Let's argue about BC's

[Topshot]

Michael,\Sounds fair enough. Nice looking ballistic program too.

It would be interesting to see you use that method to calibrate your scope at some of the intermediate points on your turret travel.

I use a similar method but have every actual MOA point marked on a special board from 0 to 40 MOA. I then set the scope on the line and wind the scope turret up.

This shows errors over the whole range of travel from 0 to 40 MOA. If I want more, then I just re-aim the rifle and do the next section.

Some parts of the turret on some scopes do have different amounts of correction needed. So if you are only checking out the turret every full turn (or where it lines back up with zero) then you may be missing out on something.

On some scopes there is also a flat spot near maximum travel where no movement of the cross hairs takes place, even though the turret still turns and clicks away. So beware of the upper end of turret travel.

 

[groper]

Michael, i have read all your posts very carefully my friend.

In post #102, are you using a Litz derived G7 BC or your own? If your using a Litz BC, dont mess with it... all you had to do was throw the chrony away and tweak the velocity instead of the BC to get your trajectory to line up as per my previous recommendations.

Your having issues with your 7mm magnum due to using a G1 BC instead of G7.

Your having issues with the caliber mentioned in post #102, because your velocity isnt correct or your fudging the G7 BC.

Anything else i missed?

 

[Michael Eichele]

Michael,\Sounds fair enough. Nice looking ballistic program too.

It would be interesting to see you use that method to calibrate your scope at some of the intermediate points on your turret travel.

I use a similar method but have every actual MOA point marked on a special board from 0 to 40 MOA. I then set the scope on the line and wind the scope turret up.

This shows errors over the whole range of travel from 0 to 40 MOA. If I want more, then I just re-aim the rifle and do the next section.

Some parts of the turret on some scopes do have different amounts of correction needed. So if you are only checking out the turret every full turn (or where it lines back up with zero) then you may be missing out on something.

On some scopes there is also a flat spot near maximum travel where no movement of the cross hairs takes place, even though the turret still turns and clicks away. So beware of the upper end of turret travel.

You bring up some very good points here in regards to differences near the upper and lower limits and other areas in between. I have often thought about that but never taken it seriously.

In the end, I know I need to break down and do it again more completely and do my NF at the same time.

Thanks for you input here.

M

 

[Michael Eichele]

Michael, i have read all your posts very carefully my friend.

Your having issues with your 7mm magnum due to using a G1 BC instead of G7.


Anything else i missed?

How carefully have you read them? I am not playing with a 7mm at all. I suspect based on that, you are missing quite a bit.

 

[groper]

My apologies michael, i had you confused with someone else who had the problem shooting the 7mm wildcat and using the litz G1 BC of .74. Again, to that person, use the Litz G7 number when wildcat or Bryan publish it, and try again...

As per your results, im assuming your shooting a 300RUM? Also a very fast caliber so your in a similar boat. However, i hear what you are saying regarding your velocities over multiple chronographs, and no way should you need to adjust the velocity by 200fps, its usually less than 50fps error from chrony to chrony day to day, so there must be other reasons.

That said, G7 numbers still do change with velocity, not nearly as much as G1, but they still do change. Litz probably tested that 168gr bullet from a 308win, not a 300 magnum, so i would expect a small change in measured G7 AVERAGE considering the difference in calibers is about as extreme as you could possibly compare. Assuming all your other variables are correct, which im sure you have checked, this would be a likely cause of at least some of the problem. What does litz quote the BC of that bullet at, and what G7 BC have you derived for it? Theres probably not a lot of difference is there?

EDIT: For example, ive modelled a 30cal bullet similar to the 168Amax and the calculated G7 BC varies from a minimum of .266 upto an extreme maximum of .284 depending on the velocity between transonic and a muzzle velocity of 3300fps, but during the vast majority of its flight regime thru to 1000yds, its G7 BC is between .274 and .269. if litz physically tested this bullet in the way he does to 1000yds, he would have found an average somewhere in between this range... So G7 does change just not by much, if your getting significantly more difference than this lets say no more than 0.01x from a derived Litz G7 BC, something else is a miss...

 

[Michael Eichele]

As per your results, im assuming your shooting a 300RUM? Also a very fast caliber so your in a similar boat. However, i hear what you are saying regarding your velocities over multiple chronographs, and no way should you need to adjust the velocity by 200fps, its usually less than 50fps error from chrony to chrony day to day, so there must be other reasons.

That said, G7 numbers still do change with velocity, not nearly as much as G1, but they still do change. Litz probably tested that 168gr bullet from a 308win, not a 300 magnum, so i would expect a small change in measured G7 AVERAGE considering the difference in calibers is about as extreme as you could possibly compare. Assuming all your other variables are correct, which im sure you have checked, this would be a likely cause of at least some of the problem. What does litz quote the BC of that bullet at, and what G7 BC have you derived for it? Theres probably not a lot of difference is there?

