Ballistic Coefficient

[TJV]

At what point in the trajectory path does BC become a relavent factor ?

 

[Falfan2017]

At what point in the trajectory path does BC become a relavent factor ?

Typically only after it leaves the barrel.

 

[Falfan2017]

Sorry couldn’t help it. Smartassery aside here’s a chart showing drops of common bullets all going 2900 FPS. You can decide when it becomes significant but they really start to diverge around 250 yards. Slower cartridges like a 7.62x39 with a low bc would fall off even before that.

 

[Tommie]

Typically only after it leaves the barrel.

Best laugh of the day. Kudos!

 

[Ned Ludd]

At what point in the trajectory path does BC become a relavent factor ?

The second the bullet leaves the bore.

What happens later in the trajectory is directly impacted by what happens early, meaning as soon as the bullet leaves the barrel. For that reason, looking at a graph of bullet trajectory over distance is misleading, making it appear as though only late in its flight does a difference in BC make a sufficient difference in trajectory to create a different line on the graph. Think about the effects of near wind, or how a slightly slow barrel twist reduces apparent BC. Both are effects that begin occur as soon as the bullet leaves the bore. At this point in the bullet's flight, even seemingly small or insignificant effects on the bullet can have a huge impact on what it does far downrange.

 

[Enosiophobia]

Typically only after it leaves the barrel.

Wanted to but couldn't.

 

[eric32]

At what point in the trajectory path does BC become a relavent factor ?

Are you trying to true your BC with a kestrel or manually thru another program?

I would start at 600-800yards for BC truing. And 600 just for velocity truing.

 

[TJV]

View attachment 1089124 Sorry couldn’t help it. Smartassery aside here’s a chart showing drops of common bullets all going 2900 FPS. You can decide when it becomes significant but they really start to diverge around 250 yards. Slower cartridges like a 7.62x39 with a low bc would fall off even before that.

I know that there is a point at which the BC becomes a factor in the performance of the bullet but specifically I wonder if the manufacturers supply that info somewhere by product. They all provide G1 and G7 numbers but none tell you when the number "kicks in" and becomes something that you should consider as you shoot. Put simply I would like the manufacturer to say something like : "the 55gr match target bullet, product # 12345 traveling at 3000 fps at the muzzle will become ballistic coefficient sensitive after reaching a distance of 350 meters."
Why do I care to know this info? Because in certain hunting situations it may affect my shoot/don't shoot decision.

 

[Sniper338]

Trigger time tells you that..

 

[Falfan2017]

I know that there is a point at which the BC becomes a factor in the performance of the bullet but specifically I wonder if the manufacturers supply that info somewhere by product. They all provide G1 and G7 numbers but none tell you when the number "kicks in" and becomes something that you should consider as you shoot. Put simply I would like the manufacturer to say something like : "the 55gr match target bullet, product # 12345 traveling at 3000 fps at the muzzle will become ballistic coefficient sensitive after reaching a distance of 350 meters."
Why do I care to know this info? Because in certain hunting situations it may affect my shoot/don't shoot decision.

Just put your hunting load into a ballistics calc like jbm and it will tell you what your drop is at different ranges. Basically a rifle is not point and shoot after 150 yards and involves holdover

 

[Sniper338]

Just put your hunting load into a ballistics calc like jbm and it will tell you what your drop is at different ranges. Basically a rifle is not point and shoot after 150 yards and involves holdover

Unless you got a creedmoore...

 

[mikecr]

Put simply I would like the manufacturer to say something like : "the 55gr match target bullet, product # 12345 traveling at 3000 fps at the muzzle will become ballistic coefficient sensitive after reaching a distance of 350 meters."

What do you mean by ballistic coefficient sensitive?

 

[Sniper338]

What do you mean by ballistic coefficient sensitive?

He wants the bullet manufacturer to put the yardage at which the BC takes effect, and or starts working on the box label.

Lol

 

[dcali]

I know that there is a point at which the BC becomes a factor in the performance of the bullet but specifically I wonder if the manufacturers supply that info somewhere by product. They all provide G1 and G7 numbers but none tell you when the number "kicks in" and becomes something that you should consider as you shoot. Put simply I would like the manufacturer to say something like : "the 55gr match target bullet, product # 12345 traveling at 3000 fps at the muzzle will become ballistic coefficient sensitive after reaching a distance of 350 meters."
Why do I care to know this info? Because in certain hunting situations it may affect my shoot/don't shoot decision.

What you want is called point blank range, and it will vary with muzzle velocity and the size of the error you’re willing to tolerate. That’s why they don’t put it on the box. There are some calculators that will calculate it for you, however.

