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SD vs BC

Have a question for the more experienced shooters or anyone with good info on this:

If you have a 140 gr 6.5 cal bullet being fired at 2850 fps with a BC of 618 and then compare it to a 162/168 gr. 7mm bullet with about the same BC, only being fired at 2650, which bullet would most likely fair the best at 1000 yards, as far as wind drift and drop?

Thanks guys
 
Go to this site and plug in your numbers. Then you can see for yourself.
http://www.jbmballistics.com/cgi-bin/jbmtraj_simp-5.1.cgi
 
Thanks much Nomad...tried going there and for some reason it is very slow loading the site...will try again later but thanks much again for pointing me there.
 
BRGUY, there are 2 things that defeat a rainbow trajectory and wind drift: 1.) B.C. and 2.) Speed... Now according to some, bullet weight is SUPPOSED to be taken into consideration in these "Ballistic Programs"... It has been my experience that IF they take bullet weight into consideration, it is not an "exact mathematical science".. As an example: I have noticed 115DTAC's drift considerably more than my 6.5 140 Hybrids and they are VERY close in both B.C. AND Speed.. I am of the opinion that the "trajectory side" of B.C. is one thing and the "wind drift side" of the B.C. equation is another.. I know I will get flak from that statement, BUT practical experience has shown me that there IS a difference between the two.. Attempting to quantify them both into 1 equation does not do the entire "B.C. Equation" service..
 
I am of the opinion that the "trajectory side" of B.C. is one thing and the "wind drift side" of the B.C. equation is another.

Don't quite understand that statement.

What people need to understand is that a particular bullet's BC may only be estimated based on design characteristics. That may be high/low as later confirmed with through-the-air testing done with multiple chronographs positioned along the trajectory.

Other points need mentioning:

1. You can take ten different bullets out of a box, and they all may have very slightly different "true" BCs. This is one reason why David Tubb, years ago, started trimming meplats, in an effort to make the BC more identical from bullet to bullet.

2. BC is dynamic. What this means is that the drag factor varies slightly as the bullet slows (changes velocity) down during its flight. The published BC is an attempt to come up with the best single number to plug into the programs.

3. Additionally, the BC may vary according to velocity (MV) in addition to being dynamic during flight.

How much does this really matter? I think if you use the Litz-derived BCs you will probably be within 1-2 quarter-MOA clicks of the predicted impact at 600 yards.
 
Forum Boss, what I am trying to say is, is that my experience tells me that there SHOULD be a B.C. equation for trajectory and another for wind drift. Attempting to blend them comes up short>>>not to a LARGE degree but I have seen a difference. I shoot a 6-6.5 x 47 with 115DTAC's and a 6.5 x 284 shooting 140 Hybrids.. They have a 585 and a 618 B.C. respectively and in my rifles the 115's are moving out at 3010 and the 140's at 3030f.p.s. I have shot them over the course of about 16months at the same ranges and on comparative days.. I have noticed whether at 600 yards or 1000 yards, I have to hold CONSIDERABLY farther to one side or the other (depending on wind direction) with the 115's than the 140's... I know I can't say I shot them under EXACTLY the same wind conditions, however, I have shot both enough to know both rifles well and the 140's drift A LOT less than the 115's..
 
Thanks much everyone for the "enlightenment" and info. I had the snese that the 6.5 would outperform but wasn't sure. When I wrote the term "SD" I was refering to Sectional Density, not Standard deviation. I'm sure most who replied here knew that but just wanted to clarify.
Best Wishes All
 
A quick correction on the launch velocity comments in re Sierra's multiple BCs. This doesn't have anything to do with launch velocities per se, but the fact that they realized full well that the G1 drag model they (and everyone else, prior to Bryan's joining forces with Berger) used for their published BCs, wouldn't match the downrange performance of those same bullets. The standard projectiles on which these two drag models are predicated are considerably different. For a BC to remain fairly constant throughout a bullet's flight, you need to select a standard that matches the shape of the bullet in question fairly closely. The G1 model simply doesn't match the profile of most of our modern bullet designs. It's a rather short, blunt, flat-based bullet, which differs considerably from out modern, sharply profiled boat tailed match bullets. Knowing that the actual trajectory of the bullets wouldn't match up well with what the G1 BC predicted, Bill McDonald and Ted Almgren used multiple BCs for the bullet's flight as it transitioned from one speed to another. It was an attempt to patch a known problem, nothing more.

