Sierra bullets: BC vs velocity
- Thread starter Brians356
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Bryan Litz Ballistics
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It's all about how the drag of a particular bullet relates to the drag of the 'standard' bullet at various speeds. Most modern bullet shapes have proportionally greater drag than the G1 standard as velocity slows down. However, some bullet shapes actually have less drag than the standard at lower speeds in which case their BC would go up at lower speeds.
When looking at the G1 standard, nearly all modern bullets G1 BC's go down at lower velocities.
The G7 standard is much closer in shape to modern bullets, so any particular bullet may be + or - from there at various speeds.
-Bryan
When looking at the G1 standard, nearly all modern bullets G1 BC's go down at lower velocities.
The G7 standard is much closer in shape to modern bullets, so any particular bullet may be + or - from there at various speeds.
-Bryan
Brians356
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Bryan Litz said:
Bryan,
Ok, thanks. But look at two very similar modern bullets, the .224 69- and 77-gr BTHP MatchKings. The former has the inverted relationship, while the latter does not:
69-gr | 77-gr
> 2800 fps: .301 | > 2700 .372
< 2200 fps: .317 | < 1700 .343
Of the 10 different .224 hollow-point bullets, 2 have the inverted relationship, and 1 has the same BC for both high and low velocity ranges. The other 7 all behave as expected.
This must be peculiar to blunt rifle bullets (HP and semi-point) as none of the pointed (spitzer or tipped) .224 bullets have the inverted relationship, but all of the SMPs and some of the HPs do. (I haven't surveyed the larger diameter bullets, just the .224s for this discussion.)
Bryan Litz Ballistics
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Look at the velocity breakpoints for the low velocity band: 2200 vs. 1700 fps.
I'm not sure that explains it but it's one thing I noticed in your post.
You may be right about the BC going up at low speeds only for the blunt bullets.
If the bullet is sharper/lower drag than the G1 standard, I would expect it's drag to be proportionally greater at low speed, hence it's BC higher. If the bullet is blunter than the G1 model, the drag is higher everywhere, and not proportionally higher at slower speeds so that would explain the BC going up at lower speeds for bullets like that.
I think I have seen cases in their data where the G1's go up at lower speeds even for very pointy bullets.
Some of this may be explained physically by the bluntness issue, but some of it may be experimental error as well. Measuring accurate BC's for a variety of speed bands is easy to do, but very difficult to do accurately.
-Bryan
I'm not sure that explains it but it's one thing I noticed in your post.
You may be right about the BC going up at low speeds only for the blunt bullets.
If the bullet is sharper/lower drag than the G1 standard, I would expect it's drag to be proportionally greater at low speed, hence it's BC higher. If the bullet is blunter than the G1 model, the drag is higher everywhere, and not proportionally higher at slower speeds so that would explain the BC going up at lower speeds for bullets like that.
I think I have seen cases in their data where the G1's go up at lower speeds even for very pointy bullets.
Some of this may be explained physically by the bluntness issue, but some of it may be experimental error as well. Measuring accurate BC's for a variety of speed bands is easy to do, but very difficult to do accurately.
-Bryan
To put it another way:
You start with a G1 BC measured at a high velocity. When you have a slender bullet, it will show a G1 form factor considerably smaller than 1. This effectively moves the G1 drag curve downward, to make it fit your bullet's drag at the high speed.
But, and this is the surprising effect, the shape of a G1 drag curve (fitted at high velocity) at lower speeds then is much too optimistic compared to real bullet drag. So you have to move the BC down (increasing G1 drag) to compute realistic velocities.
If a bullet is not slender but blunt, its drag shape is more similar to the shape of G1 drag. In this case, you have less variation in BC over a larger velocity range. It may even be necessary to increase BC somewhat.
The base line is: G1 drag fitted to a slender (!) bullet (at a high velocity) will give too optimistic drag figures at lower velocities. To compensate for this, a smaller BC (simulating increased G1 drag) has to be used in the lower velocity band to get more realistic results. The thing hard to understand is that G1, although created for blunt shapes, can be too optimistic for slender shapes. But it is a fact. When using G7, you are using a drag model that is much more similar to the real drag of modern slender bullets (as Bryan Litz proved by his measurements). Usually only one BC will be needed.
Sierra uses 3 or 4 BCs. Military machine gun firing tables were computed using 20 or more different BCs. Today individual drag curves for each bullet type are used.
While I was typing my response, Bryan posted his. I fully agree with his remark that drag at low velocities is very, very difficult to measure. Even a 20 percent change in drag may result in a velocity drop smaller than the accuracy of the equipment. My personal view is that subsonic velocities (measured or computed) simply cannot be trusted.
You start with a G1 BC measured at a high velocity. When you have a slender bullet, it will show a G1 form factor considerably smaller than 1. This effectively moves the G1 drag curve downward, to make it fit your bullet's drag at the high speed.
But, and this is the surprising effect, the shape of a G1 drag curve (fitted at high velocity) at lower speeds then is much too optimistic compared to real bullet drag. So you have to move the BC down (increasing G1 drag) to compute realistic velocities.
If a bullet is not slender but blunt, its drag shape is more similar to the shape of G1 drag. In this case, you have less variation in BC over a larger velocity range. It may even be necessary to increase BC somewhat.
The base line is: G1 drag fitted to a slender (!) bullet (at a high velocity) will give too optimistic drag figures at lower velocities. To compensate for this, a smaller BC (simulating increased G1 drag) has to be used in the lower velocity band to get more realistic results. The thing hard to understand is that G1, although created for blunt shapes, can be too optimistic for slender shapes. But it is a fact. When using G7, you are using a drag model that is much more similar to the real drag of modern slender bullets (as Bryan Litz proved by his measurements). Usually only one BC will be needed.
Sierra uses 3 or 4 BCs. Military machine gun firing tables were computed using 20 or more different BCs. Today individual drag curves for each bullet type are used.
While I was typing my response, Bryan posted his. I fully agree with his remark that drag at low velocities is very, very difficult to measure. Even a 20 percent change in drag may result in a velocity drop smaller than the accuracy of the equipment. My personal view is that subsonic velocities (measured or computed) simply cannot be trusted.
Also some thing I have been wondering about for a long time. Thanks for the info gents!
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