If this question has been answered on this forum, I apologize in advance. However, I did a little searching on the forum and still don't have a definitive answer to my question, which is: Does gyroscopic stability decline at longer ranges. To be more specific, I'm considering a bullet with just adequate (let's say marginal) stability at 100 yards (say an Sg value of something like 1.2-1.3). Can I expect it to retain adequate stability at, say, 500 yards? Or can I expect it to fall into an unstable condition at that range? If I'm understanding this issue properly, the rotational spin (in RPM) will likely remain fairly close to that at the muzzle, while, of course, the velocity will drop off considerably. Will the retained rotational spin keep the bullet stable at long range?
Bullet Stability Drop-off at Long Ranges
- Thread starter South Pender
- Start date
I have experienced that very thing with my 284 Shehane shooting the 180 gr ELDM bullet at 2925 fps out of a 9 twist barrel. It shot great out to 1000-1100 yards (2.75” (3) shots @ 1000 yards in the dark with a PVS-27 Nightvision clip on) and then accuracy went all to pieces around 1200 yards and the trajectory was as if the Ballistic Coefficient fell off. I attributed that to the stability gave way finally after 1000 yards. Thinking a 8 twist would be more suited to the 180 gr ELDM. I do believe a marginal stability will hurt you at extreme long range.
Loudenboomer
Silver $$ Contributor
Pass the popcorn.
DaveTooley
Gold $$ Contributor
I've shot with an Sg of 1.1 to 1K. No problem. Bullet design comes into play. More so as you approach transonic velocities. Some bullets are just, technical term here, wonky. Doppler data to back this up.
Gyroscopic stability only increases downrange. The Miller Stability number is a ratio of the angular momentum of the bullet compared to the drag force that wants to overturn the nose of the bullet. It turns out that the angular velocity (RPMs) of the bullet decays much more slowly than the linear velocity of the bullet, so the ratio keeps getting bigger as the bullet travels downrange.
dellet
Gold $$ Contributor
Basically it’s a trick question, that requires a very simple complicated answer. Without using the big words and scientific jargon, you have to accept that there are basically two definitions or results of bullet “Stability” or not.
Maximum efficiency of flight.
Bullets sideways in the target.
There are also different ways to predict mathematically how any given bullet will perform.
Stability calculator
Stability predictor
The most common is the Stability predictor. It uses four inputs, bullet length, weight, velocity and twist. I call this a predictor because it makes a lot of assumptions. Simply put, it doesn’t factor in bullet shape, it assigns one. The formula seems to work great until you take the exact same bullet, load it backwards and forwards expecting the same results on target. This type of “Stability calculator” does not factor in actual bullet shape, and why a bullet assigned a factor of 1.2, May out perform a bullet assigned 1.4.
A Stability calculator that includes a drag function will require specific bullet measurements. With this type of calculator, the above mentioned anomaly of a bullets assigned 1.2 out performing a bullets assigned 1.4, disappears. It will generate different numbers if you reverse the inputs and use the base for the tip. It knows if your shooting the bullet backwards or forwards.
If you’re willing to accept that very basic premise, then the idea that there is no such thing as a marginally stable bullet, helps the next idea. Really more than idea, experiential results, without scientific formulas.
There’s no such thing as marginally stable, it either is, or is not.
Stability/drag is important for long range precision. It’s more about maintaining the most efficient bullet flight from muzzle to target. Since velocity drops off faster than spin, the longer it travels, the better it flies. In theory.
Stability/wobble throws a curve to the normal idea of a bullet being more stable as velocity declines, when bullet spin to velocity ratio increases.
The idea of a bullet starting out with a slight wobble and then “going to sleep” down range, starts becoming a reality you can see with sub-sonic shooting. Most people never worried about this until they bought a suppressor and blew the baffles out with a bullet that showed perfectly stabilized by a Stability predictor. When people started checking for keyholes no farther from the muzzle than where the muzzle blast would not tear the paper, interesting things started showing up.
Setting up targets every 25 yards out to 2-300 yards shooting sub-sonic allowed you to track stability. Round holes at 25, a slight smear at 75, full on key hole at 150, completely sideways at 200. The fact is that the bullet was unstable at the muzzle, it never went to sleep, the wobble just kept increasing instead of decreasing until it tumbled. Sometimes the opposite. A slight smear at 10 yards, a round hole at 100.
It’s a long way of saying, both answers, an increase or decrease in “stability” over distance can be correct. The type of formula used to predict that is very important. Actual bullet shape is a must, as is actual field testing.
There are plenty of examples of bullets that have exceptional performance at 100, 200, 600 yards that people won’t consider using at 1000. It’s not because of spin rate falling off. Something else in the bullets shape and balance points, cause a less than ideal efficiency of flight. That’s why I say a “stability predictor” with limited inputs is more a general idea. Where a “stability calculator” that includes bullet shape will make a more accurate prediction and help make the numbers make sense. Knowing the actual drag curve is helpful.
