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? about ballistic programs

 
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  #1  
Old 04-26-2009, 08:02 PM
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? about ballistic programs

Do you guys tweak your BC to make your charts match what your shooting or your velocity?
Seems to me that if you chrony the load, then your kicking a dead dog by changing the MV in the program.
Since we can verify everything else, doesn't it have to be our actual BC that is off? and changing the MV would still make you off POA at extended range because BC has a larger influence on the bullet than velocity does. Am I thinking right?
RR
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  #2  
Old 04-26-2009, 08:15 PM
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Re: ? about ballistic programs

You are correct. You can change your velocity instead of your BC to match an end range but NOT the mid point ranges.

This is why you change the BC. True BC's are almost never what are published. This is because you dont use the same rifle that was used in the test for that bullet. Your twist, barrel quality, velocity, number and depth of grooves and more all play a part of BC.

Test it, adjust it and be happy! Yes, this is how it is done.
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  #3  
Old 04-27-2009, 10:28 AM
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Re: ? about ballistic programs

I believe that the scope adjustments are usually responsible for 'field data' not matching 'ballistic program' data.

It's true that published BC's are not always accurate, sometimes they're off by up to 10%, 5% is more common. To a smaller degree the barrel you're shooting the bullet from may affect the BC too, but only by 1 or 2% max.

When someone observes that they need a BC that's 30% or 50% higher than advertised to get their field data to match the program, there's probably something else at play. I think in most cases it's understanding the scope adjustments.

For example, we all use 1/4 MOA clicks, right? So if you need 16 clicks to hit your point of aim at 400 yards, that's 4 MOA * 4 * 1.047 = 16.75" of drop at 400 yards, right? Now you plug in all the variables into the ballistics program and find that the bullet you're shooting requires a BC of 0.800 to match your observed drop. What if that bullet has an advertised BC of 0.400? Is it possible that bullet is just twice as good from your rifle? Probably not.

Go back to the part where we calculated drop from clicks. We arrived at 16.75" by applying the assumption that the crosshairs moved exactly 1/4 MOA per click as advertised. Has this been verified? What if the crosshairs actually move .265 MOA per click instead of .25 MOA? In that case the actual scope adjustment for 16 clicks would be 16*.265 = 4.24 MOA, which correlates to 17.75" of drop at 400 yards (instead of 16.75"). This is enough to cause an error of about 15% in a derived BC if you aren't aware of the error in scope clicks.

Furthermore, if the target was actually 393 yards or 407 yards, that could also cause a 1" error in drop and the same 15% error. And, can your rifle really group well enough to resolve 1" of drop at 400 yards?

This isn't an attack. There are a lot of things to think about when you try to derive BC's from drop data, and I'm just illuminating some of them in response to the question asked. The calculation is very sensitive to error.

Let's say that you find a number that you can plug into the BC field of a ballistics program that results in a predicted trajectory that matches up to your observed drop (using scope adjustments). If you don't know the exact, true value of your scope adjustments, the number you're using isn't a BC, it's just a number that you input which causes output to be generated which happens to offset the error in your scope clicks. If a bullet has an advertised BC of .4, and you need to input .6, there's probably something wrong.

So what if something is wrong? Who cares? The point is to be able to calculate a trajectory that will enable you to hit targets with your scope, right? Who cares if it's not the actual, true BC, it works. Well, kinda.
It can work for you and your particular scope to predict drop, but the retained velocity (important for knowing when the bullet goes subsonic), kinetic energy (important for lethality considerations), and wind deflection will all be very wrong. By accepting the use of a BC that's not the true BC, you're flushing all the other output generated by the program down the toilet.

Furthermore, no-one else will necessarily be able to use your BC. Not because the bullet actually flies different from the two rifles, but because the rifles have different scopes, which move different amounts when '30 MOA' is dialed in. One scope might move 29 MOA, the other might move 31 MOA and the two shooters, if not aware of their scope adjustment error, would perceive different drops (different BC's) from their rifles even though the bullet may actually flies exactly the same from both.

