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Thoughts on BC

 
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  #1  
Old 03-21-2003, 09:27 AM
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Location: Elizabethville Pa
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Thoughts on BC

I was doin some browseing the other day and i came across this and found it preatty intresting.. I would like to here you thoughts on this..


B.C. vs. B.S.? What does that mean? The answer is extremely simple, but the rationale is extremely complex.

B.C. is Ballistic Coefficient, and naturally, B.S. is Bull ****.

We all know what bull **** is, and if some of you out there don't, I suggest that you go ask your dads ... But how many of us know what a ballistic coefficient is? Not too many! Some reloaders/shooters feel that the B.C. of a bullet was handed down by God, to Moses, but they were lost when the first set of tablets were thrown to the ground in anger.

Let's describe what the B.C. really means. It is a number, assigned to a hypothetical perfect bullet, under ideal and constant atmospheric conditions, at a constant velocity. There are three major considerations that we MUST look at here. "Perfect", "Ideal", "Constant". None of these apply to any man or environment know to man, or at least inhabited by man.

First, what is the "perfect" bullet? It is a bullet which is three-calibers long, and ogival head of two-calibers radius, and of homogenous construction with equal and concentricity of the mass around the center from tip to butt. Got that? Name one bullet that meets those specifications!

This bullet must be fired from a source that will establish and guarantee that a constant velocity of that bullet will remain from the moment of launch until the moment of impact. Got that? Name one projectile that meets that requirement!

In addition, all this MUST take place at exactly sea level, at a temperature or 59-degrees F., 29.58-inches of mercury barometric pressure and 78% humidity. Oh, and absolutely no movement of the air... Name one place on earth that has those qualifications, 24-hours-per-day, 7-days-per-week, 52-weeks-per-year... Can't, can ya!

Now if we could find that "perfect" bullet, and launch it and maintain it at the "constant" velocity, under the "ideal" conditions, we would be able to assign a ballistic coefficient of 1.000 to that bullet. That's a hell of a lot of work to get a rating of "1"! (Hell, my first wife was a "9", and my second wife was a "6"!)

Now, what does all this mean in our everyday world of reloading and hunting? It means that the B.C. of a bullet means absolutely NOTHING! That's right, not a damn thing. (Excuse my language, but I really get upset when I discuss the B.C. of a bullet.)

All bullet manufacturers (other than Sierra) assign a B.C. rating to each and every one of their bullets under the "ideal" or "standard" conditions I have listed above. How do they derive their numbers? No, they don't travel to the moon, they do it on computers. Someone, somewhere computed how long it would take the bullet with the B.C. of "1" to travel a specified distance. For simplicity's sake, let's say it took one second. Then, they fired one of their less than ideal bullets the same distance and measured the amount of time it took that bullet to travel that distance. Let's say it took 1.3 seconds. That bullet would then rate a B.C. of "0.768". There are NO bullets currently available to the general shooting public that have a B.C. higher than .768! Bullets available to modern man have "rated" B.C.s of between .120 and .768. (And please remember, this value is only valid at a specific initial velocity.) Sierra saw the light a few years ago, and now assign three different "approximate" B.C.s to each of their bullets. Call them "high velocity", "standard velocity" and "low velocity". They found some bullets behaved better at lower velocities than at higher velocities...thus they have a higher B.C. at lower velocity (contrary to what many "experts" will try to tell you), and some exhibit just the opposite. And yes, Virginia, there is a Santa Claus ... some bullets perform better at medium velocities!

So what does this really mean? Let's say you live in Florida. You are enjoying your 90-degree temperatures, your 90% humidity, and your gentle gulf breeze. You decide to work-up a new load for your favorite .30-06. As remarkable as it may sound, your first load prints groups of 5 shots at 200 yards of less than 1 inch! You are happier than a pig is ****!

That fall you go to Colorado to hunt mule deer. You take your favorite .30-06, and those great new loads, with you. You are high in the mountains, you spot a great 18-pointer at around 400 yards, with a body weight of at least 500 pounds, squeeze off a shot, and miss the sucker by almost 5 feet ... but instead hit a scrawny little 95 pound doe with a severe case of hair-loss! You get so pissed you take a second shot, knowing that you are limited to only one kill, but what the hell, no one saw you ... and the second shot hits the game warden that was standing around 3 feet from the poor old doe! (Unfortunately you didn't kill the warden...so off to court and jail you go...)

How could that have happened? Here's an explanation, as best as I can do.

First of all, the 180 grain bullet you selected had a factory rated B.C. of 0.431 (at the standard conditions). Well you tested the loads at 90 degrees instead of 59 degrees, he humidity was around 90% instead of 78%, barometric pressure that day was around 29.53" Hg. All these factors increased that factory rating of 0.431 to a little over 0.529. A difference of almost 25%!

Oh, we ain't even near being done yet!

