appropriate BC for a series of different ranges depending on your desire for accuracy, in the case of splitting hairs.

Many who compete, and who insist on still using regular drag models like G1/G7 will often enough have different BC numbers in their dope for varying ranges, or more precisely, match a BC to particular mach ranges they expect the bullet to be running at.

When a bullet goes from supersonic to subsonic velocity, the shock waves around the bullet dissipate, with a significant decrease in the drag coefficient. A shock wave is the result of air compression at supersonic flow bacause the air cannot get out of the way of the bullet. This happens at the ogive, tail, and anywhere along the length of the bullet where there is a change in surface curvature. Each shockwave generated is a source of drag and contributes to the drag coefficient at any given speed. In subsonic flight the shockwaves are no longer present, thus reducing the aerodynamic drag and leading to an increase in apparent BC because no air is being compressed. It takes energy to compress air, so subsonic bullets expend much less energy because they are not compressing air.

 

[nralifer]

At least the published Doppler radar generated drag curves show that the coefficient of drag increases in the transonic zone then suddenly drops as the bullet becomes subsonic. Since BC is the sectional density divided by form factor, the latter reflecting the drag coefficient of the model bullet and the tested bullet I was just making sure it was OK

Could be margin of error in addition to using G1. G1 or any G7 (g7 is more realistic) are still estimates. Hence why you have to true the data. If G1 or G7 were spot on you wouldn't need to true with today's technology. G1 and G7 are a guesstimate or a standard we start with, it's not the end all be all. Then we'll introduce CDM's which are the actual drag models that are proprietary unless you guysbwant to chip in for doplar radar and we can sell the data like OTHERS do. Which I agree with (selling it, that is).

So not really a curve ball .

May I ask, what was the wind doing on both occasions?... that will change your BC. Most guys using G1 will have a trued calculator out to 1200. Then you have another set of data trued out 2000, etc... because the BC is changing you need to true and have several trued up profiles for the same bullet and load at several distances.

Not really understanding what you are saying. Do you own a high powered Doppler radar? I fully understand that BCs, either G1or G7 are estimates since both can vary for the same bullet shot out of different rifles illustrating the fact that each rifle is unique in so far as the BC of the bullets shot from it. From published drag, both the G1 model and the G7 model have both similarities and differences. The similarity

Could be margin of error in addition to using G1. G1 or any G7 (g7 is more realistic) are still estimates. Hence why you have to true the data. If G1 or G7 were spot on you wouldn't need to true with today's technology. G1 and G7 are a guesstimate or a standard we start with, it's not the end all be all. Then we'll introduce CDM's which are the actual drag models that are proprietary unless you guysbwant to chip in for doplar radar and we can sell the data like OTHERS do. Which I agree with (selling it, that is).

So not really a curve ball .

May I ask, what was the wind doing on both occasions?... that will change your BC. Most guys using G1 will have a trued calculator out to 1200. Then you have another set of data trued out 2000, etc... because the BC is changing you need to true and have several trued up profiles for the same bullet and load at several distances.

BCs are indeed estimates, especially those published by bullet makers. They are first estimates, something to start with in developing a trajectory curve for the bullet they purchased. The phenomenon we saw is indeed real, and is backed up by published radar drag curves showing that for both G1 and G7 models the drag coefficient dramatically reduces when the bullet speed drops below supersonic. This change in drag will reflect itself in the observed BC calculated by the Oehler 88 which measures an average BC for the entire flight of the bullet. Not clear to me how you use that information other than to realize one needs to measure BCs in the subsonic speed range to calculate a trajectory of the bullet at subsonic speeds. Doing that, though, presents some potential problems because the RPMs of a bullet shot at subsonic speed from the same barrel as the same bullet shot at supersonic speed which has transitioned to subsonic, are not likely the same, so stability issues could come into play. Litz's method of using His radar data to generate custom drag models is very practical in that he is able to measure BCs every yard along the bullet path. This enables him to fit the drag curve of any boat tail bullet to the G7 model.

