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Old July 5, 2020, 03:03 PM   #1
Swifty Morgan
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Explain On-the-Fly Ballistic Coefficient Changes

The other day, I was shooting with a pro doing my spotting. The ballistic coefficient on my ammo box was 0.462. After a few shots, he told me to change it to 0.420 in my ballistic calculator.

How did he know to do that?
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Old July 5, 2020, 03:32 PM   #2
Bart B.
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What ammo make, cartridge and bullet was used?
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Old July 5, 2020, 04:30 PM   #3
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Federal Gold Medal Target, Sierra Matchking, 168-grain.
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Old July 5, 2020, 05:07 PM   #4
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There are 2 ways to do your truing.
Velocity, or BC.
Either can only be done while shooting longer distances.

If i'm correct in my thinking, you were hitting low before he told you to change the BC in your calculations, and redial.

Essentially telling the calculator the bullet doesn't fly as well as the advertized BC would have you think.
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Old July 5, 2020, 05:14 PM   #5
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The muzzle velocity was determined using an accurate chronograph, and the spread was very low at 8.1 fps. Something else had to be wrong, so an error in the published BC would make sense.

I don't understand how he came up with 0.420, though. Maybe he was familiar with this ammo? It's very popular.
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Old July 5, 2020, 05:39 PM   #6
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The other day, I was shooting with a pro doing my spotting.
Sounds like he truly was. Keep his number handy.
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Old July 5, 2020, 05:46 PM   #7
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Army sniper and civilian competitor. Must know a few things.
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Old July 5, 2020, 07:20 PM   #8
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Here's a hint your kindergarten teacher never told you:

*Nothing* in ballistics is linear. Air flow is both laminar and turbulent and the ratio and values change with speed, attitude and more than a dozen other variables.

In general, BC drops as speed drops.

As such, a single number for a BC is likely a 'best case': Hornady says the G1 on their 750 gr AMax is 1.05 (G1), but Brian Litz measured it at .991

In general, and that's more like 'much of the time' a BC is going to be higher at much higher speeds. Until you start chasing X-15 rocket planes, then the tertiary and quaternary effects take over and ballistics math as we commonly use it doesn't work any more.

But kudo's to your instructor, you found an excellent one.
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Old July 5, 2020, 07:36 PM   #9
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He told us a lot of stuff which goes against "expert" advice. He told us to ignore at least one thing they teach military snipers. He said the military has to get people trained fast, so corners get cut.
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Old July 5, 2020, 08:47 PM   #10
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That's a very popular bullet and my guess is he has experience with it and what he found to be more accurate.

