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ballistic coefficients?

 
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  #29  
Old 08-25-2003, 04:17 PM
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Re: ballistic coefficients?

Speed,

The BC number is just a number to plug into the program that will modify the trajectory curve, either flattening the arc or steepening it. Your actual fired drops, when they are know, should match the arc at each range that data was collected at, or be very close. It's easy to see if the data points match at both ends and don't in the middle, so changing the BC to a higher number on the program will flatten the programs curve in the middle or lowering it will steepen it to match yours. When you get it to match in the middle ranges, and the beginning and ending ranges have moved and don't match up well, you'll likely have to use a different drag model, G5, G7 etc for one to follow yours from beginning to end more closely. The numbers on the other models will be lower but, just keep changing them until the arcs match up best you can get them, the number is a referance only relative to use with that drag model and is meaningless to compare to the others, use it as a referance number for that load... This all is based on the fact you have a definite and solid MV average you can count on. Changing the MV will alter the curve just as well as the BC and will cause you grief if you don't nail it down first.

I use Exbal, but it does lack the other drag models even though it lets you enter multiple BC's at different ranges.

The Sierra program, if it doesn't give you an option to change to a G5, G7 curve etc, it uses the G1, almost guaranteed, as it's what they recommend using anyway. It isn't always the most accurate matching curve tho.
http://www.perry-systems.com/

Jeff, sounds like a sweet rifle. Good luck with the load there, even tho don't look like you even need any. [img]images/icons/grin.gif[/img]

Used RSI's demo a while back but can't remember how I liked it it's been so long. I'm hooked on Exbal right now pretty good, reticle analysis and all, I'm sure I'll be using it for quite some time to come.
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  #30  
Old 08-26-2003, 06:54 PM
MAX MAX is offline
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Re: ballistic coefficients?

Jeff & Brent, thanks for your comments. My recollection is that the projectile used for establishing the G1 was a 1" X 1 lb form with a tangent ogive of 1-2 calibers.

For a reality check on my part, would not BC comparisons based on different G functions be an apple/orange thing? The G1 form is...crude, from an aerodynamic perspective. BC calculation is part weight, part form, so if you're on the wrong "scale" does the data have any particular validity? This was the point that Jameison was making I think.

The mention above regarding changing BC's(Seen in the Sierra Manual also), and some other comments I see from time to time are making me appreciate the Coeficient of Drag approach used by the 'Aeroballisticians', as McCoy called himself and his peers.
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  #31  
Old 08-27-2003, 01:54 AM
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Re: ballistic coefficients?

Max,

Hope I wasn't confusing anyone, or possibly misleading them, sorry to simplify so much, so to clarify some...

For a reality check on my part, would not BC comparisons based on different G functions be an apple/orange thing?

Absolutely.
the number is a referance only relative to use with that drag model and is meaningless to compare to the others


The G1 form is...crude, from an aerodynamic perspective. BC calculation is part weight, part form, so if you're on the wrong "scale" does the data have any particular validity?

I think it comes close enough for most SR hunters, that's why they keep passing the G1 off as reality with all bullets, or ignoring it's shortcomings to sell bullets. Until people start exposing their error in using the G1 drag table for a specific bullet and correcting it with a more correct one to use, regaurdless of the lower number it produces, it will be the standard they all keep us fixed to.

The G1 works to a point, just have to decide at what point it isn't working well enough and another is I guess.

The mention above regarding changing BC's(Seen in the Sierra Manual also), and some other comments I see from time to time are making me appreciate the Coeficient of Drag approach used by the 'Aeroballisticians', as McCoy called himself and his peers.

Can you explain on that a little more Max?
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  #32  
Old 08-27-2003, 09:37 AM
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Re: ballistic coefficients?

Brent,

Great post on the answers. I have 7 different ballistic programs at home. I found out later, most of them were written by the same individual for different companies. He just had to make them look different. From what I found out, these were sold to the bullet companies under their label.

Jim Ristow who wrote the RSI program was the head ballistic engineer at Aberdeen Proving grounds for the army for many years.

I understand that the new Chey-Tac .408 development is now using a new type state of the art ballistic program that uses Doppler radar and can track a bullet over 2500 yards. They're able to track the bullet over the entire range with a visible reference on the computer screen that can be measured at any point.

Now I like to bring 100 rounds or so to that range and play for an afternoon to see what my loads are really doing. [img]images/icons/smile.gif[/img] [img]images/icons/grin.gif[/img]
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Mathew 5:16

Distance is not an issue, but the wind will make it interesting!
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  #33  
Old 08-28-2003, 12:41 AM
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Re: ballistic coefficients?

