Applied Ballistics for LR shooting ??s

Discussion in 'General Discussion' started by jmason, Jul 21, 2009.

  1. jmason

    jmason Well-Known Member

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    These questions are ones I have from reading Bryan's book. They are geared for Bryan to answer if he would be so kind, but please feel free to chime in if you can help.

    What does the image of a "G5" drag model bullet look like?


    I'm getting a sense from other things I've read that a Nosler ballistic tip is not quite a direct match to the "G7" model. If that's true than you feel the "G7" is better representative than the "G1" and feel it's a better standard for model manufacturing to go by, correct?

    If that's true does the G5 actually match BT type bullets better in shape since they aren't as long as a VLD type?

    My questions are born by comparing info, and requested info from your book and LoadBase 3.0 mobile.

    Do you have any intention of a mobile platform of "Point Mass Ballistics solver"?
     
    Last edited: Jul 21, 2009
  2. BryanLitz

    BryanLitz <b>Official LRH Sponsor</b>

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    J,

    Good questions.

    It's true there are several other standard projectile shapes other than G1 and G7. It is possible to define a BC for any bullet in relation to any of those standards. Ideally, from a technical point of view, you should choose the standard that exhibits the least variation. For long range bullets, G7 is much better than G1, but in some cases, G5 can be a slightly better match than G7.

    To put it in perspective, using G7 referenced BCs will usually erase 90% of the velocity dependence of a G1 referenced BC. In some cases, a particular bullet may have an additional 5% improvement if referenced to some other standard (like G2, G5, etc).

    Although it may technically be a better solution to use G5 in some cases, I offer the following arguments against that practice:

    1. If you start using many multiple standards to define BC's, you loose the ability to compare bullets based on BC. For example, if 'bullet A' exhibits a slightly better match to the G7 model and it has a G7 BC of .237, and 'bullet B' exhibits a slightly better match to the G5 model and has a G5 BC of .342, you can't say which is better based on BC (not without converting, which is a lengthy and complicated process to do right). My point is that it's better, from a standardization point of view to adopt one standard rather than using multiple different standards. You're still eliminating most of the velocity dependence by using the G7 standard.

    2. The experimental data isn't always good enough to determine which match is better. My experimental data is very good at generating a reliable average BC (regardless of the standard used). However, the variation in BC, which is determined from the exact shape of the drag curve, is much harder to nail. So if you have a bullet with 0.006 variation in G7 BC and the same bullet has 0.010 variation in G5 BC, can you say for certain that the G7 is a better match? Not with my data. My data is good enough to resolve that a G7 (or G5) BC has much less variation than a G1 BC because the difference is so huge. However, the difference in the shapes of the drag curves between G7 and G5 is so subtle, and honestly, my data isn't good enough to say for sure which is a better match for each bullet. Naturally, since the difference is small, it has a minor effect on the accuracy of the trajectory you'll calculate.

    You may notice that the above 2 considerations both compromise on what's technically correct. However, they both do so in order to make a solution possible that's better and more useful than what we already have.

    There's nothing technically wrong with using a G5 BC for a bullet that matches that standard better, as long as you have reliable data that indicates that the projectile actually does match that standard better AND you understand that you can't compare a G5 BC with a G7 BC.

    My prescribed advice of using G7 BC's for long range bullets is a balance of many considerations including what's technically right, and what's a practical solution that the majority of shooters will be able to apply. A solution that's perfect in it's technical completeness will probably not get off the ground because it's overly complicated and has too many 'gotchas'.

    Regarding ballistic tips in particular, it wouldn't surprise me if they match G5 better than G7 for the following reasons:
    1. Tangent ogive vs secant (the G5 model is tangent, G7 is secant)
    2. Short steep boat tail. (the G5 model has a short BT, but it's not as steep).

    Having said that, I would expect that you will erase most of the velocity dependence by using a G7 compared to G1, even if G5 is slightly better. It probably amounts to a fraction of an inch at 1000 yards, an inconsequential amount.

    Here's an image of the G5 standard projectile:

    [​IMG]

    The dimensions in the figure are in 'calibers'.

    Really good questions, I hope I've helped to clear it up.

    -Bryan
     
    Last edited: Jul 21, 2009

  3. jmason

    jmason Well-Known Member

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    That's perfectly clear and understandable to me! Thank you!

    The software I'm currently using in the field is LB 3. It wants velocity readings from multiple ranges in the "analyzer" module. I assume what it's trying to do is use the G1 drag function to reverse engineer the G1 to to be accurate. Does that sound right?

    Do you have any intention of a mobile platform of "Point Mass Ballistics solver"? It seems to me (I haven't got into your software yet) that your software is covering "all the Bases" as long as you (the user) had the ability to save your input and show all reliant options on one screen I think it would be a great option for shooters. With the info you make available it would release the user from having to gather a ton of field data (that most likely will not be accurate). The most main stream 3 offerings require this of the user. Some more involved than others and some require methods that could cause as much error as they would correct. At the very least all 3 of these end up forcing the user to "fudge" something to get the software to match the "real world" drops. If that wasn't true certain values like "BC" would not be adjustable, and certain functions like "trajectory validation" wouldn't exist.

