Over-Stabilization of Bullets — Why Is Too Much Spin a Problem?
On the Applied Ballistics Facebook page, Ballistician Bryan Litz regularly offers a “Tuesday Trivia” question about ballistics. Today’s brain-teaser is a true/false question about bullet stabilization. On shooting forums you often find heated arguments about “over-stabilization”. Bryan wants readers to consider the issue of over-stabilization and answer a challenge question…
Is This Statement TRUE or FALSE?
“The problem with ‘over-stabilizing’ a bullet (by shooting it from an excessively fast twist rate) is that the bullet will fly ‘nose high’ on a long range shot. The nose-high orientation induces extra drag and reduces the effective BC of the bullet.”
True or False, and WHY?
Click the “Post Comment” link below to post your reply (and explain your reasoning).
Diagram from the University of Utah Health Sciences Library Firearm Ballistics TutorialBullet Movement in Flight — More Complicated Than You May Think
Bullets do not follow a laser beam-like, perfectly straight line to the target, nor does the nose of the bullet always point exactly at the point of aim. Multiple forces are in effect that may cause the bullet to yaw (rotate side to side around its axis), tilt nose-up (pitch), or precess (like a spinning top) in flight. These effects (in exaggerated form) are shown below:Yaw refers to movement of the nose of the bullet away from the line of flight. Precession is a change in the orientation of the rotational axis of a rotating body. It can be defined as a change in direction of the rotation axis in which the second Euler angle (nutation) is constant. In physics, there are two types of precession: torque-free and torque-induced. Nutation refers to small circular movement at the bullet tip.
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Tags: Applied Ballistics, Bryan Litz, bullet, Stabilization
I vote for True. If the bullet is rotating fast enough the wind resistance can be insufficient to force the bullets noise to follow the trajectory of the bullet’ Center of Gravity as if begins to arc downward.
At long ranges all bullets become “more stabilized” since the forward velocity is shed much more rapidly than the rotational velocity. By the time the bullet velocity has dropped to half the muzzle velocity, very little of the rotational velocity is gone. The bullet is behaving as if it had just been fired at its lower velocity from a barrel with twice the twist rate.
I’m going to say false for now, as I have not experienced it. Not a real good WHY, I guess, but I believe the primary down side to over stabilization are bullets coming apart in flight.
The statement is theoretically correct, although rarely a practical fact since few would use such an overstabilized combo at the ranges the problem would occur…
I’ve seen it myself pushing 6.5 mm 85 gr pills through a 1:7.25 barrel, where identically oriented keyholes appeared on the targets at 500 meters and on. Accuracy was very far from impressing, but I did hit the targets often enough to see the trend. I’d expect the same result shooting light/short .224 pills in a 1:7 – not that an unlikely combo with all the fast-twist AR’s out there – given, of course, that the target was big enough to actually catch a bullet or two…
The main concern isn’t the accuracy but the increased drop and wind drift resulting from the greater area (combined with a worse form factor/less arodynamic shape, thus reducing our beloved B.C.) presented to the wind and the following (more rapid) loss of speed. There are no formulae that can imitate these conditions, so IF your rifle/bullet combo suffers from overstabilization the only way to get a decent longrange drop table is from D.O.P.E. – no ballistics app in the world can predict the flight path. I don’t know exactly how much more drop I was getting, but from a 300 m zero I doubled the expected amount of clicks and bottomed out the scope just to kick up dirt short of the 800 m targets. “Kentucky windage” got me on paper there, at 900 m I could not get a single bullet onto the 1.2 x 1.6 m target (that’s 4′ x 5’4″ measured in various body parts of long since deceased kings) – with a load that shot sub MOA @ 100 m and 1-1.5 MOA @ 300 m…
I remember reading on the Lija Barrels site that bullet imbalance is a major factor. If the longitudinal center of mass does not align with the bore, the bullet will move laterally when it leaves the bore. The faster it is spinning (tighter twist), the farther the lateral jump. If the bullet is at an angle within the rifling, it will not be as accurate for one of several reasons(I believe that the technical term is “the bullet is all Katty-wumpus in the bore”). Again, the tighter the twist the greater the problem.
Dr. Mann did a lot of interesting experiments in the late 1800’s with unbalanced lead bullets and rifling twist. I am not sure how much of his data is pertinent to the excellent-quality match bullets of today.
This statement is false because faster rotation will result in less yaw, not more. Much like the wobble in a football thrown easily versus a “laserbeam” thrown with much faster rotation.
It is much like a “Spinning top” if spun fast enough it will stabilize if not it will wobble.
The statement is true, but predominantly in the context of longer range shooting (in excess of roughly 500 m + or -).
