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Applied Ballistics for LR shooting ??s

 
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  #1  
Old 07-21-2009, 09:20 AM
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Applied Ballistics for LR shooting ??s

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"?
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Last edited by jmason; 07-21-2009 at 10:22 AM.
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Old 07-21-2009, 02:27 PM
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Re: Applied Ballistics for LR shooting ??s

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:



The dimensions in the figure are in 'calibers'.

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

-Bryan
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Last edited by BryanLitz; 07-21-2009 at 06:24 PM.
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Old 07-22-2009, 07:25 AM
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Re: Applied Ballistics for LR shooting ??s

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
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Old 07-22-2009, 07:39 AM
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Re: Applied Ballistics for LR shooting ??s

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
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Old 07-22-2009, 08:06 AM
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Re: Applied Ballistics for LR shooting ??s

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
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Old 07-22-2009, 08:24 AM
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Re: Applied Ballistics for LR shooting ??s

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.
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Old 07-22-2009, 09:44 AM
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Re: Applied Ballistics for LR shooting ??s

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
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