 

[waveslayer]

At least the published Doppler radar generated drag curves show that the coefficient of drag increases in the transonic zone then suddenly drops as the bullet becomes subsonic. Since BC is the sectional density divided by form factor, the latter reflecting the drag coefficient of the model bullet and the tested bullet I was just making sure it was OK

Not really understanding what you are saying. Do you own a high powered Doppler radar? I fully understand that BCs, either G1or G7 are estimates since both can vary for the same bullet shot out of different rifles illustrating the fact that each rifle is unique in so far as the BC of the bullets shot from it. From published drag, both the G1 model and the G7 model have both similarities and differences. The similarity

BCs are indeed estimates, especially those published by bullet makers. They are first estimates, something to start with in developing a trajectory curve for the bullet they purchased. The phenomenon we saw is indeed real, and is backed up by published radar drag curves showing that for both G1 and G7 models the drag coefficient dramatically reduces when the bullet speed drops below supersonic. This change in drag will reflect itself in the observed BC calculated by the Oehler 88 which measures an average BC for the entire flight of the bullet. Not clear to me how you use that information other than to realize one needs to measure BCs in the subsonic speed range to calculate a trajectory of the bullet at subsonic speeds. Doing that, though, presents some potential problems because the RPMs of a bullet shot at subsonic speed from the same barrel as the same bullet shot at supersonic speed which has transitioned to subsonic, are not likely the same, so stability issues could come into play. Litz's method of using His radar data to generate custom drag models is very practical in that he is able to measure BCs every yard along the bullet path. This enables him to fit the drag curve of any boat tail bullet to the G7 model.

Exactly, so what's your question?...

 

[nralifer]

Exactly, so what's your question?...

Do you have a high power Doppler radar? And, if you do, are you able to track bullets through the transonic ad int the subsonic?

 

[waveslayer]

Do you have a high power Doppler radar? And, if you do, are you able to track bullets through the transonic ad int the subsonic?

No, that's why I use CDM's or build out several profiles as I stated above.... still not understand your point... BC's change... yep they do all the time. Hence why you build several profiles depending on the distance. If I had a doppler radar would you ask me another question?

Labradar count?

 

[nralifer]

No, that's why I use CDM's or build out several profiles as I stated above.... still not understand your point... BC's change... yep they do all the time. Hence why you build several profiles depending on the distance. If I had a doppler radar would you ask me another question?

Labradar count?

Not clear you know how a Doppler radar works. The PDMs that AB constructs actually are CDMs constructed using measured G7 BCs at every yard of bullet flight out to 1500 yds. Likely further. Even the lab radar can measure velocity changes at one yard increments except that it's a much lower power radar and will not track bullets much beyond 150 yds. Since each PDM published in AB's bullet library is unique to the bullet/rifle combo that was used to generate the PDM specified, each PDM's average BC will differ slightly from the BC of the next PDM. I would chose the PDM whose BC matches the one from your rifle. The lab radar can be used to measure a G7 BC for a bullet shot from your rifle.

 

[waveslayer]

Not clear you know how a Doppler radar works. The PDMs that AB constructs actually are CDMs constructed using measured G7 BCs at every yard of bullet flight out to 1500 yds. Likely further. Even the lab radar can measure velocity changes at one yard increments except that it's a much lower power radar and will not track bullets much beyond 150 yds. Since each PDM published in AB's bullet library is unique to the bullet/rifle combo that was used to generate the PDM specified, each PDM's average BC will differ slightly from the BC of the next PDM. I would chose the PDM whose BC matches the one from your rifle. The lab radar can be used to measure a G7 BC for a bullet shot from your rifle.

I'm not clear on your conclusion that I'm confused on Doppler... no lack of understanding how Doppler works, where did I say or show any lack of what it is?... and no Labradar is not the same.. Doppler radar will follow a bullet from start to finish. The FDA required labradar to dumb down their product, hence why it took so long from its launch at SHOT show several years ago. When I met the owner it was almost 2 full years for it to be released. I believe he's from Canada.. anyways. The CDM's are unique to that bullet not the rifle. You adjust the drop Scale Factor to align the CDM to your rifle or if you use 4DOF you use the Axial Form Factor. The bullet has slight variations... the gun will change the CDM slightly if you have a muzzle break, can, powder of choice etc... so you true the CDM using the drop scale factor. Which I'm not confused about. I'm still scratching my head on why you think I'm confused when we are saying the same thing....