So you take your loads, with their B.C. of 0.529 up to the mountains. The temperature is now 20 below zero, you are at 5,000 feet, and the humidity is only 35%. That Florida bullet with a B.C. of 0.529 now rates around a stinking 0.282. Impressed? You should be!

Do you wanna know why you missed? That magic number of 0.529 is now down to 0.282, and we haven't finished yet! Were you aware that combustibles behave differently at different temperatures? That bullet launched at, let's say, 2,600 fps in Florida is now only leaving the barrel at around 2,400 fps! And remember you fired your groups in a very gentle gulf breeze, you are now firing those bullets across 400 yards of cross winds of up to 30 mph! (And you were probably shooting either uphill or downhill.)

How much of this miss is due to the B.C.? Very Little! Your .30-06, when sighted in while you were in Florida dropped 50 inches at 400 yards, when launched at 2,600 fps. Now that you are here in Colorado, the bullet is leaving the barrel at only 2,400 fps, it will drop 60 inches over the same 400 yards. The gulf breeze you enjoyed in Florida had no effect on accuracy, however the 30 mph crosswinds up in the Colorado mountains will cause that bullet to deflect almost 48 inches. So now your bullet is 4 feet to the side of your target, and at least 1 foot lower than you expected! That's exactly where the doe was standing! And the warden? Well, he was hiding next to a tree watching you, a few feet from the doe, and in your anger and haste, you pulled the crosshairs of your $39 scope just a tad to the right on your second shot! Well, the thick cross hairs of that cheap scope cover around 12 inches at 400 yards! You moved it two widths of the cross hair, and the warden caught that 180 grainer! So how come the doe died and the warden didn't? The doe weighed around 95 pounds, the warden 275. The doe had one layer of skin to protect her, the warden had his down jacket, wool liner, felt shirt, insulated underwear, and his beer-gut to protect him.

When you get out of jail the first thing you do is take your .30-06 to a range in Colorado. To your amazement, at 200 yards, 5 shots grouped around 2 inches ... that's how much the difference in B.C. actually made ... a stinking 1 inch at 200 yards! And that was for a B.C. difference of 50%! (And I have been witness to many a heated argument and discussion of B.C.s that varied as little as 1%!!!

The answer to the question is that B.C. is B.S.! If you want your rifle to shoot where you point it, test your loads in as near the same environment as where you will be hunting, under the same conditions, at the same range. (I sight all mine in at 300 yards, and yes, I do go out when it is 20 below zero and do a test target every year, with every rifle, before hunting season.)

6.5 Bandit
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  #2  
Old 03-21-2003, 10:44 AM
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Join Date: Jul 2001
Posts: 1,459
Re: Thoughts on BC

Bandit,

A very interesting and colourful descriptions of what many here know. The BC is simply a number based on the conditions you put into the computer. It is not fixed like mass or vel.

The BC of a bullet will change due to atmospheric conditions, muzzle vel, and even twist rate. One thing you didn't include was that different barrels launching the "same" bullet at the same vel may not have the same BC. This is due to the damage done to the bullet surface by the rifling.

You will hear no argument from me or from others that do alot of LR shooting that any load must be fully field tested. Drop tables must be confirmed by actual shooting and as much shooting must be done in different conditions to ensure dependable performance. Not to mention some practise for the shooter.

That is why I use the first shot sighter method for really LR shooting. In one instant, you eliminate all the variables and get a result (bullet impact) that represents the environment that you are in now. Real world, real time performance.

Run this one through a ballistic program and see what type of BC number you get. 165gr 30 cal bullet leaving at 2700fps needs 28 min. of adjustment from a 100yd zero to hit at 1000yds. That is what my '06 needs with SST bullets. With Interlock 165gr BTSP and the same load/rifle, it needs 44 min to do the same thing.

Doesn't make much sense does it? That drop has been verified several times.

Jerry
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  #3  
Old 03-21-2003, 01:14 PM
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Join Date: May 2001
Posts: 2,369
Re: Thoughts on BC

Be careful of near truths that head toward mostly false.

I don't have a lot of time so I'll pile on and say only a few things...someone else will carry through and either correct me or add.

A bullet with a B.C. of 1.0 is NOT the perfect bullet, it's mearly the "standard" bullet at the time the "standard" was established.

It's a numeric value needed to complete an equation/prediction. It can change and that's not a problem. The major point is that a bullet with a B.C. of .8 will not normally convert into a bullet with a B.C. of .25 on a whim, it's B.C. will remain better than a bullet with a "normal" B.C. of .478.

[ 03-21-2003: Message edited by: Dave King ]
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  #4  
Old 03-21-2003, 02:25 PM
*WyoWhisper*
 
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Re: Thoughts on BC

Wanna know what I do..

I take my rifle my load and shoot it at know distances. 500 yards, 700 yards 1000 yards 1200 yards.
I write down the conditions for that day and how many clicks I needed to get to that distance on target.. then I go back to my program and play with the BC until I get my exact values at those conditions..