Also, if you've arrived at a BC with a ballistics program and you didn't input the measured atmospheric conditions (air temp, pressure, humidity), then the BC you arrive at is also not useful to anyone else, or even for yourself on any other day.

If you've arrived at a BC from observed drop data, and you haven't checked your chronograph against at least one other unit, then that's a potential source of error. Chrono's are usually good at measuring shot-to-shot variation (which is important for developing loads) but if they have a systematic bias (ie, always reads 30 fps slow for example) then any BC you derive assuming you know the muzzle velocity will be wrong.

The range to the target needs to be known exactly. If your laser rangefinder says 1007 yards, and you treat it as 1000 yards, you will have error.

Some of the above error sources are more picky than others, but they can all add up to a perceived BC that's very far from the actual BC. This may be fine if you're just looking for a number that helps you hit targets with your specific scope, but unless you've measured your scope adjustment and taken all the other steps to remove error from your test, then all you can say is that it's a number, not a BC.

-Bryan
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  #4  
Old 04-27-2009, 03:26 PM
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Re: ? about ballistic programs

Quote:
Originally Posted by bsl135 View Post
I believe that the scope adjustments are usually responsible for 'field data' not matching 'ballistic program' data.

It's true that published BC's are not always accurate, sometimes they're off by up to 10%, 5% is more common. To a smaller degree the barrel you're shooting the bullet from may affect the BC too, but only by 1 or 2% max.

When someone observes that they need a BC that's 30% or 50% higher than advertised to get their field data to match the program, there's probably something else at play. I think in most cases it's understanding the scope adjustments.

For example, we all use 1/4 MOA clicks, right? So if you need 16 clicks to hit your point of aim at 400 yards, that's 4 MOA * 4 * 1.047 = 16.75" of drop at 400 yards, right? Now you plug in all the variables into the ballistics program and find that the bullet you're shooting requires a BC of 0.800 to match your observed drop. What if that bullet has an advertised BC of 0.400? Is it possible that bullet is just twice as good from your rifle? Probably not.

Go back to the part where we calculated drop from clicks. We arrived at 16.75" by applying the assumption that the crosshairs moved exactly 1/4 MOA per click as advertised. Has this been verified? What if the crosshairs actually move .265 MOA per click instead of .25 MOA? In that case the actual scope adjustment for 16 clicks would be 16*.265 = 4.24 MOA, which correlates to 17.75" of drop at 400 yards (instead of 16.75"). This is enough to cause an error of about 15% in a derived BC if you aren't aware of the error in scope clicks.

Furthermore, if the target was actually 393 yards or 407 yards, that could also cause a 1" error in drop and the same 15% error. And, can your rifle really group well enough to resolve 1" of drop at 400 yards?

This isn't an attack. There are a lot of things to think about when you try to derive BC's from drop data, and I'm just illuminating some of them in response to the question asked. The calculation is very sensitive to error.

Let's say that you find a number that you can plug into the BC field of a ballistics program that results in a predicted trajectory that matches up to your observed drop (using scope adjustments). If you don't know the exact, true value of your scope adjustments, the number you're using isn't a BC, it's just a number that you input which causes output to be generated which happens to offset the error in your scope clicks. If a bullet has an advertised BC of .4, and you need to input .6, there's probably something wrong.

So what if something is wrong? Who cares? The point is to be able to calculate a trajectory that will enable you to hit targets with your scope, right? Who cares if it's not the actual, true BC, it works. Well, kinda.
It can work for you and your particular scope to predict drop, but the retained velocity (important for knowing when the bullet goes subsonic), kinetic energy (important for lethality considerations), and wind deflection will all be very wrong. By accepting the use of a BC that's not the true BC, you're flushing all the other output generated by the program down the toilet.