There is no BC that matches a rifle in the PDM's. Hence why you have to true your ballistics solver... true velocity out to 600 despite what your chrono says, past that you true the BC... no posted BC is perfect, some are close, but you have to true it. Which I have said over and over... so again. Respectfully I'm not the guy that has the questions....

 

[nralifer]

I'm not clear on your conclusion that I'm confused on Doppler... no lack of understanding how Doppler works, where did I say or show any lack of what it is?... and no Labradar is not the same.. Doppler radar will follow a bullet from start to finish. The FDA required labradar to dumb down their product, hence why it took so long from its launch at SHOT show several years ago. When I met the owner it was almost 2 full years for it to be released. I believe he's from Canada.. anyways. The CDM's are unique to that bullet not the rifle. You adjust the drop Scale Factor to align the CDM to your rifle or if you use 4DOF you use the Axial Form Factor. The bullet has slight variations... the gun will change the CDM slightly if you have a muzzle break, can, powder of choice etc... so you true the CDM using the drop scale factor. Which I'm not confused about. I'm still scratching my head on why you think I'm confused when we are saying the same thing....

There is no BC that matches a rifle in the PDM's. Hence why you have to true your ballistics solver... true velocity out to 600 despite what your chrono says, past that you true the BC... no posted BC is perfect, some are close, but you have to true it. Which I have said over and over... so again. Respectfully I'm not the guy that has the questions....

So that we understand each other, my understanding of a PDM is that it is a drag model calculated from 10 shots fired in AB's radar beam by the owner of the rifle. I know this because me and 4 friends shot our guns through the radar beam during last year's Nightforce ELR challenge. So each PDM is rifle and load specific. On this point we may differ. In AB's bullet library there are a whole bunch of PDMs listed for the Berger 245 gr EOL Hybrid 1.769" long bullet. Out of 14 that I looked at, 12 had the bullet length specified as 1.769" and 2 were specified as 1.0" in length but listed as weighing 245 gr. The G7 for each PDM can be seen by clicking on "Custom" and choosing G7. For the 14 PDMs the G7 BCs specified varied from a high of 0.438 to a low of 0.401. The CDM for that bullet has a G7BC of 0.412. Only 2 out of the 14 PDMs had a G7 of 0.412 and 2 others had a G7 of 0.411. The conclusion I draw from these data is that the G7BC, as measured using the most accurate method available, can vary considerably from rifle to rifle firing the same bullet. Only 4/14 PDMs agreed with the G7 BC listed for the CDM. Given this information, truing the dope to fit the trajectory calculated by a CDM or PDM is necessary to achieve the most accurate firing solution. I think we both agree that a BC for any given is not set in stone. Looking at AB's PDM BCs have clearly illustrated to me the role of the rifle's influence on the BC of the bullets shot from it. One thing that is true, the G7 BC is a reasonable measure of the aerodynamic efficiency of a bullet and a reliable way to compare bullets in this regard.

 

[Euler]

I'll just leave this here

 

[Pdvdh]

Individual barrels and barrel twist rates can affect bullet BC values, much more than than the 1-2% suggested by Bryan Litz.

I use LabRadar velocity decay to establish bullet BC from my barrels. Identical bullets fired in a different twist rate barrel, with barrels chambered with the identical chamber reamer, travel with BC value differences much greater than the 1-2% referenced in the above Litz sit down discussion. In my experience...

Once you've nailed down muzzle velocity, why would you tweak MV in your ballistics program? Why juggle MV in your ballistics program to match down range bullet POIs, after confirming your bullet's MV with LabRadar or Magnetospeed?

The culprit will be the error in BC input value. So tweak BC value. Not MV.

 

[just_jon]

I true velocity using a kestrel. It will provide a recommended distance for initial validation that is wihtin 85% of transonic range. It has worked well for me, but additional field testing at other distances is needed for conformation.