    If I have the G7 function available to me in the software is it safe to say I can use the "average" BC from your book(as long as my MV is close to the max velocity you listed) and not change the BC from there? Meaning other inputs should be adjusted but don't change the BC.

    Thanks,

    Grass Hoppa
     
  4. jmason

    jmason Well-Known Member

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    There's a lot of info in your book for me to try to retain (considering my level of understanding) so please don't be up set when I ask what's been answered in your book.

    Is a bullets "drag coefficient" based on any particular "drag function" or is it based on velocity measurement at different ranges alone?

    Grass Hoppa
     
  5. BryanLitz

    BryanLitz <b>Official LRH Sponsor</b>

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    J,

    More good questions!

    I do plan to develop a mobile version of ballistic software, but it's not a high priority at the moment. It will probably be a winter project (now that I'm in Michigan, I'll have about 9 months of winter a year...)

    When I get serious about it, you can bet I'll be on here soliciting advice from users on what features they want. I know how to write the program so it's accurate, but 'user friendly' means different things to different people, and I'll want it to be 'friendly' for as many users as possible.

    LoadBase uses a Pejsa solution, which is a strange bird. The details are complex, but basically it doesn't use BC directly like conventional solutions, it converts and massages the G1 curve into something that looks more or less like a G7 or G5 curve thru the use of the retardation coefficients (the 'n' exponent).

    It is safe to say that you can just use the average G7 BC as a reliable input to your software. If the output doesn't match what you see in the field, you should question some other component of the system.

    Drag coefficient is a native, raw property of a projectile. Each projectile has a unique drag coefficient at each Mach number, and a unique drag curve over a given range of Mach numbers. The drag curve of a projectile is related to the drag curve of a standard projectile via the form factor. The form factor is used (along with sectional density) to calculate the BC. So form factor and BC are relative quantities (relative to whatever standard you're comparing to), but drag coefficient is not a relative quantity.

    Clear as mud, right!

    -Bryan
     
  6. jmason

    jmason Well-Known Member

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    That answers my ?s. I don't completely understand but I don't need to. I now know what I wanted to.:)

    BTW and IMO LB3 does the best job of putting all a shooter needs on one plate/ screen for the field solution and that includes all the drift variables. You can decide weather or not to use them. I really don't care for the way any of them try to quantify your actual field trajectories to get it to mate with the program though. Again, that's just me. I prefer the scientific methods over my info gathering for reverse engineering. I don't need to fully understand those methods I just know they work.
     
  7. jmason

    jmason Well-Known Member

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    In your opinion is there some kind of rule of thumb or scale fpr the G7 bc as far as good, better, etc.?

    Like with the G1

    .1-.3= ok
    .3-.49 better
    .5-.7 really good
    .7 + awesome
     
  8. BryanLitz

    BryanLitz <b>Official LRH Sponsor</b>

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    As a 'rough' rule of thumb, G7 BC's are approximately 1/2 of G1 BC's.

    So if you consider a G1 BC of .600 to be high, that's roughly comparable to a G7 BC of .300.

    Speaking in terms of bullets suitable for long range shooting, I would give the following qualitative scale for G7 BCs:

    G7 BC below .150 is pretty low
    G7 BC of .150 to .200 ~ low
    G7 BC of .200 to .250 ~ nominal/low
    G7 BC of .250 to .300 ~ nominal/high
    G7 BC of .300 to .350 ~ high
    G7 BC of .350 to .400 ~ really high
    G7 BC above .400, very rare and crazy high

    -Bryan
     
  9. jmason

    jmason Well-Known Member

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    Sensei-

    Is it safe to say that there is always some amount of "Dispersion" that can in no way be accounted for from shot to shot?

    If that's so how on earth does the guy in the article "bugholes from a bipod" pull it off. I can see how at short range dispersion can be so subtle to allow for bug hole groups but How can it be at say 700 yards? Especially given the disproportionate magnification of it. I'm not arguing, just curious of you thoughts.

    BTW this does help explain why my shots "pin wheel" around the 10 ring at 900 and 1000 yards. Some is me, some is my load, some is wind, and some is dispersion that's on top of the rest correct?

    Grass Hoppa
     
    Last edited: Jul 23, 2009
  10. jmason

    jmason Well-Known Member

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    Sensei-

    I finished your book!! I'll need to read it again to retain all the new info.
    I have just a few more comments and questions.

    I now know how the pronounce the word "o-give", I've heard that word butchered and personally butchered it.:D

    Is the term "form factor" the same as "drag coefficient"?

    I'll be trying to exercise this new knowledge by collecting "accurate" data for LB3. I think that doing this will help me pull this all together better. I'm more of a hands on type. Would you be opposed to assisting me in analyzing data if need be? I want my data to be 100%.

    When you work on your field software another thing that the others don't currently do is share file from PPC to PC. So that means double data entry. I don't know what programming hurtles that causes but would also be a great feature.