Furthermore, no-one else will necessarily be able to use your BC. Not because the bullet actually flies different from the two rifles, but because the rifles have different scopes, which move different amounts when '30 MOA' is dialed in. One scope might move 29 MOA, the other might move 31 MOA and the two shooters, if not aware of their scope adjustment error, would perceive different drops (different BC's) from their rifles even though the bullet may actually flies exactly the same from both.

Also, if you've arrived at a BC with a ballistics program and you didn't input the measured atmospheric conditions (air temp, pressure, humidity), then the BC you arrive at is also not useful to anyone else, or even for yourself on any other day.

If you've arrived at a BC from observed drop data, and you haven't checked your chronograph against at least one other unit, then that's a potential source of error. Chrono's are usually good at measuring shot-to-shot variation (which is important for developing loads) but if they have a systematic bias (ie, always reads 30 fps slow for example) then any BC you derive assuming you know the muzzle velocity will be wrong.

The range to the target needs to be known exactly. If your laser rangefinder says 1007 yards, and you treat it as 1000 yards, you will have error.

Some of the above error sources are more picky than others, but they can all add up to a perceived BC that's very far from the actual BC. This may be fine if you're just looking for a number that helps you hit targets with your specific scope, but unless you've measured your scope adjustment and taken all the other steps to remove error from your test, then all you can say is that it's a number, not a BC.

-Bryan

Can't buy that bsl, because
Nosler 160 accubond published BC of .531, when my tests were over my actual BC was .62, and I didn't make any adjustments during the tests, I measured my actual drops (put my target next to the top of 12 feet of cardboard), then changed scopes, with the BC of .62 my POI and POA were dead on.
Then I ran some loads for my 6.5 gibbs, first bullet was a 139 gr scenar, the published BC matched perfectly, 3rd shot at game resulted in a deer kill at 781, first killed a groundhog, second shot missed a turkeys neck by about 1.5" at 503 yards. Bullet performance was so bad I switched to the berger 140 vld published BC of .64, at 3340 fps I was always shooting high till I ran the 12' of cardboard test BC came out at .7.
RR
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  #5  
Old 04-27-2009, 03:45 PM
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Re: ? about ballistic programs

It depends on why your trajectory doesn't match the calculations.

Is the manufactures published BC wrong? Some are most are close.

Is your chronograph wrong? Maybe if you chronograph is triggering on the muzzle blast shockwave instead of the bullet.

Did you enter the scope/sight centerline over the bore correctly in the ballistics program? That's very important if you calulated clicks/moa to match. To nit pick that should be the distance the line of sight passes over the centerline of the bore >AT THE MUZZLE<.

It's not likely the calculations are significantly in error. Just about all software for PCs uses the same ballistic tables and basic equations
published by Robert F McCoy of the US Army Aberdeen Balistics Research Labs. There a notable exceptions like Art Pejsa's program.

So what's left? The atmosphere of course.

I mostly shoot at 5600' elevation. If I don't set the air density correctly the calculated trajectories are significantly different from actual trajectories. Air density can be calculated from temperature, standard barometic pressure, elevation, and humidity or it can be measured directly with an air density gauge which actually weighs a volume of air.

Last edited by LouBoyd; 04-27-2009 at 04:07 PM.
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  #6  
Old 04-27-2009, 04:06 PM
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Re: ? about ballistic programs

Lou,
with todays chrony's, how far can they be off? the photoeyes are 2' apart (on mine) it times the bullet through that 2' I get an accurate reading or an error, maybe its off 8 fps, and that amounts to how much at 1K?
As I said in my first post, we enter all the data, if we have it right, the only thing it can be is our actual BC. Is this correct?
RR
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  #7  
Old 04-27-2009, 07:03 PM
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Re: ? about ballistic programs

Quote:
Originally Posted by Ridge Runner View Post
As I said in my first post, we enter all the data, if we have it right, the only thing it can be is our actual BC. Is this correct?
RR
To be technical, NO. For practical applications, yes. This practical application is why I answered yes originally and I stand by this.

As a side note, I believe Bryan and RR are both correct.