 

[Depart123]

I know that there is a point at which the BC becomes a factor in the performance of the bullet but specifically I wonder if the manufacturers supply that info somewhere by product. They all provide G1 and G7 numbers but none tell you when the number "kicks in" and becomes something that you should consider as you shoot. Put simply I would like the manufacturer to say something like : "the 55gr match target bullet, product # 12345 traveling at 3000 fps at the muzzle will become ballistic coefficient sensitive after reaching a distance of 350 meters."
Why do I care to know this info? Because in certain hunting situations it may affect my shoot/don't shoot decision.

With most bullets and cartridges that a person would shoot long range with, say...bullets with a minimum .475 G1 BC travelling 2600+ fps, velocity is the main component to trajectory out to about 500 yards.

What I mean by that, is that you can shoot bullets with fairly wide ranges in BC and not see much difference in trajectory. After that, differences in BC begins to have a fairly dramatic effect on trajectories.

 

[Ned Ludd]

Another way to think about the same question is in the context of exactly what makes up a "hummer" barrel. Hummer barrels show up in [lucky] people's hands from time to time, and seem to outperform what should otherwise be identical barrels from the same manufacturer. These barrels seem to tune very easily, with extremely good precision, and seem to give tuned loads with various bullets more wind resistance than they should theoretically have. If anyone knows with absolute certainty what constitutes a "hummer" barrel, I am not aware of the explanation. However, one theory that has been tossed around is that bullets leave the bore of a hummer barrel with slightly less yaw/pitch than they would out of an otherwise identical barrel that is not a hummer. Because of that, it has been suggested that the bullet may actually behave as though it has a higher BC than the box value would indicate.

I have no evidence whether this theory is correct, but it does seem to be in agreement with other theories regarding barrel twist rates and "effective" bullet BC. If a bullet is fired from a barrel with a sufficient twist rate to gyroscopically stabilize it just enough to prevent keyholing or other phenomena associated with gyroscopic instability, it will typically behave as though it has less BC than it actually does, partially due to more pitch/yaw straight out of the muzzle. According numbers commonly used by Bryan Litz and others, this would mean gryoscopic stability coeficients somewhere in the neighborhood of perhaps 1.2 to 1.4. Sg values of around 1.1 or less is where you start to observe oblong holes in close range targets, or even bullets going sideways through the target (keyholing). According to Litz, if you increase barrel twist rate until an Sg of 1.5 or greater is achieved, that will help dampen pitch/yaw as the bullet exits the bore and allow the bullet to fly with its full intrinsic BC. The point being, the effects start as soon as the bullet leaves the bore, or possibly even before the bullet leaves the bore. Anything that alters bullet behavior that early in its trajectory will have an even greater effect at the target than something that affects its trajectory equally, but much closer to the target (i.e. farther downrange). That is why I stated earlier that the effect of changing BC can be observed on bullet trajectory very early in flight, and is essentially taking place as soon as it leaves the bore.

It is certainly much easier to shoot smaller groups at 100 yd than at 600 yd in perfectly calm conditions, but that is largely due to a decreased effect of angular dispersion at a shorter distance. In other words, if group spread is your Y-axis value, reducing the distance shrinks effectively down the magnitude of the Y-axis values. Nonetheless, it is not difficult to observe a difference in group width (windage) in moderate to strong wind conditions, even at distances of only 50 to 100 yd, between two bullets with markedly different BCs, such as a lightweight .224" 52 gr bullet and a much heavier (higher BC) .308 200 gr bullet. The differences in their trajectories begins as soon as they leave the bore and are subject to the effect of the wind, even if there is some minimum distance, closer than which it is not easy for us to discern the difference.

 

[ballisticxlr]

There is an answer... While it's true that it matters the moment the bullet leaves the barrel, that's purely academic and not helpful for OP. A rule of thumb you can use is, after about 500yrds things get interesting regardless. You can see that on the chart above, wildly differing chamberings, wildly differing BC's, similar velocity. All are performing pretty closely to each other right up until about 500yrds, then the curves really start departing from each other. In many years of competition shooting, this is basically what I've seen. You'll notice that the one with the worst BC, the .224, peters off closer to 400 but it's still right there with the others. After 500 though they all depart vigorously from each other's trajectory and you see the best BC with the least drop.

 

[Webster]

I know that there is a point at which the BC becomes a factor in the performance of the bullet but specifically I wonder if the manufacturers supply that info somewhere by product. They all provide G1 and G7 numbers but none tell you when the number "kicks in" and becomes something that you should consider as you shoot. Put simply I would like the manufacturer to say something like : "the 55gr match target bullet, product # 12345 traveling at 3000 fps at the muzzle will become ballistic coefficient sensitive after reaching a distance of 350 meters."
Why do I care to know this info? Because in certain hunting situations it may affect my shoot/don't shoot decision.

