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.)