Your statement is true, but you want to limit �supersonic velocities� to mean something other than �supersonic velocities?� That would be a different statement.
Go look at Bryan's book and check his data points and you see that he only tests bullets from Mach 1.2 to about Mach 2.7. That's the same velocity range I'm using. Obviously, he understands how such bullets will be used. Are you willing to concede there's a practical velocity range or do you just what to argue for the sake of arguing?
OK, even staying well within your Mach 1.2 limit (since you have the luxury of not worrying about where it really gets hard) your own data still shows error of 5.1� with one method and zero with the other at only 1300 yds. How exactly does five inches of error become �just as good as� zero inches of error? What does �just as good� mean to you, exactly?
The part you're missing is neither one can be exact because of random variations such as MV. Most rifles a person can carry on a hunt produce groups of at least 0.5 MOA which is 6.8 inches at 1300 yards. From an engineering standpoint any signal that's below the level of noise is insignificant as it can't be reliably measured. If you want to brag about how incredibly accurate your rig is and that you can reliable shoot less than 0.5 MOA groups at 1300 yards go ahead and make that claim. Most of us know better.
Wrong. It�s a value. Not a predictor of other values. A curve is many values of one variable plotted against another, a single value is only a single point on the curve. Even an average tells you nothing beyond the average value�a vertical line and a horizontal line can have the same average value.
You need to brush up on your math. I gave you this equation yesterday. Form factor is the actual bullet's drag coefficient divided by the standard bullet's drag coefficient at a given velocity.
Now substitute the standard bullet's drag coefficient in the numerator for the actual bullet's drag coefficient. Now you have the drag of the standard bullet divided by the drag of the standard bullet and any number divided by itself is 1. Thus,
all standard bullets have a form factor of 1.0 at all velocities. An actual bullet that perfectly matches a standard bullet also has a form factor of 1.0 at all velocities. An actual bullet that has an average form factor of 0.9972 is going to be a close match to the standard bullet over the velocity range the actual bullet was tested at. This is just the cold hard facts of math.
A given velocity is only a single point on a curve. It tells you nothing about how good a match the bullet is for the rest of the curve.
In general that's true, but we're talking about two specific curves here and perhaps I know something you don't. Below is a chart showing the G1 and G7 drag profiles where G1 has a BC of 1.000 and G7 has a BC of 0.500. In the center is a velocity zone between Mach 1.7 and 2.6 (1900 to 2900 fps) where the two drag functions coincide in a nearly exact relationship of 2 to 1 and then diverge above and below that velocity range.
![[Linked Image]](http://farm4.static.flickr.com/3230/5840704020_8b6e35be14_z.jpg)
What this means is no bullet of any shape or construction can exhibit a change in G1 BC within this velocity range without also changing by the same percentage relative to the G7 BC. In this important velocity range G1 BCs are as good a predictor as G7 BCs.I don't expect you to take my word for it so you can test this for yourself on the JBM site by picking any G7 BC and doubling it to get an equivalent G1 BC for this velocity range. For example, using a G7 BC of 0.279 at 2900 fps MV the velocity at 650 yards is 1901.5 fps. For a G1 BC of 0.558 at 2900 fps MV the velocity at 650 yards is 1905.1 fps. Compare drop and you'll find that it remains within 2 inches out to 1100 yards even though the velocity has dropped to about Mach 1.2 at that range.
Looking at the chart you'll see that on the left in the low velocity zone the G1 line is below the G7 line, but on the right in the high velocity zone the G1 line is above the G7 line. This means we can extend the velocity range in which the G1 closely matches the G7. For example, on the JBM site I increased the MV to 3200 fps using the G1 and G7 BC values from before. At 800 yards the G1 BC has a velocity of 1929.6 fps and the G7 BC has a velocity of 1931.0 fps. Furthermore, drop remains within 2.2 inches out to 1400 yards.
In theory G7 BCs are a better predictor for VLD bullets in the transonic velocity zone, but that's only important for the long range shooters who believe they can maintain accuracy at such ranges. From the "Determining a Load�s Maximum Range" topic, that seems to be very few shooters. I say in theory because checking Bryan's book (1st edition) I don't see any data points below about Mach 1.2 and for most bullets, the lowest data points are around Mach 1.5, so he doesn't have any real data for VLD bullets going subsonic, at least not in the 1st edition. I don't consider that an oversight, only that being a long range shooter himself, Bryan realizes that long range shooters stay above about Mach 1.2.
It�s really very simple. While Sierra doesn�t make as many VLD-shaped bullets as Berger, one I�ve used a lot is the 30 cal 210 SMK with listed G1 BC�s of: .645 @ 1800 fps and above, .630 between 1600 and 1800 fps, .600 between 1400 and 1600 fps, .530 @ 1400 fps and below.
In my above comparison, if you shoot this bullet from a big magnum at 3500 fps, for a large portion of the flight from muzzle to subsonic the bullet will have a G1 BC of .645+ (actually higher if you look at Bryan's data). This will increase its average G1 BC from the muzzle to subsonic.
If you shoot it from a smaller round with a MV of 2000 fps, it will spend very little time at .645 (actually it'll stay lower according to Bryan's data) and thus will have a much lower average G1 BC for the entire flight from muzzle to subsonic.
Two markedly different G1 BC values. Which would you print on the box? Which would you declare as "accurate?"
If you take the G1 BC of 0.645 in the 1800 fps range and divide it by 2 for a G7 BC of 0.323 and plug them into JBM with a MV of 3500 f/s you'll find that the drop is within 2.7 inches out to 1000 yards even though the MV is outside the G1 � G7 drag convergence velocity zone. Bryan publishes a G7 BC of 0.316 for this bullet and plugging that number into JBM the drop is within 0.8 inches at 1000 yards and down to 0.3 inches at 1300 yards. If I reduce the MV to 3000 fps then the G7 BC of 0.323 is within 0.1 inches at 1000 yards and at 1.2 inches at 1300 yards.
A simple dividing of the G1 BC by 2 results in a G7 BC that gives trajectory predictions that are well within the normal group size of any hunting rifle out to 1300 yards for loads with muzzle velocities of from 3500 to 2800 fps using this bullet.
I expect Bryan's G7 number is more accurate outside the MV and range envelope I defined, but I didn't have to measure anything to get results that are practically the same for most long range shooting.
As for loading this SMK bullet to a MV of 2000 fps, its pure hypothetical. I don't know of any long range shooter in their right mind who would waste their time on such a load
The reason G1 has been so successful is not because it produces higher values, as some suggest, but because it's almost as accurate as G7 for VLD bullets in the velocity range where such bullets are used and it's more accurate than G7 for non-VLD bullets. G1 is the best all around standard and this allows shooters to compare bullet ballistics between all types of bullets from most manufactures.
The equal TOF technique I've been explaining takes advantage of G1's nearly exact 2 to 1 match with G7 over much of the usable small arms velocity range and its better match to non-VDL bullets.
Think about the chart I posted and what else it means. It has implications beyond what I've reveled so far, but at this point I don't think anyone but ballisticians care.