Interestng thing happened.. you can switch scopes and have the same results reguardless of the fact they were both 1/4 min. click scopes....

I have noticed that manufactuers BC are stated higher than they really are..

soo.. BS yes... nothing compares to actually testing the product under diferent conditions
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  #5  
Old 03-21-2003, 03:40 PM
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Join Date: Jan 2002
Location: Wenatchee, WA
Posts: 804
Re: Thoughts on BC

WyoWhisper,

I've noticed the same thing w/ regards to published B.C. for several manufacturers. I've not gone thru and tested things out to determine the actual B.C. on a given day at a given distance, but what struck me as odd was that for similar bullet weights, most manufacturers's B.C. values are higher than Sierra's. But Sierra continues to have a very commanding presence, some might even say dominating, in reality. What I've noticed is that Sierra gives several values, depending on velocity ranges, as the B.C. does change as the bullet travels down range. Other companies, for example Hornady (not picking on them, I actually like their bullets) list only one value, and I'm guessing its probably the highest B.C. they could legitimately claim. Never mind that a given cartridge out of *your* gun may possibly never push it fast enough to realize that B.C., but that's what they list.

Oddly enough, in the Sierra Infinity software, even Sierra just uses the published B.C. listed by the bullet manufacturer, which often makes their products look less attractive on paper, until the actual field testing takes place [img]images/icons/wink.gif[/img]

Monte
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  #6  
Old 03-21-2003, 04:20 PM
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Join Date: May 2001
Location: Pennsylvania
Posts: 1,757
Re: Thoughts on BC

And to add to what Ric said----some bullet manufacurers are "lower" then they originally posted or advertised the BC to be.

A case in point is the 300 gr 338. It works out to .787 BC and it holds true to any ballistics program I have used. Another one is the 240 Gr 30 Cal MK which was first posted very low until many of us called them to say we are getting MUCH better results then the posted BC. It was then raised to .711 after a remeasure.

Overall I don't feel the posted BC of the majority of bullets, is BS at all. It's an indication of what the bullet is designed to do and the performance to expect. The manufacturers seem to be VERY close in their calculations of rated BC.

It may be off as compared to your ballistics program but, it may be your program or a variety of things to consider.

The higher BC bullets are usually the ones that are sleek, small Hollow point or sharp point, boat tail designs and are usually VERY accurate bullets. They will out shoot another bullet of the same weight if that bullet does not have the same type design.

Even though some bullets are the same weight does not mean they will shoot the same at extended distance. The design of the bullet means "everything."

The senario with the 400 yard shot in Florida compared to a 400 yard shot in Colorado would mean that the bullet would shoot flatter and faster thus increasing the BC of the bullet, "NOT" lowering it.
The shot would have gone high to hit the doe and the Warden if the same load was used in Colorado as it was in Florida.

BCs are based on 3 velocity ranges and the highest BC is usually the fastest velocity range.

The construction and dimensions of the bullet determines the BC it will produce. The higher the BC the bullet has, the better it will float and retain velocity and energy at extended range and that's not BS.

We have tested too many bullets in the last 30 or 40 years and have seen the differences at longrange. We know which one's die way out there and they are the ones that do not have a BC within .100 oir .200 of another one most of the time. A bullet with a .500 BC will NOT compare to one with a .600, .700 or an .800 BC---Even if the lower BC is started faster the higher BC bullet will catch it in no time and usually within 500 yards.

Give me the higher BC bullets everytime for my hunting.

Practice is the word no matter where you intend to hunt and what BC bullet you are using.

I was wondering where 6.5 shoots "all" his rifles at when the temperature is "minus 20 degrees" and every year? Alaska? Arctic?

I NEVER hunt in minus 20 degree weather here in PA or Colorado. It does feel like 20 below sometimes though. It's been MANY years since the temp was that low in this area or what I have read about here in PA.

That's the reason I don't hunt the real long ranges with the 139 to 142 gr 6.5 bullets in my 6.5/300 Weatherby.

The 30 cals with the 220 gr 240 gr and the 338 300 gr will shoot MUCH flatter and with a lot less clicks at extended range. I'm speaking out past 1000 yards.

So to sum this up, if you want a good bullet that will reach super longrange and retain the velocity and energy better, look for a heavy, highest BC bullet and you will see the difference and again, that's not BS.

Later
DC [img]images/icons/wink.gif[/img]
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  #7  
Old 03-21-2003, 05:20 PM
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Join Date: Mar 2003
Location: Elizabethville Pa
Posts: 123
Re: Thoughts on BC

Thanks for the replies all.. I just want to make one thing clear i did not write any of that article.. The only thing that was me was...

"I was doin some browseing the other day and i came across this and found it preatty intresting.. I would like to here you thoughts on this"..

This artical was on stevespages.com..

But i was thinking along the same lines as Darryl when he said the 400 yard shot in Colorado would make that shot high and not low.. By no means am i a bc expert since im just learning what bc's are all about..

6.5 Bandit
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