Short version is that some bullets that are labeled “marginally stable” that have problems at distance, were actually probably never truly stable to begin with. It just takes longer for the instability to show up.
Maximum efficiency of flight.
Bullets sideways in the target.
There are also different ways to predict mathematically how any given bullet will perform.
Stability calculator
Stability predictor
The most common is the Stability predictor. It uses four inputs, bullet length, weight, velocity and twist. I call this a predictor because it makes a lot of assumptions. Simply put, it doesn’t factor in bullet shape, it assigns one. The formula seems to work great until you take the exact same bullet, load it backwards and forwards expecting the same results on target. This type of “Stability calculator” does not factor in actual bullet shape, and why a bullet assigned a factor of 1.2, May out perform a bullet assigned 1.4.
A Stability calculator that includes a drag function will require specific bullet measurements. With this type of calculator, the above mentioned anomaly of a bullets assigned 1.2 out performing a bullets assigned 1.4, disappears. It will generate different numbers if you reverse the inputs and use the base for the tip. It knows if your shooting the bullet backwards or forwards.
If you’re willing to accept that very basic premise, then the idea that there is no such thing as a marginally stable bullet, helps the next idea. Really more than idea, experiential results, without scientific formulas.
There’s no such thing as marginally stable, it either is, or is not.
Stability/drag is important for long range precision. It’s more about maintaining the most efficient bullet flight from muzzle to target. Since velocity drops off faster than spin, the longer it travels, the better it flies. In theory.
Stability/wobble throws a curve to the normal idea of a bullet being more stable as velocity declines, when bullet spin to velocity ratio increases.
The idea of a bullet starting out with a slight wobble and then “going to sleep” down range, starts becoming a reality you can see with sub-sonic shooting. Most people never worried about this until they bought a suppressor and blew the baffles out with a bullet that showed perfectly stabilized by a Stability predictor. When people started checking for keyholes no farther from the muzzle than where the muzzle blast would not tear the paper, interesting things started showing up.
Setting up targets every 25 yards out to 2-300 yards shooting sub-sonic allowed you to track stability. Round holes at 25, a slight smear at 75, full on key hole at 150, completely sideways at 200. The fact is that the bullet was unstable at the muzzle, it never went to sleep, the wobble just kept increasing instead of decreasing until it tumbled. Sometimes the opposite. A slight smear at 10 yards, a round hole at 100.
It’s a long way of saying, both answers, an increase or decrease in “stability” over distance can be correct. The type of formula used to predict that is very important. Actual bullet shape is a must, as is actual field testing.
There are plenty of examples of bullets that have exceptional performance at 100, 200, 600 yards that people won’t consider using at 1000. It’s not because of spin rate falling off. Something else in the bullets shape and balance points, cause a less than ideal efficiency of flight. That’s why I say a “stability predictor” with limited inputs is more a general idea. Where a “stability calculator” that includes bullet shape will make a more accurate prediction and help make the numbers make sense. Knowing the actual drag curve is helpful.
Short version is that some bullets that are labeled “marginally stable” that have problems at distance, were actually probably never truly stable to begin with. It just takes longer for the instability to show up.
South Pender, don't look to gyroscopic stability. The shape of the bullet base is the culprit.
When the bullet speed drops below Mach 0.9, the flow of air around the bullet base changes. This causes what you describe.
It is known that sharp corners in the base area may reduce this effect (balle D), rounded corners increase it (typical 7.62 NATO). Straight bases are less affected than boattails, which of course is bad news for long range shooting.
It takes a multimillion dollar radar to follow rifle bullets beyond the point where velocity has dropped below Mach 0.9. Therefore, the only way for you is to test and compare several bullet candidates on the range.
If you look for more information: search for "Magnus moment" (do not confuse with Magnus effect).
When the bullet speed drops below Mach 0.9, the flow of air around the bullet base changes. This causes what you describe.
It is known that sharp corners in the base area may reduce this effect (balle D), rounded corners increase it (typical 7.62 NATO). Straight bases are less affected than boattails, which of course is bad news for long range shooting.
It takes a multimillion dollar radar to follow rifle bullets beyond the point where velocity has dropped below Mach 0.9. Therefore, the only way for you is to test and compare several bullet candidates on the range.
If you look for more information: search for "Magnus moment" (do not confuse with Magnus effect).
DaveTooley
Gold $$ Contributor
Here's some expensive dopplar data on a familiar 30 cal bullet.
Fast14riot
Gold $$ Contributor
You lose about 3% of BC for every 0.1 below 1.5SG, greatest effect is at trans and sub sonic.
Great responses, guys. Thanks for this information. If I'm reading this right, it looks as if the effect at long range is primarily on the BC, rather than stability.
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