Is a bullet stabilized?
- Thread starter SteveOak
- Start date
Ned Ludd
Silver $$ Contributor
You've got gyroscopic stability and dynamic stability, which are two different things. You need to specify which type of stability to get a proper answer to your question.
Gyroscopic stability (Sg) is affected by bullet length, barrel twist rate, and velocity. Once the bullet exits the bore, Sg actually increases as it travels downrange because the bullet's linear velocity decreases much more quickly than its rotational velocity. Insufficient twist rate for a bullet of given length will show up as oblong holes or keyholing at the target after very short distances (within 100 yd or less) after leaving the barrel.
Dynamic instability is a different animal and may be due to the design of the bullet itself. A prime example of this is the excessively steep boattail angle of the 168 gr SMK bullet, which can render it unstable at typical .308 Win muzzle velocities after about 700-800 yd. This effect can sometimes be overcome to a certain degree by using a faster twist rate and/or muzzle velocity, thereby increasing the distance before dynamic instability becomes a major factor.
Because dynamic instability can show up at much longer distances than that at you will typically start to see problems on the target with gyroscopic instability, the answer to your question is that it really depends on what's causing the instability as to how far downrange you might expect it to show up. For example, a 168 SMK fired at typical .308 Win velocity out of a 12-twist barrel might fly just fine out to 700 or 800 yd because the gyroscopic stability from a 12-twist barrel is more than sufficient. However, that same load/bullet may well be going sideways through the target at 900 or 1000 yd due to dynamic stability. If you fired the same load/bullet out of a 17-twist barrel, it would probably go through the target sideways at 100 yd.
If you know the velocity, bullet dimensions, and barrel twist rate, Berger's calculator will give you a pretty good idea of where you stand with respect to gyroscopic stability:
http://www.bergerbullets.com/twist-rate-calculator/
Gyroscopic stability (Sg) is affected by bullet length, barrel twist rate, and velocity. Once the bullet exits the bore, Sg actually increases as it travels downrange because the bullet's linear velocity decreases much more quickly than its rotational velocity. Insufficient twist rate for a bullet of given length will show up as oblong holes or keyholing at the target after very short distances (within 100 yd or less) after leaving the barrel.
Dynamic instability is a different animal and may be due to the design of the bullet itself. A prime example of this is the excessively steep boattail angle of the 168 gr SMK bullet, which can render it unstable at typical .308 Win muzzle velocities after about 700-800 yd. This effect can sometimes be overcome to a certain degree by using a faster twist rate and/or muzzle velocity, thereby increasing the distance before dynamic instability becomes a major factor.
Because dynamic instability can show up at much longer distances than that at you will typically start to see problems on the target with gyroscopic instability, the answer to your question is that it really depends on what's causing the instability as to how far downrange you might expect it to show up. For example, a 168 SMK fired at typical .308 Win velocity out of a 12-twist barrel might fly just fine out to 700 or 800 yd because the gyroscopic stability from a 12-twist barrel is more than sufficient. However, that same load/bullet may well be going sideways through the target at 900 or 1000 yd due to dynamic stability. If you fired the same load/bullet out of a 17-twist barrel, it would probably go through the target sideways at 100 yd.
If you know the velocity, bullet dimensions, and barrel twist rate, Berger's calculator will give you a pretty good idea of where you stand with respect to gyroscopic stability:
http://www.bergerbullets.com/twist-rate-calculator/
SteveOak
Gold $$ Contributor
Thank you for your reply Ned. Lots of good info there.
To clarify my question. I will shoot this bullet only at 500 meters so the performance of this bullet at that range, is what I am trying to determine. I am looking for the bullet to be gyroscopic and dynamically stable to that distance.
After studying your post my take away is that the round holes I am seeing at 300 and 500 meters indicate that the bullet is adequately stable gyroscopically at those distances.
To determine if the bullet is/remains dynamically stabilized I would need to shoot it at longer distances. Since this bullet seems to be adequately stable at 500 meters and I only shoot this bullet only at 500 meters the question of dynamic stability is not relevant or has been adequately answered by theround holes at 300 and 500 meters.
I would be interested in shooting this bullet at 1,000 yards to see what happens and for fun but for the purposes of my use for this bullet, it seems that I am good to go.
To clarify my question. I will shoot this bullet only at 500 meters so the performance of this bullet at that range, is what I am trying to determine. I am looking for the bullet to be gyroscopic and dynamically stable to that distance.