Also as a side note, to back up Bryan on this point, scope imperfections can and will give you a percieved BC that is more or less than reality. This really is of little to no consequence (at least out to 1K AND minus accurate impact velocities. More on that later). As long as you are doing things consistently and taking into account the actual air density.

Before everybody lights the flame thrower, let me elaborate.

Lets say you go to the range and develop your load, get your average velocity, set your zero and then go to "find" your BC at long range. You set up at your zero for quick confirmation. Then your program tells you based on the published BC (.507) that your 1000 yard drop is 211" low. Knowing your scope is calibrated in .25 MOA values, you click it 81 times. But because there is an imperfection in your scope and the value is really .260 MOA you only needed 78 clicks. This will cause an impact of about 7.5" High so your "percieved" BC works out to about .535. As long as you use .535 AND adjust in the field in the same manner you did during the tests, you will be scarry close. Technically, it is wrong but for ALL practical use, it works. This is the method I prefer over ALL other methods. I have used the double chrony to verify the actual BC of a bullet but when it comes to setting up on a real world long range shot, I always use the numbers found in the drop tests. If there are imperfections in my scope, they get absorbed into my BC or better yet, my "percieved" BC.

As another side note. I have used double chronies to come up with actual BC's and are usually close to published. Recently I have worked up the 208 AMAX, 190 Hornady BTSP, 180 Nolser ballistic tip. I found the 208 in my 300 RUM was .671. The 190 BTSP was .530 and the 180 BT was .524, yet it always "seems" to work out differently when matching real drops to the scope. However, I dont care. I use the drop data to figure my shots and I do NOT use data figured from double chronies due to these issues. Granted, I may not get an accurate picture as far as downrange impact velocities etc....This is why I also derive info from double chronies.

Now for a success story.

In 2002, I spent a couple weeks getting ready for a sheep hunt. I just got a new 308 and NF BR scope. Being somewhat new, I was using .125" for a click value instead of .125 MOA. Never the less, I went out and performed the drop tests. I would adjust my scope until I found the middle of the target. At 800 yards I was at 174 clicks. So in my mind it was 174" of drop. I played with the calculator and adjusted my BC untill my drops matched my data. This worked out to .484 using the 168 SMK. The reality was that later I found out I was wrong and my real BC was actually .450. Never the less, I was using "wrong" data and still killed a ram at 763 yards with ONE bullet. I was still using numbers based on real world tests, albiet one factor was wrong, it was washed out. YOU CAN ETHICALLY KILL GAME OUT TO 1K USING THIS METHOD!

In my case, that old BR scope was really .125 MOA. The error lied with me and not the scope. However the error was still hidden in the BC. A dead ram was the result.

Now if you want to get technical, you can test you scope values by cranking the dial and shooting at 100 yards. The target MUST be exactally 100 yards from your scope. You can find the real MOA or inch value per click using this method. By using this method, double chrony tested BC's are much more accurate when plugged into the software. For me, I have shot enough to know that things arent always what they should be. This is why I still advocate finding values from drop tests, at the very least, verifying them. The funny thing is that after "verifying" and things are off, I end up adjusting my BC anyway.

I said it once, I will say it again. Find your numbers via drop tests, use those numbers and be happy!

Also, to be fair to RR, I have seen different values based on double chronies with the same bullets but using different guns. You cannot predict with 100% ACCURACY a BC based on form factors or computer models. You can get close but and you can even find a darn good average. But you cannot buy into the thought that they will be the same from gun to gun.

This is a very good topic and really highlights the importance of gathering BC data from more than one type of test as well as the importance of regular practice and verification sessions. You cannot get an accurate picture of what is going on in one or even two sessions. Thanks RR!
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Long range shooting is a process that ends with a result. Once you start to focus on the result (where the shot goes, how big the group is, what your buck will score, what your match score is, what place you are in...) then you loose the capacity to focus on the process.

Last edited by Michael Eichele; 04-27-2009 at 07:45 PM.
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