Bryan and I have had this conversation several times, but it all comes down to this; the G1 simply became the default drag model used for all published BCs for commercial bullets. At the time, very few shooters had the slightest clue what BC was, or how it was used, so it was something of a moot point. It served as a good reference point for ballisticians of all companies to compare their projectiles with others in the industry. With the advent of PCs and the ability of the average shooter to get into this area far more deeply than ever before, the problems with the old G1 model have become better known and understood by those same "average shooters." Today, it is far more prudent for a manufacturer to provide better drag models, such as the G7, for bullets that more closely match that profile.
 
I'm not going to go completely into the math, but for two bullets having the same speed, or close to the same, the heavier one will have more momentum. And, the heavier one with the higher momentum will take more force to move it from F=Ma, for similar acceleration away from a stable path, the force required to impart that acceleration is proportional to the mass of the projectile. Or, for a given wind with a force on the bullet being proportional to cross sectional area of the bullet, the heavier bullet will depart from it's path less than a lighter bullet. Of course, the heavier bullet will have a larger cross sectional area, but the mass is a square function where the area is linear so a bullet 1.2 times heavier will not move 1.2 times as much, but maybe 1/15 times as much.

Make sense?
 
Hornady has a good ballistic calculator on their sight. Jbm has just about everything on theirs. The only reason I bought an iPod was to get a reliable ballistics calculator in my hand, that doesn't need internet connection to work. With one a man can answer many of his own questions.
 
Thanks again all much....Kevin, you had some very insightful stuff as always as did most all of the others.
This all came about as a question for me when a friend who is new to long range shooting was asking me to suggest which would be a better cartridge to build on, the 7 08 or the 6.5 Creedmoor. One factor he mentioned was that his wife would be shooting it also. That leaned me toward the Creed but I also felt it should outperform at long range as well and so I was suggesting the Creed as a better choice. I wanted to hear from all of you to make sure I was going the right way.
Best Wishes.
 
22BRGUY,

What you've got here is the essence of the debate; everyone's focused on BC. I'm certainly not saying it isn't important (it is, or certainly can be for much of our shooting), but it's only ONE factor that needs to be considered. Look at the big picture, figure out how important the high BC truly is for your needs, and add in those other factors as well. Guaranteed, you'll make a better and more informed choice than simply being dazzled by BCs.
 
ShootsDots,

The 6.5 should have with less drift. It has .33 better BC and is going 20' faster. That adds up to 20" less drop and 10.4" less drift at 1000 yds.

22BRGUY,
If bullets have the same true BC, and shot at the same speed, they will shoot the same. But some bullets Makers do stretch the BC numbers.

Mark Schronce
 
KevinThomas said:
22BRGUY,

What you've got here is the essence of the debate; everyone's focused on BC. I'm certainly not saying it isn't important (it is, or certainly can be for much of our shooting), but it's only ONE factor that needs to be considered. Look at the big picture, figure out how important the high BC truly is for your needs, and add in those other factors as well. Guaranteed, you'll make a better and more informed choice than simply being dazzled by BCs.

Thanks again Kevin and I understand that there are many many more factors coming into play that can be as important or even more important than the BC. In this case a new to long range shooter was trying to decide which cartridge would most likely offer the best long range terminal performance, the 7 08 or the 6.5 Creedmoor. As I mentioned, his wife was also going to be shooting this new custom rifle. That said, right away I thought recoil should be a consideration. Then looking at the BC/SD thing it looked like the Creed would or should outperform the 7 08. The 7 08 is by no means a slouch here but I think the Creed would be a little better in this instance. If you feel otherwise on this please share your thoughts and why you think so as I and all others truly respect your knowledge and experience.
 

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