He also could have been throwing out an educated guess to help you get your drop table trued up with actual drop. It doesn't take that much time playing with variables on a ballistics calculator to get a feel for how a given cartridge and bullet will be affected by making changes.
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Old July 6, 2020, 08:46 PM   #11
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Originally Posted by Swifty Morgan View Post
Federal Gold Medal Target, Sierra Matchking, 168-grain.
Thats about the most known box of ammo ever loaded. Thats how he knew.
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Old July 7, 2020, 05:50 AM   #12
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Interesting--did you take shots at a different distance with the new BC entered to confirm if the projected trajectory was more accurate?
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Old July 7, 2020, 06:09 AM   #13
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Hey Swifty, I see that in your location it states Florida. So I am thinking that since your so close to sea level that he was thinking that the air is denser due to higher barometric pressure and that it is why he lowered the BC on the app so that compensation added the extra clicks on the scope for elevation.
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Old July 7, 2020, 07:15 AM   #14
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Hey Swifty, I see that in your location it states Florida. So I am thinking that since your so close to sea level that he was thinking that the air is denser due to higher barometric pressure and that it is why he lowered the BC on the app so that compensation added the extra clicks on the scope for elevation.
Most good apps compensate for barometric pressure.
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Old July 7, 2020, 07:23 AM   #15
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Here is your answer straight from Sierra:
"The gun can affect the measured BC value in two important ways: spin stabilization and tipoff moments. A bullet is gyroscopically stabilized by its spin, which is imparted by the rifling in the barrel. If a bullet is perfectly stabilized by its spin, then its longitudinal axis (which is also its spin axis) is almost perfectly aligned with its velocity vector. If a bullet is not perfectly stabilized (which is usually the case), the bullet may not be tumbling, but its point undergoes a precessional rotation as it flies. In previous editions of Sierra’s Reloading Manuals we have described this precessional rotation and have called it “coning” motion to aid in mental visualization of the motion. As the bullet flies, the point rotates in a circular arc around the direction of the velocity vector. Coning motion results in increased drag on the bullet, and any firing test method then yields an effective BC value for the bullet that is lower than the theoretical value. The rifling twist rate in the gun barrel and the muzzle velocity together control the spin rate of the bullet, and therefore control its degree of stability."
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Old July 7, 2020, 07:52 AM   #16
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Most good apps compensate for barometric pressure.
I don't use apps so I would not know, in that case the logical thing would be to have Swifty ask the man how he knew that.
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Old July 7, 2020, 08:06 AM   #17
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1. Advertising.
2. The bullet is not an exact match for the form factor used to figure BC.
3. When in doubt, print the higher number, see #1.
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Old July 7, 2020, 08:51 AM   #18
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1. Advertising.
2. The bullet is not an exact match for the form factor used to figure BC.
3. When in doubt, print the higher number, see #1.
And it could be that .462 is a G1 BC. G1 is not very reliable on a boat tail.
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Old July 7, 2020, 10:09 AM   #19
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Right, see #2 above, why Sierra shows velocity brackets. But strangely, G7 is not as much of an improvement as I would have expected... and it is a lower number for the same bullet, therefore not good under #1.

It is a lot like Quickload, you have to tweak the "fixed" values to get it to "predict" what you are actually getting.
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Old July 7, 2020, 11:07 AM   #20
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Quote:
Interesting--did you take shots at a different distance with the new BC entered to confirm if the projected trajectory was more accurate?
I'm trying to remember exactly what happened. I believe we made two adjustments. Anyway, I ended up shooting accurately.
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Old July 7, 2020, 11:55 AM   #21
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I've chronographed 4 different weight Sierra 30 caliber match bullets getting 100 yard zeros recording environmental conditions. Metallic sights were used as their angular LOS adjustments are easy to calculate exact windage and elevation values per click.

Zeros at 300, 600, 800 and 1000 yards on another range in different conditions calculated with Sierra Infinity software were within a half MOA of what was actually needed.

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Old July 7, 2020, 12:10 PM   #22
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Quote:
Originally Posted by Swifty Morgan View Post
The other day, I was shooting with a pro doing my spotting. The ballistic coefficient on my ammo box was 0.462. After a few shots, he told me to change it to 0.420 in my ballistic calculator.

How did he know to do that?
Here's what Sierra says for their 30 caliber 168 grain HPMK G1 BC:

.462 @ 2600 fps and above
.447 between 2600 and 2100 fps
.424 between 2100 and 1600 fps
.405 @ 1600 fps and below

https://www.sierrabullets.com/produc...gr-hpbt-match/

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Old July 7, 2020, 02:50 PM   #23
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Box BC's are the G1 BC at a typical muzzle velocity for the bullet. G1 BC's usually decline with velocity, though you see a few bullet shapes where it can go up. and the 168 has some of that behavior.

I put an Excel plot together to show the drag coefficients for the G1, G7, and 168-grain SMK. You can see the shape of the G7 is much closer to that of the 168-grain SMK than the G1 is.



A ballistic coefficient is chosen to scale a bullet's drag to match that of a reference projectile as closely as it can. Here, your eye quickly tells you that if you apply the right scale factor to the 168 SMK drag curve to cause it to match either of the other curves at Mach 4, it's not going to match as well at Mach 1. Less obvious, without a magnifying glass is that matching them at Mach 2 won't have them matching at Mach 1.5, either. This is what multiple velocity range BC's are intended to improve upon (actually, you want multiple Mach number ranges, but there is an assumption you are shooting in a standard atmosphere). You can also find the best average BC for any particular range, which is what the coach was doing here.