Jeff in TX,

Not sure if you've seen the Pressure Trace ballistics lab Jim Ristow's come out with or not but, ones on the way to the house here now for testing. A copy of RSI Shooting Lab is on its way too. I spoke with Jim for what seemed like half a day on the phone, quite a guy that shared tons of info with me on just about everything ballistics and reloading. I'll be setting a barrel up with two strain gages to run the Oehler 43 and the PT simultainiously for several different test comparisons.
For anyone that hasn't looked at them, it's really worth a look. It may prove that it's a much more usefull, and even possibly a more accurate pressure testing and load development diagnostic tool than the Oehler 43 is, and that's saying alot too! [img]images/icons/wink.gif[/img]
The price for this instrument is so reasonable, it's almost too good to be true.

Jim said the same thing as Max and Jeff did about the G1's standard bullet, it was a 1.0" dia 1 lb. projectile with a pretty blunt looking nose, like .45 ball. Not even close to todays long and pointed bullets, much less the better bullets out there we're all acoustomed to using.

RSI Shooting Lab "does" have a BC conversion capability which Jim made sure of. It and Oehlers are also the only programs he said that uses the same algorythims developed by Uncle Sam and our money by the best minds out there, the others all use Ingalls except the 6DOF program.

I think the Doppler data for the 408 CT and other LRBT bullet designs were the start of yet another drag curve, and Warren is protecting his interests and keeping it secret too, I've heard. He still advertizes them based on a G1 model like everyone else too...

Jim said the referance standard for any drag model always has a 1.0 BC, with a 1 lb. projectile in the specific bullets form (shape). A change in form, and all goes out the window basically. Comparing a 1.0" dia. 1 lb. round nose projectile to a 240gr SMK with its sleek design is what gives the super, super lightweight in comparison 240gr bullet the BC that comes real close to matching the 7000gr projectile. In reality, if a 1 lb. 1.0" dia. projectile with the shape of a 240gr SMK was the standard, the 240 would have a super small number in comparison to "that" referance standard... BUT, the trajectory curve would NOW match up, and that's the important part here.

A day at Aberdeen, life'd never be the same at my range again after that range session. [img]images/icons/grin.gif[/img]

Good day fellas, been an interesting and enlightening day here. [img]images/icons/smile.gif[/img]
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  #34  
Old 08-28-2003, 07:01 AM
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Re: ballistic coefficients?

Brent,

I've spent many an afternoon talking with Jim and suddenly you realize you've been on the phone for 1, 2 or 3 hours and your brain is really hurting from all the info he's given you. I thought I really understood ballistics and shooting, but he can humble you quickly.

He is a wealth of info. I did not know he was coming out with the new program. When you get done testing with it, let me know how it works. [img]images/icons/smile.gif[/img]

[ 08-28-2003: Message edited by: Jeff In TX ]
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Mathew 5:16

Distance is not an issue, but the wind will make it interesting!
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  #35  
Old 08-28-2003, 08:32 PM
MAX MAX is offline
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Re: ballistic coefficients?

I'll give it a try Brent, but one thing for certain, I ain't no Robert McCoy!

BC evaluates the relationship between weight, form and diameter. It is a ratio with no dimension, a comparison of "X" to a standard(G something) with known ballistic characteristics. Because the factors relating to the calculation have direct relationships, ie. BC is proportional to SD for a given form, inversely proportional to diameter for given form and weight, etc. etc. It is fairly convenient to use(remember, figures don't lie, but liars do...), but it's accuracy is dubious(garbage in-garbage out) in field conditions.

The high priests of modern ballistics began to look at this issue in a strictly aerodynamic sense. What matters to them is COEFFICIENT OF DRAG. It matters not what the size in weight or diameter(in most cases). The Cd is purely a function of form, and it is expressed in Mach Numbers. One of the points of this is that Mach is Mach, while FPS may be one Mach # today, and another tomorrow. The drag curve, when plotted looks pretty much the same, but when you ID a point where it says Cd=.23 at Mach 3.8 it is a reliable fact. Doesn't matter if it's -20 or 120* F, it's still a Cd of .23 at Mach 3.8

Anyway, after all of this, additional factors are applied to evaluate what the projectile will do, items such as weight, GS, etc., and perhaps a big reason the industry hasn't embraced this concept is that a) they have a system in place and it's comfortably slippery. b) the equations used in the Aerodynamic approach have names like "differential" and "quadratic", and it's really neat if you have a Cray handy to crunch your numbers! Too, if you want the true Cd for a given form, you have to test it. Extrapolation is fuzzy science.

Anyway, that's my take on this FWIW, and I'm sure I need to read it a few more times.

"Modern Exterior Ballistics", by Robert McCoy

If you happen to buy this book, take a good look at the depicted/graphed Cd values as speed increases beyond the Mach 4 regions. It makes my madness seem a little less severe, my personal musings more worthy. [img]images/icons/wink.gif[/img]
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