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This is in response to a statement in the Sierra Manual post. "Originally posted by ricciardelli: Since we don't shoot in a vacuum, the BC is essentially useless, except at the exact speed, altitude and barometric conditions specified... I don't know if the idea already exists or has been proven impossible." What about developing a "Ballistic Coefficient Index?" This way we could establish the relative BC of all bullets. Put them on a scale of 1 to 100. There would probably be no BCIs of 1, but a BCI of 100 would be the most fantastic match bullet imaginable. This is based on the premise that a bullet which has a BC 2x better than another in one condition is 2x better in all other conditions. If that doesn't hold true then my idea is worthless. If it does hold and there's no other kinks we could all say to each other "XYZ bullet has a BCI of 50" and know that in whatever conditions we're shooting XYZ is in the middle the pack in terms of BC. What does everyone else think about this? Has this already been developed, or has it been tried and failed due to inconsistencies? | ||
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a BC index is not needed at all, due to the fact that all companies should, regardless of the velocity subranges, be computed at ICAO atmospheric conditions. On the other hands, taken into account that BC varies in different conditions, and concluding that it is worthless... well what a long shot !!!! In general terms, if we compute trajectories using BC as provided by the companies, (G1 drag function) we will obtain more than accurate results for everyday use. That's is a very fact to check. | |||
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If I'm not mistaken the BC numbers we get from the bullet companies are already an index based on a theoretically perfect projectile accelerated at standard temperature and barometric pressure. Of course each company may have a different idea of what the "perfect" projectile is. | |||
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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. 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. 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. | |||
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quote:Or we could use the yards traveled when the bullet has lost 1/2 of its energy. Probably would want to normalize so that 1.000 would be 1,000 yards. Of course this would be under some normal conditions. Since most long range shooting uses bullets that travel around 3,000 ft/sec we could use that velocity as the normal. We could then call this the 'G1' function. If we did this we could just glance at the BC for a bullet and see what distance it would be a good choice for. Like a BC of .400 would be a good choice out to 400 yards. OOPS, This is what we have!!! Ignore what I said. JerryO | |||
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BC is already a indexed number. The "standard" bullet has a value of 1.00 and is 4 times more aerodynamic than a bullet with a BC of .250 and 1/2 as aerodynamic as one with a BC of 2.00. 50BMG bullets are normaly in the BC range of 1.00 and better. 20mm and bigger rounds have quite high BCs. If you were shooting in a vacuum the BC would be meaningless, a wadcutter would have the same velocity decay rate (BC) as a spitzer boattail. | |||
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And the reason they have trajectory charts for the bullets at different MV is mainly it isn't a simple number process.Example a 30 cal bullet with a BC of .610, in Hornady book,started at 2800 fps, zero 100 yds, at 300 yds will drop 12.7 inches.Another with a BC of .130 will drop 25 inches, about twice as much, even though only having 1/5 the BC.It didn't drop 5 times as much. Ed. | |||
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Tailgunner has it closest. BC is based on the drag curve of a Standard Projectile. In the case of the G1 BC, the standard projectile weighs 1lb, has a diameter of 1" and is round nosed (IIRC, 2 Caliber Ogive) and flat based. By Definition, this projectile has G1 BC of 1.00. BC for other projectiles are taken as a ratio of comparison versus this Standard Projectile. This G1 Standard Projectile is not typical of what we're shooting today and definitely not anywhere near being a "perfect projectile". The G1 Drag Curve is not "hypothetical" it was arrived at empirically by firing such a projectile. In previous manuals, Sierra listed several BC's depending on velocity. This is because G1 models the behavior of current projectiles poorly. Therefore Sierra had to break up the curve into different BC's per velocity range. The more appropriate Drag Curve for the modern bullets we currently shoot would be the G7 or G5 which are based on Standard Projectiles with long pointed ogives and boat tails. Unfortunately, G7 BC's of the bullets we're shooting are typically 1/2 the number of a G1 BC for the same projectile. That makes for poor marketing for bullet manufacturers, and we're stuck with drag curves that don't work as well as they should (particularly at the longer ranges). [ 02-15-2003, 12:12: Message edited by: Chris F ] | |||
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quote:Now would you care to comment on how far a bullet (that leaves the muzzle at 3,000 ft per sec) will travel while loseing 1/2 its energy? Perhaps with something like BC's of .250, .400, & .610. Anyone? JerryO | |||
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I can't see what Mr. Ricciardelli's problem is, since the formulas for correcting for temperature and altitude are in easily available works like Sierra #4 and Art Pejsa's "Modern Practical Ballistics". Moving on to JerryO's question, the answer is 1000 times the ballistic coefficient. This is so surprizing that I double checked with two ballistic programs. Since energy is velocity squared times bullet weight, and we can let weight = 1, and ignore it from now on, 3000^2 = 9,000,000 / 2 = 4,500,000. Take the square root and we get 2121 fps for 1/2 the energy at 3000 fps. If we use the G1 drag curve and the old Standard Metro atmosphere, by coincidence, a velocity of 2121 fps is reached at 250 yards with a .250 BC bullet, 400 yards with a .400 BC bullet, and 610 yards with a .610 BC bullet. The range will be about 2% less with the slightly denser ICAO atmospheric standard. Here's the easiest way to look at it. The ranges at which bullets of different BCs slow to the same velocity is proportional to their BCs. A pointed 180 grain .30 calibre bullet has a BC of about .4 and a round nose, same weight & calibre, has a BC of about 2/3rds .4 or .267. Result, the pointed bullet is travelling as fast at 300 yards as the round nose is at 200 yards, if the muzzle velocities are the same. Bear in mind that things get complicated in the trans sonic range of 1000-1400 fps. Bye Jack | |||
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quote:Agree 100% BTW, I don't see either the need for " BC Index " what's that ?? | |||
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I'll throw my hat in with JackM and Gustavo. | |||
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