Now you are getting it.

The numbers I worked up were with the 300RUM yes as well as 2 different 308 win barrels. All in all, there were 3 vastly different velocities tested here. Hence the change in BC's. The faster they were driven, the higher the BC went. I have done this with other loads as well, just keeping it simple here by using one good example.

The part where you just said G7 numbers still change with velocity is why I was seeing the differences that I have been seeing. The first set was 2550, the second 2750 and the third nearly 3250 plus or minus a few FPS for chrony error. What I have been trying to point out is that BC's change with velocity be it G1 or G5 or G7 and that if you use a number derived by somebody other than yourself even if they are the best in their field, you may or may not find that value to work. There is no doubt that when Litz tests a bullet, his findings are going to be very close. Very close for the velocity and parameters in which he tested it. But you cannot expect to take a value that was determined by a 308 win and have it work in a 30-378. This has been my case and point all along.

To take a set published BC and adjust your velocity to fit may still offer a close match for trajectory, but in many cases the difference will equate to innaccurate impact velocities which is critical for determining if a given bullet is going to expand at a given range. The trajectory may fit but the end velocity will not. Many times, it will not be close at all.

 

[groper]

What was the change in G7 BC you derived from 2550fps to 3250fps?

 

[Michael Eichele]

What was the change in G7 BC you derived from 2550fps to 3250fps?

.248 to .290

Is that a steep change? Yes. It is what it is. The velocities were checked again and again. the zeros were checked again and again. The drops were checked again and again. All the variables were accounted for and these were the results.

 

[groper]

Please bear with me here, but did you verify the trajectory at multiple distances, such as 500yd, 800yd, 1000yd, or did you just figure the BC from a single long range distance, of say 1000yds?

What im eluding to is this - Theres no way the G7 BC can change that much, even with the extreme velocity differences youve tested. So, lets take a look it things more closely;

Assuming you tested the 2550fps load and the chrony was reading 60fps fast, your actual velocity was 2610fps... from verifying the dope at a distance of 1000yds, your derived BC goes from .248 upto .27 as per my ballistic calc.

Do the same by your 300RUM load and assume the chrony threw it 50fps slow so the actual velocity was 3200fps instead of the 3250fps. Your BC goes from .29 to .27 verified by the 1000yd dope in the calc. Now if you go back and check your dope at say 500yds and 800yds, using the .27 BC and your new velocity, your trajectory still fits within 0.1moa across its entire flight- less than half a 1/4moa click- for your entire 300RUM trajectory to 1000yds... The maximum trajectory error of 0.3moa for the slow caliber or a single 1/4moa click.

Now in reality, perhaps the true average BC of the chronographed 2550fps load was about .265 and the magnum load chronoed at 3250fps the true average BC was about .275... the new trajectories line up so closely that the velocity disagreement is less, and the trajectory's match even better. It would then be safe to say that bullet had an average BC of between .265 and .275 for the extreme velocity differences of each caliber in .30cal. - Which is in good agreement with what the theory says the velocity difference would be in an average G7 BC. What did Litz quote the G7 for this bullet out of interest, do you know it?

Minute details can make a significant difference, and controlling the variables to within 1/4moa via drop tests in a real world environment is virtually impossible - which is the real reason people end up with different derived BC`s... it also shows, that using a Litz derived G7 BC and not touching it, will get you **** close, regardless of what caliber your shooting, provided you get the other details correct and allowing for a small chronograph error.

The reason Litzs G7 averages are so accurate, is in the methods by which he derives them. By measuring time of flight between 5 different points along a 1000yd trajectory, he measures the rate at which velocity decays and therefore eliminates the need to know an accurate muzzle velocity, extreme spread shot to shot, elevation to target, even aiming errors, zero errors, scope errors and drop errors are gone... the only source of errors i can think of, is making mistakes with the environmental conditions... So If anyone thinks you have a more accurate method than his, think again... the only method im aware of that would be more accurate than his, is via dopplar radar measurements at a full blown ballistic range proving ground such as Yuma or Aberdeen etc. Sure you can manipulate his BC if you have a super wildcat thats flight regime is vastly different from normal, but changing it by more than .01 is a mistake and you should look elsewhere to fix things...

 

[eddybo]

Groper,

I have seen lots of people explaining to you exactly the experiances they have to lead them to think that even the G7 form factor is imperfect.

I have not seen your experiances with the G7 form factor or any form factor to lead you to think that the G7 BCs are correct to 1%.

What leads you to this apparently firmly held conclusion? How many rifles have you set up? using what bullets? at what velocities? ranges tested at?

 

[ericpetritz]

Groper,
I believe Bryan published an average G7 BC of 0.240 for the 178 grain, .308 Amax.