After studying your post my take away is that the round holes I am seeing at 300 and 500 meters indicate that the bullet is adequately stable gyroscopically at those distances.
To determine if the bullet is/remains dynamically stabilized I would need to shoot it at longer distances. Since this bullet seems to be adequately stable at 500 meters and I only shoot this bullet only at 500 meters the question of dynamic stability is not relevant or has been adequately answered by theround holes at 300 and 500 meters.
I would be interested in shooting this bullet at 1,000 yards to see what happens and for fun but for the purposes of my use for this bullet, it seems that I am good to go.
Ned Ludd
Silver $$ Contributor
Although a good indicator, the relative "roundness" of holes in the target might be subject to different interpretation by the individual viewer. Nonetheless, they are certainly a positive sign and as long as your bullet is predicted to be stable with the twist rate of your barrel, I'd say the two pieces of evidence together suggest lack of stability is not a factor.
Yes it does indicate this.
When a bullet is gyroscopically unstable, it barely makes it point first through chronograph screens(10yds).
And given dynamic stability, which is a matter of either sound or bad bullet design, gyroscopic stability goes up further & further down range.
It's a mystery to me why there would be any bullets offered that have dynamic stability issues. I don't know why they would be designed, made after identifying the issue, or bought/used by shooters at all.
SteveOak
Gold $$ Contributor
The bullet I am inquiring about is not a present offering of the bullet maker. He has been working with me to develop a bullet for a particular application. The first path we took did not offer an improvement. Increasing the weight was another path. This resulted in a bullet that is 125gr and 1.39" long.
Left to right are a Sierra MatchKing 107, a Berger 115gr Hunting VLD and the 125.
Left to right are a Sierra MatchKing 107, a Berger 115gr Hunting VLD and the 125.
Using my calibrated "visual form factor" eyeball gage that bullet is definitely dynamically stable.......unless you ran the lead clear up into the nose.
opinionby
al
LOL
What boat tail angle are you using as it looks a little steep to me?
The majority of the bullet designs we have tested have demonstrated either dynamic instability or neutral dynamic stability early or late in their flight. This has been shown by flight follower recordings and/or radar derived drag coefficients showing large changes. Many bullets are not consistant in their behaviour giving large variations from bullet to bullet. Depending on if it is nutational or precessional instability and the time at which it occurs it does not always manifest itself with dispersion problems
dcali
Bullet Maker
I think the boattail's steepness is just shadow. Look at the right side - it looks more reasonable to my eye.
In your experience, are there characteristics that promote good stability in small caliber bullets? Things like boattail length, nose length, ogive shape, etc? It seems like the math lets us down to a degree when it comes to predicting dynamic stability. There is a ton of publicly available literature on the impact of various features on drag coefficients, but I have not seen much that does the same for the coefficients that drive dynamic stability. Are we left to simply test and measure?
Dynamic can be tested at 500 if you know your velocity at 1000 and get your 500 meter velocity to match . You'll be a little off due to the bullets slightly higher spin .
I and others have shot Sierra 168's from 22 and 24 inch 1:12 twist barrels that made nice round holes in 1000 yard targets shooting near 1 MOA. Load was rebulleted 7.62 M118 match ammo or handloads. Muzzle velocity about 2600 fps. Barrel bore and groove diameters were spec or less
Same load or M852 ammo shot in oversized bore/groove barrels (a few or more tenths over spec) often went subsonic about 850 yards and changed directions shooting 3 MOA or more at 1000. No full length keyholes ever reported I know of and bullet holes still reasonably round. Of course, the M852 ammo with Sierra 168's was nothing to brag about in the first place. This is why Sierra 175's replaced the 168 with its better boattail shape.
M852 ammo lots had 168's from several lots mixed. M118LR ammo lots with 175's had one lot of bullets.
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8mm barrels, or were you shooting them in .338" barrels?
That's a ridiculous question. It gets a matching answer.
We shot them in .410 gauge barrels. Spiral grooved sabots holding the bullets. Match grade extra high base shells.
One tenth over spec puts it in .410 category, lol
Haha, I think he made a typo, tenths out spec a good one, .002" out of spec would be a shit sandwich.@Milo 2.0
Don't worry, think he knows more about Sierra's results then even Sierra does
And nothing at all based from current years. Reminisces he depicts and mediates from "back in the day".
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