    Your book has opened my eyes to ballistics. It also resolves a lot of uncertainties/confusion I had before. I have a 100% better understanding of how things work. Now that you've resolved this part of my game the only variable left is my abilities. Those are works in progress! I am defiantly a more confident, and educated shooter for having read this book. Thanks Bryan!!

    Fell free to use all or any part of this thread as a testimonial as you see fit.

    Grass Hoppa
     
  11. BryanLitz

    BryanLitz <b>Official LRH Sponsor</b>

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    J,

    Congrats on finishing the book! And thanks for the testamonial words, I just might use that in a future upgrade to my book selling page.

    It's actually not as long as it seems, those last 175+ pages are not really to be read like the rest of it.

    To answer your question, no. Form factor and drag coefficient are not the same thing.

    Every shape has a unique drag coefficient at a given Mach number (velocity). Form factor is how the drag coefficient of one bullet relates to the drag coefficient of a standard bullet.

    Let's say that projectile A has a drag coefficient of .250 at Mach 3, and that the 'G7 standard projectile' has a drag coefficient of .242 at the same Mach number. In this case, you would say that 'projectile A' has a G7 form factor of .250/.242 = 1.033.

    Now if you take that same 'projectile A' and define it's form factor in relation to a different standard projectile, then it's form factor is different. For example, the drag coefficient (cd) of the G1 standard projectile is .513 at Mach 3. So the G1 form factor of projectile A is .250/.513 = .487.

    So drag coefficient is a property of a projectile, and form factor is how the drag coefficient compares to a standard projectile.

    The above example is on page 524 in the appendix.

    Going from form factor to BC is easy, you just divide the sectional density by form factor. For example, if projectile A is a .308 caliber 155 grain bullet, then it's sectional density is: 155/7000/.308^2 = .233. To define this bullets BC in relation to the G7 standard, you would use the G7 form factor of 1.033. So the G7 BC is: .233/1.033 = .226. If you want to define this projectile's BC in reference to the G1 standard, then it's .233/.487 = .478.

    You make a good observation about porting data from PPC to PC. I'll keep that in mind when I start work on my mobile software.

    As for helping you with your analysis, I'm happy to answer questions when I have time. However, if you want me to actually get involved in doing any work that takes time, well, that's why I started my ballistics consulting business.

    Now, Grass-Hoppa, go forth and put holes in distant targets!
     
  12. jmason

    jmason Well-Known Member

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    Sensei-

    Thanks for the explanation! Not looking to take up any time. I'll post my data here (if I have an issue)and get everyone's thoughts. I may ask you to look but just for quick thoughts. I want to figure it out on my own if I can.

    What are your thoughts on my dispersion ??s a couple posts up? (last question I promise:D)

    Grass Hoppa Out...........
     
  13. BryanLitz

    BryanLitz <b>Official LRH Sponsor</b>

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    Regarding dispersion,
    It's a maddening fact but there will always be some variation from shot-to-shot that will induce dispersion that's outside our control as shooters. Some examples of 'unavoidable' sources of dispersion include:
    Atmosphere (wind)
    MV variation
    sighting error (you can only resolve the aim point with a limited amount of certainty)
    bullet imperfections
    loaded ammo imperfections/misalignment
    variations in how the rifle is supported during recoil
    muzzle blast effects (launch dynamics)

    All of the above variations will have some amount of random uncertainty, whether you can measure it or not. Shooting 10" groups at 1000 yards is crazy, crazy good in light of all the variables involved. Still, BR shooters routinely shoot under 10" groups by tightening everything up even more and using heavier rifles to further dampen the harmonics of recoil.

    The best way to understand your individual components of dispersion is experimentation. Just start systematically changing one variable at a time and noting the effect on group size. When you've converged on the combination that produces the least dispersion with a given rifle, then you know that any remaining dispersion is unavoidable.

    As a matter of practical consideration, I recommend setting a reasonable dispersion goal depending on your equipment. For example, if you're shooting a light weight factory rifle, try getting it within 1 or 1.25 MOA. Once you've achieved that, rather than burning up the rest of the barrel life doing endless experiments to shave of a fraction of dispersion, go out and practice with it. The dispersion is a known quantity, so you can use that knowledge to limit your shots to a given range. Concentrate on maximizing your effectiveness within that range.

    If your rifle is a top of the line custom, maybe you can realistically expect 1/2 MOA. Once you've achieved that, finalize the load and practice with it, especially at long range! This policy will make you a more effective LR shooter/hunter than making dozens of trips to the 100 yard range trying to make a .5 MOA rifle shoot .4 MOA then going hunting with your .4 MOA rifle and taking your first long shot at an animal. You might have a false confidence in your a .4 MOA rifle, but if you haven't practiced at long range, I don't like your chances.

    So that's how I view dispersion. BR shooters live and die by dispersion, so they're always changing things trying to minimize it. As a practical shooter, I say KNOW the dispersion of your equipment, accept it, and practice with it.

    -Bryan