You need to print out a drop chart using something like the free Berger ballistics program. There is no such thing as the BC kicking in. No matter what the BC the rate of drop increases with distance.

 

[Laurie]

For the long-range shooter of certain cartridges / calibres it's not the where does BC kick in? issue but perhaps related to it / being confused here with it is the issue of how far from the muzzle does the bullet become transsonic; then next step subsonic? Or turned around for most people intending to shoot a specific distance, it becomes what MV / BC combination do I need to stay out of trans speeds at this distance? (Transsonic is loosely defined as below 1.2 MACH / 1,350 fps under standard atmospheric conditions, but work done by the US Army back in 30-06 / early 7.62 days with the old 173gn FMJBT FA Match suggests 100 fps less, ie 1,250 fps retained velocity is the crunch point where decreasing stability sees wind effects and group sizes seriously increase.) F/TR shooters being restricted to 308 and 223 and their match bullet makers have put great efforts into taking this pair out of the trans zone with the better combinations - and succeeded too to a degree that would have seemed fantasy maybe 12 or 15 years.

According numbers commonly used by Bryan Litz and others, this would mean gryoscopic stability coeficients somewhere in the neighborhood of perhaps 1.2 to 1.4. Sg values of around 1.1 or less is where you start to observe oblong holes in close range targets, or even bullets going sideways through the target (keyholing). According to Litz, if you increase barrel twist rate until an Sg of 1.5 or greater is achieved, that will help dampen pitch/yaw as the bullet exits the bore and allow the bullet to fly with its full intrinsic BC. The point being, the effects start as soon as the bullet leaves the bore, or possibly even before the bullet leaves the bore. Anything that alters bullet behavior that early in its trajectory will have an even greater effect at the target than something that affects its trajectory equally, but much closer to the target (i.e. farther downrange). That is why I stated earlier that the effect of changing BC can be observed on bullet trajectory very early in flight, and is essentially taking place as soon as it leaves the bore.

In his Modern Advancements .... books, Bryan is now going much further Ned with experiments using the 0.308" Berger 185gn Juggernaut and super-fast twists. Bryan appears to find long-range stability benefits in using 1:8 pitch rifling with this bullet. Running this model through Berger's stability program, it is already at a healthy 2.07 Sg in 1:10 at 2,750 fps MV and the 8-twist increases that to 3.23. We'd have shuddered with that level of spin a few years back when GB 7.62 'Target Rifle' (sling shooting with Palma type rifles) competitors routinely used 1:14 twist barrels to cope with poor quality 7.62 issue ball ammunition, but it's not nearly as 'Far Side' as it seems. The 155.5 Berger BT that I still shoot occasionally in a 1:10 barrel at 3,050 fps produces 2.07 Sg under standard conditions, but increasing altitude to 7,500 fps ASL (ie Raton NM) and 90*F (August 2013 for the US FCNs and FCWC matches) also increases it to 3.23 Sg. Not only did my bullets stay in one piece, but the rifle and load exceeded my capabilities (Yet again! ).

 

[Ned Ludd]

The real question is whether someone is actually shooting a sufficient distance that the small effect provided by these extreme twist rates really is a benefit. If not, there are likely more downsides to using them, including reduced precision and gun handling issues due to increased torque.

I have actually been leaning the other direction recently. Litz used to recommend a gyroscopic stability coefficient (Sg) of at least 1.4. Later he increased the minimum recommended Sg to 1.5. According to these suggestions, if you aren't generating an Sg of 1.5 (or greater), the bullet is not achieving its maximum intrinsic BC. For some time, I have been using the Berger 90 VLD and and 200.20X bullets out of 7.0-twist, and 11.0-twist barrels, respectively. Both of these twist rates are insufficient to produce Sgs of 1.5 with the two respective bullets at the velocities I achieve in .223 Rem and .308 Win cases.

However, LabRadar velocity data suggests they are achieving BC values very close, if not identical, to those predicted on the box for unpointed bullets, or with a predicted 4-6% BC increase from the box value for pointed bullets. I understand that the LabRadar is not the preferred instrument for calculating BC, primarily due to its limited range for measuring velocity drops. Nonetheless, the LabRadar data suggests that these bullets are not giving up a significant amount of BC relative to the published values, even when fired with Sg values closer to 1.4, rather than 1.5. I suspect any variance or limitations of the values I have been obtaining using the LabRadar data are small enough that they're not worth talking about. I certainly can't shoot the difference. So in my hands, running bullets at twist rates predicted to provide Sgs around 1.4 appears to provide some advantages, including obtaining very close to the box value BC, along with less recoil and lower potential for jacket failure.