 

[Michael Eichele]

Please bear with me here, but did you verify the trajectory at multiple distances, such as 500yd, 800yd, 1000yd, or did you just figure the BC from a single long range distance, of say 1000yds?

400, 500, 600, 700, 800, 900, 1000

What im eluding to is this - Theres no way the G7 BC can change that much, even with the extreme velocity differences youve tested. So, lets take a look it things more closely;

Assuming you tested the 2550fps load and the chrony was reading 60fps fast, your actual velocity was 2610fps... from verifying the dope at a distance of 1000yds, your derived BC goes from .248 upto .27 as per my ballistic calc.

Which is in good agreement with what the theory says the velocity difference would be in an average G7 BC. What did Litz quote the G7 for this bullet out of interest, do you know it?

I have no clue. I dont care to either.

Minute details can make a significant difference, and controlling the variables to within 1/4moa via drop tests in a real world environment is virtually impossible - which is the real reason people end up with different derived BC`s... it also shows, that using a Litz derived G7 BC and not touching it, will get you **** close, regardless of what caliber your shooting, provided you get the other details correct and allowing for a small chronograph error.

I guess I could buy that itis all chrony error if the 'errors' were consistent. One bullet matches perfect, another higher and another even higher, another lower etc.......so they are not consistent. Knowing that BC's change with velocity, I see that as the major variable here. Remember, I use multiple chronies. All are within 30 FPS of eachother at 2900 FPS, 2 are scary close.

All in all, what I have been doing is working very well.

Thanks for the thought provoking debate.

M

 

[BryanLitz]

Focusing on Michael's G7 BC results with the 178 Amax:
.248 from a low velocity round (.308 Win I think) and
.290 from a 300 RUM

.248 is only +3% from my value of .240. This is essentially a match, considering that the error only results in ~10" difference in predicted trajectory at 1000 yards. This is about as accurate as a drop test can be expected to be given all the uncertainties mentioned (chrono error, range error, etc) as well as unaccounted for effects like vertical wind, and coreolis (which could be up to ~3" of vertical in this case).

Now the same design bullet is fired from a different, higher velocity rifle. It appears to produce a G7 BC of .290. Since the weight and caliber of the bullet is the same, the only difference affecting the BC could be drag. Going from .248 to .290 implies that the drag (form factor) reduced by 15%. That's a huge amount. That's the difference between a bullet profiled like the 178 Amax (which is a relatively high drag bullet), and something even sleeker than a 7mm 162 Amax (which is an exceptionally low drag bullet).

When groper insists that the G7 BC can't be that high (.290) it's the same as saying the drag of the 178 Amax can't be as low as a 7mm 162 Amax. And he's right. That's a 15% difference, and bullets don't magically fly with 15% less drag from one rifle to another. You don't ever have to pull a trigger to know that.

I think Michael and the others know that the bullet isn't flying with such a different BC, but are still left wondering why their tests imply the drastic difference. This is the question I'd like to focus on. Because to focus on the question of why the BC really is 15% different between rifles is a waste of time; because it's not that different.

Michael provided information that describes a fairly detailed and well executed test. Some basic questions I have about the test are:
1) Wind conditions. Is there any terrain that would generate vertical wind over the range you're testing? We all know what the smallest amount of wind can do to horizontal displacement. It's just as bad in the vertical plane if it's there.
2) The test that produced a BC of .290; was it ever repeated? If so, how closely was the .290 repeated?
3) Are you accounting for the velocity drop between the muzzle and chrono? A 10 fps drop in 5 yards (from muzzle to chrono) correlates to ~4" difference in predicted drop at 1000 yards.

Michael and most everyone else is deriving BC's from drop tests. One of the most critical measurement instruments in that experiment is the scope, so I'd like to ask some questions about that.

I'll question the assumption that a scope that's calibrated (meaning click values measured) when mounted to one rifle has the same calibration factor when mounted to a different rifle. Here are my reasons:

1) I suspect that when a scope is mounted with different hardware (rings, bases, etc) and clamped in different places on the tube, that it can effect the amount of internal deflection to the 'mechanism' that moves the reticle in response to clicking the turret.
2) Let's say on rifle 'A', the wind zero for the scope is in the mechanical and optical center of the scope. You establish a calibration factor for this wind zero. Now mount the same scope on rifle 'B'. It happens that the wind zero for rifle 'B' is 10 MOA (for example) to the right. Now, when the plunger pushes on the ROUND erector tube which houses the reticle, it's not pushing directly on the bottom of the ROUND cylinder, but instead is pushing up on the edge of it. I think this might result in a different amount of reticle movement per click compared to the situation where the plunger was pushing directly on the center of the round tube.

