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Applied Ballistics for LR shooting ??s

 
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  #8  
Old 07-22-2009, 10:19 AM
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Re: Applied Ballistics for LR shooting ??s

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
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And: Accuracy and Precision for Long Range Shooting

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  #9  
Old 07-22-2009, 01:58 PM
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Re: Applied Ballistics for LR shooting ??s

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
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Last edited by jmason; 07-23-2009 at 07:52 AM.
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  #10  
Old 07-23-2009, 11:48 AM
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Re: Applied Ballistics for LR shooting ??s

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.

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
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  #11  
Old 07-23-2009, 05:04 PM
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Re: Applied Ballistics for LR shooting ??s

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!
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Author of: Applied Ballistics for Long Range Shooting
And: Accuracy and Precision for Long Range Shooting

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  #12  
Old 07-23-2009, 05:45 PM
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Re: Applied Ballistics for LR shooting ??s

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)

Grass Hoppa Out...........
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  #13  
Old 07-24-2009, 09:49 AM
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Re: Applied Ballistics for LR shooting ??s

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
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And: Accuracy and Precision for Long Range Shooting

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