How exactly do you believe he measures a bullets BC AT Mach 1.2? How would you measure the BC of a bullet AT Mach 1.2? Shoot it at that velocity over an �instant BC generator?� Or measure the velocity drop or TOF a ways above and below so you get an average at that velocity?
Go look at Bryan's drag coefficient charts where he calculates BC. They all show
his data points. Please read the chapter where Bryan explains how to read his data.
Your scientific method is flawed. That other variables may not remain constant does not excuse the inaccuracy of another. It's not just a copout of your ballistics argument, it's not a scientifically valid one. Have you heard of a thing called a �scatter plot?� Real Engineers use them all the time. So do shooters�they just call them �groups.�
Every shot is subject to random variations in MV and barrel movement and these variations produce "groups" as you acknowledged even when shooting indoors using a machine rest. When a manufacture brags about how accurate their rifles are they say they are Sub-MOA, meaning they produce "groups" of less than 1 MOA. Check out the
Weatherby site if you don't believe me. You can argue that variations less than 0.5 moa are significant all day long, but most shooters and manufactures know better.
Which part of �a� velocity do you not understand? You are talking about a CURVE in which the X-Axis is VELOCITY. To populate that curve you either need an equation that defines drag as a function of velocity�which you don't have�or you need to measure the value (drag) at different velocities. Bryan did the later. You are doing neither. You are �assuming� data which does not exist to draw your curve. Your curve is an imaginary one.
Anyone can take a single sentence out of contest and argue against it. I did say "at a given velocity" in that sentence and then went on to expand the definition for velocity ranges in the following sentences. How did you miss them?
Again, you are describing a curve for which you have no data. You are using imaginary data to populate the curve. Try using actual measured data for the 107 SMK as suggested. The numbers are right in front of you in black and white. Actual measurements. You can only burry your head in the sand so long.
I picked the 115 Berger because Bryan has 18 data points for it as compared to only 6 data points for the 107 SMK. The more data the more accurate the form factor and variation data is going to be. Had Bryan tripled his testing of the 107 SMK the results would likely have been different. Furthermore, Burge publishes the G7 BC for their bullets and Sierra doesn't. For a valid and fair comparison I wanted to use a manufacture's own published values. Nevertheless, I'll used another bullet you picked in another test below.
First, note how widely divergent they are at the 3500 fps muzzle velocity you began the thread with (and remember what happens at the muzzle affects the entire rest of the flight).
Of course, what happens outside the convergence velocity zone affects the rest of the flight, but it can be calculated and for those of us who understand significance, useful conclusions can be made.
Secondly, you need to acknowledge that real life bullets may not follow either curve, so that chart means nothing with respect to the above. In fact, virtually all bullets will diverge from one of the curves at least a little bit. It would be a rare bullet indeed that followed one of the curves exactly . This is why actual values must be measured. Bryan isn't doing all those measurements just for his health.
Of course actual bullets seldom follow either curve, but you are missing the point. In the convergence velocity zone there is only one curve. If a bullet doesn't follow it then it's not following either the G1 or G7 drag function. The topic is about how the G1 compares to the G7, and in the convergence velocity zone, they have the same curve.
Have you ever heard of the 6.5 Grendel? 6mmAR? BR? SPC? Etc?
Yes, but none of them can use this .308 cal. bullet, and about the heaviest bullet anyone makes that could be stuffed into 6.8mm Rem SPC is the 150 grain SGK.
While not the first choice for hunting, where you actually need to kill something, a bullet doesn't need to be going fast to punch a hole in a piece of paper. Dave T was using the 240 SMK out of a .308 for a while�.
I said I don't know of any
long range shooter in their right mind who would waste their time on such a load. If Dave T was using the 240 SMK in a 30-378 Weatherby he might get it up to 2600 fps or so. Maybe you could use that for long range hunting, but that's different than launching a VLD type bullet at 2000 fps.
What rounds to you believe people are using to shoot the 115 Berger at 3500 fps? I guarantee you it's a pretty small percentage that get launched that fast.