The above two possibilities are only my speculation, grasping for possible causes as to why a scope's vertical adjustment calibration might not be the same when mounted to different rifles. Keep in mind I'm not an optics expert.

Back to the BC's. I'm not here to stomp my foot and declare that anyone's wrong because they got different numbers than me and my numbers are accurate. In this case, we can throw my numbers away completely and focus on the fact that someone did testing that implied the same bullet had a BC that was different by 15% when fired from two different rifles. I think we all accept that that's not actually the case, but the question we all want answered is: how and why would a test imply that?

Could it be velocity effects? I don't think so, and here's why. Using my latest test results, I've compiled the following correlation between G7 BC and velocity for the 178 Amax. Note that the BC values given are the instantaneous values at the corresponding speed. In order to get the average G7 BC for a trajectory, you average the BC's over the flight range of interest.

velocity	G7 BC
3300	0.255
3200	0.254
3100	0.253
3000	0.252
2900	0.250
2800	0.249
2700	0.247
2600	0.245
2500	0.243
2400	0.241
2300	0.239
2200	0.236
2100	0.234
2000	0.232
1900	0.231
1800	0.230
1700	0.230
1600	0.231
1500	0.233
1400	0.243
1300	0.242
1200	0.223

The average G7 BC from 3300 to 1500 fps (roughly the range of flight speeds over 1000 yards for the 300 RUM) is .241. The average for the lower velocity round (2800 to 1200 fps) is .237. So everyone who've been looking to velocity effects to explain the difference in BC is right that there is a difference, but according to the numbers above, the difference is only 1.7% for the average BC, not the 15% that's implied by the test. Could my numbers be off a little? Sure. But I don't think they're off that much. The implied .290 BC is coming from somewhere else.

Some more context. I've tested the 155, 168 and 178 Amax's. These bullets have essentially the same ogive and boat tail, which dictates that they should all have close to the same G7 form factor. The measured form factors for these 3 bullets are: 1.100 for the 155, 1.101 for the 168, and 1.118 for the 178. In other words, for bullets that have the same geometry as it relates to drag, they all have essentially the same measured form factor (drag) within 2%. Note the 178 has a little more drag which might be due to the longer bearing surface (more skin friction drag). The G7 BC's of these bullets are calculated by dividing the sectional density by the form factor. A G7 BC of .290 for the 178 Amax implies a G7 form factor of .924! This gets back to the notion that the .290 BC implies a level of drag that's simply unrealistic for a bullet shaped like this (and the 155, 168, etc).

I can think of many scenarios that would cause a bullet to fly with more drag than it should (stability problems, rough/worn barrels, bullet deforming in the barrel, etc). However, there is no way a bullet flies with less than it's minimal drag.

So we're back to the testing. I'm interested in Michaels answers to the questions above. My interest lies in understanding (for myself and helping others to understand) the science of ballistics. How and why some observations appear to contradict what's known about ballistics.

Those who deliberately enter false values into a ballistics program in order to get it to match what they shot in the field will get an answer that is more or less useful. But those who go to the trouble of verifying all the inputs are correct will get a prediction that is much closer to reality, is much more accurate over a wider range of conditions and distances, and will be more accurate on the other outputs (retained velocity, tof, wind drift, etc). If you're happy to knowingly enter false values, so be it. It's not my job to convince everyone that science works. But for those who are interested in getting to the bottom of things and doing it right, let's talk.

-Bryan

 

[MontanaRifleman]

Groper,

I have seen lots of people explaining to you exactly the experiances they have to lead them to think that even the G7 form factor is imperfect.

I have not seen your experiances with the G7 form factor or any form factor to lead you to think that the G7 BCs are correct to 1%.

What leads you to this apparently firmly held conclusion? How many rifles have you set up? using what bullets? at what velocities? ranges tested at?

Eddybo, you make a good point about experience. That being said, I would wager that Bryan may have more experience than all the rest of us in this thread put together, not to mention his background and that this is his profession, not just a hobby. He also has better facilities, equipement and procedures. I'm not at all saying his findings are infallable, but from where I'm sitting, if there's a descrepency in results, guess whose results I'm going with first? You guessed right

BC's do NOT change from bullet to bullet. Bullets of same kind are for all practical purposes identical in shape and mass and have exactly the same drag coeficient. Nor do they change shape or mass to any significant degree going through a bore unless you overpush the thinner jacketed ones. That's just a fact and one doesn't need experience to know that. One needs only to observe.

If there's a desrpency in results it has nothing to do with the rifle. The descrepency is somewhere in the equipment and procedures used.

If your's or anyone elses numbers work and you hit the target consistantly then that's what it's all about. Use them. I'm not saying that it's impossible that you or anyone else may have gotton more accurate results than Bryan, but if I was a betting man, you know who I would put my money on.

Have a great week and keep the critters in fear