Here, in a standard atmosphere, for a 168-grain SMK fired at 2605 fps (2550 fps at 78 feet from the muzzle, the military test barrel spec for M852) with its velocities after different yardages from the firing line determined by the drag function the BRL measured for this particular bullet, are the best G1 BC's for the first 600 yard increments of range. (Note that this bullet has a dynamic instability (attempts to overcorrect) starting at about 1400 fps and below, and so it can start to tumble at about 700 yards when fired from a 10" twist).

Sierra 168-grain SMK. 2605 fps MV in ICAO standard atmosphere (Mach 1 - 1116.8 fps)

0-100 yd .42704
0-200 yd .42659
0-300 yd .42775
0-400 yd .42788
0-500 yd .42694
0-600 yd .42443

100 fps slower:
Sierra 168-grain SMK. 2505 fps MV in ICAO standard atmosphere (Mach 1 - 1116.8 fps)

0-100 yd .42582
0-200 yd .42776
0-300 yd .42808
0-400 yd .42747
0-500 yd .42609
0-600 yd .42209
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Old July 7, 2020, 04:46 PM   #24
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I was getting 2500 at the muzzle, so the numbers make sense.
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Old July 8, 2020, 07:40 AM   #25
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Quote:
Box BC's are the G1 BC at a typical muzzle velocity for the bullet. G1 BC's usually decline with velocity, though you see a few bullet shapes where it can go up. and the 168 has some of that behavior.

I put an Excel plot together to show the drag coefficients for the G1, G7, and 168-grain SMK. You can see the shape of the G7 is much closer to that of the 168-grain SMK than the G1 is.



A ballistic coefficient is chosen to scale a bullet's drag to match that of a reference projectile as closely as it can. Here, your eye quickly tells you that if you apply the right scale factor to the 168 SMK drag curve to cause it to match either of the other curves at Mach 4, it's not going to match as well at Mach 1. Less obvious, without a magnifying glass is that matching them at Mach 2 won't have them matching at Mach 1.5, either. This is what multiple velocity range BC's are intended to improve upon (actually, you want multiple Mach number ranges, but there is an assumption you are shooting in a standard atmosphere). You can also find the best average BC for any particular range, which is what the coach was doing here.

Here, in a standard atmosphere, for a 168-grain SMK fired at 2605 fps (2550 fps at 78 feet from the muzzle, the military test barrel spec for M852) with its velocities after different yardages from the firing line determined by the drag function the BRL measured for this particular bullet, are the best G1 BC's for the first 600 yard increments of range. (Note that this bullet has a dynamic instability (attempts to overcorrect) starting at about 1400 fps and below, and so it can start to tumble at about 700 yards when fired from a 10" twist).

Sierra 168-grain SMK. 2605 fps MV in ICAO standard atmosphere (Mach 1 - 1116.8 fps)

0-100 yd .42704
0-200 yd .42659
0-300 yd .42775
0-400 yd .42788
0-500 yd .42694
0-600 yd .42443

100 fps slower:
Sierra 168-grain SMK. 2505 fps MV in ICAO standard atmosphere (Mach 1 - 1116.8 fps)

0-100 yd .42582
0-200 yd .42776
0-300 yd .42808
0-400 yd .42747
0-500 yd .42609
0-600 yd .42209
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Your numbers are fascinating to me; the first thing that jumps out to me is that the BC's is "dynamically adjusting" up and down--which to me suggests it is not a linear function of strictly of velocity/range. Are these based on actual recorded velocity/range numbers?

I have read of "optimal spin stabilization" required of bullets to reach a certain range--like a 300 win mag large bullet needing a minimum of two to three hundred yards to achieve optimal spin stability/accuracy. I have never understood how a projectile without control surfaces achieves that--I even wrote Bryan Litz once asking him about that and how exactly that happens (admittedly I'm a nobody amateur compared to the likes of him, but he does cite that in his literature) but unfortunately never heard back from him.
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