Thing is, you could lower the MV to 3200 or even 3000 and my example still works out better for the equal TOF BC.
So you fudged the numbers to reduce the error to �insignificant� levels by your standards�. You keep claiming something is �more accurate� but then keep defending its increased error as �insignificant.� If it was more accurate you wouldn't have to keep doing that.
Using the 0.308 210 gr SMK bullet you picked I ran the numbers to see how well Bryan's G7 BC of 0.316 matches Sierra's multiple G1 BC values. I also tested the equal TOF G7 BC to see how well it matches Sierra's multiple G1 BC values as compared to Bryan's G7 BC value.
I wanted to use the JBM site for this comparison, but it doesn't support multiple BC values. They have a Drag Function Array Conversion tool, but that's not the same thing. I used a program that does support multiple BC values in the way Sierra specifies to get both the drop and velocity values. Just so there's no question about accuracy, I also ran the G7 BC values on JBM and have included those in the graphic below.
I picked 3200 fps for the MV as that's about as fast as any standard caliber, such as the 30-378 Weatherby Mag, is going to push this 210 grain bullet. I took the calculations out to 2000 yards to get numbers well into Sierra's lower velocity range as that seems to be where you think there's a problem. I outputted the numbers to Excel to make it easier to compare both drop and velocity. The equal TOF G7 BC was calculated relative to Sierra's multiple G1 BC values from a MV of 3200 fps out to 1500 yards.
The drop values are in columns A through D and the velocity values are in columns F through I. The ballistics program's values are in rows 3 through 11 and the JBM values are in rows 16 through 24. The Sierra multiple BC value columns C and H have a yellow back ground. Bryan's G7 0.316 BC values are in the column just to the left of the Sierra column and the equal TOF G7 BC values are in the column just to the right of the Sierra column. The green background shows which values are closest to the Sierra values at the same range.
![[Linked Image]](http://farm4.static.flickr.com/3325/5842572884_278c182f69_b.jpg)
You can see how close the JBM values come to the ballistics program values, which gives confidence that the values for Sierra's multiple G1 BC are also accurate.
If we assume Sierra's multiple G1 BC values best matches their own bullet's drag characteristics, then it's clear that the equal TOF G7 BC value better matches that bullet then does the Bryan's G7 BC value for both drop and velocity for most ranges.
Below the drop and velocity values you'll see the results from the JBM Drag Function Array Conversion tool. Note that the tool uses the velocity zone conversion technique and comes up with the same value Bryan did by taking the simple average of the zone values. It's the same technique Bryan uses for evaluating his data, but the down range values show that the equal TOF conversion technique produces a BC that better matches Sierra's values.
This is more evidence that measuring velocity and then TOF over long range produces more accurate values when measure BC, which was Ken's conclusion.At standard conditions, launched at 3500 fps its TOF is 2.205 s to 1600 yds (stays above Mach 1.2). This equates to an average G1 BC of .637. That is the exact measurement method you advocate in the beginning of the thread.
Of course JBM doesn't support Sierra's multiple BC values, so your TOF, and thus, your BC is off. The correct TOF is 2.186 at 1600 yards for an equal TOF G1 BC of 0.644.
At standard conditions, launched at 2500 fps its TOF is 1.550 s to 950 yds (stays above Mach 1.2). This equates to an average G1 BC of .621.
The correct TOF is 1.532 at 950 yards for a G1 BC of 0.641. You got to use the right tool, and when dealing with Sierra's multiple BC values JBM is not it.
Being this 0.308 210 grain bullet will likely be launched at no more than 3000 fps I would use that for the MV and calculate the equal TOF BC to 1350 yards where velocity is near Mach 1.2. That value is a G1 BC of 0.641. Calculating from 2500 fps out to 950 yards I still get 0.641. That's the best single G1 BC for how this bullet will be used by most shooters.
As for G7, well I've already shown above that the equal TOF G7 value is more accurate then Bryan's value. Of course, I realize the differences are insignificant, but the question is, do you?