The issue is that an excessively stabilized projectile will want to maintain it’s axis in flight. As the projectile is drawn toward the earth under gravity, the projectile will continue its forward path as it comes down off the trajectory arc in an aircraft like, belly flop approach which introduces multiple torsional influences on the ogive of the bullet, and these begin to change things rapidly. A lesser (assume better balanced)twist will allow the projectile to nose over in a javelin type nose first approach for a longer period of time and distance. Nose first will always be the most ballistically efficient flight path.
At short range (less than 100 m), hunting heavy and dangerous game, we are better served with heavy, and yes, theoretically over stabilized bullets because of the terminal impact benefits. At intermediate ranges for more normal target and hunting situations, it’s not such a big issue. Its all about matching the task to the need, rather than about blanket statements.
False, but the only experience I have with this happened when I was marking target for a man shooting a 284, 140 grain projectile in a 1-7.5 twist long barrel at high speed. The bullet where not making out to the 600 yard target. 50 to 70 % of them where exploding before they where hitting the target,the other one — 3 out of 10 –where accurate shot. I was not believing the man when he told me what was happening on the radio, but I guess he was wright. So from my experience, IF it is really what was happening, I could say that NO over stabilization will not reduce BC. Or reduce accuracy. Bullet hole where clean and round,not showing sign of bullet yaw or movement in flight. Jacket rupture, bullet exploding in flight and heavy fouling are what the man got from shooting a over stab. bullet. Just my experience it might not always be true.
I vote false. The bullet leaves the barrel with a very slight positive angle-of-attack, AOA, which can be increased or decreased based on the crosswind at the muzzle and twist rate. The change in AOA due to cross wind is what results in the typical 10 o’clock (for a right-to-left wind) to 4 o’clock (left-to-right wind)vertical displacement at longer ranges. If this vertical displacement was due to increased drag caused by higher AOA the resulting effect on target would be the opposite. Maybe the best example I can think of is John Whidden winning the Palma Trials with a 10 twist Palma barrel and 155 grain bullets. It was against “conventional” wisdom, but it worked very well for John.
Yes. At longer ranges as the drop rate increases the bullet will fall in a parabolic arc. An over stabilized bullet’s nose will not follow the arc but maintain an attitude parallel to the barrel centerline. Overspinning will also exaggerate any balance imperfections the bullet has, resulting in decreased accuracy.
First, I am not a ballistician nor am I an engineer, so my reply is based on my ideas and that is it. I say it is TRUE but with a qualifier. Assuming rifle is correct, square boltface, straight barrel etc and the run-out on the loaded round is nil, then if the bullet is absolutely concentric and spins on it’s axis then it should go out to the target completely stable. Now with tremendous pressure coming down the barrel right behind the projectile, as it leaves the bore, the thrust might force the bullet into a small / short “wobble” that theoretically should almost immediately correct itself due to the centrifical force placed on the bullet by the spin imparted to it by thr rifling. After it corrects itself it will stay stable way out there. Let’s see how close I come to the real answer.. LOL!!
I would think that you could not “Over stabalize”a bullet. The main problem I see,as the post above states, if you spin a bullet to fast it will come apart in mid flight thus making your point moot.
False. Bullets do not football or javelin at all. Bullets land at the same angle they leave the bore assuming they aren’t tumbling. So if your barrel is 10 degrees relative to the ground, the bullet will be at a 10 degree angle at impact.
The answer is true.The center of gravity cannot be changed,but,excessive speed can change the center of moment and that will screw you up.
Answer is: Depends…
I have A 284 and shoot berger 180 I have a 1 in 9 and 1 in 8 Berger book reads 1in9 or faster
@Fred Garvin: you just called it false, followed by what can only be described as proving it true…
For practical purposes, the answer is FALSE
As a bullet arcs on a long range trajectory, it’s axis is torqued (by aerodynamic forces) to constantly align with the oncoming airflow. When a spinning object has its axis torqued, the object reacts by pointing its axis primarily ‘out of plane'; 90 degrees from the applied force. This results in a nose-right orientation (for right twist barrels) known as the yaw of repose. The yaw of repose steers the bullet ever so slightly to the side resulting in gyroscopic (spin) drift.
The bullet nose will point slightly above its velocity vector (pitch), but that pitch is only about 1/10 of the yaw of repose which is not enough to cause a practical vertical drift (less than 1/2″ at 1000 yards). Typical yaw of repose remains below 1/60th of one degree, while pitch is on the order of 1/600th of one degree. This small amount of pitch and yaw is not enough to induce a measurable amount of additional drag, even for highly stabilized bullets.
All of the above applies to stable projectiles in supersonic flight on ‘flat fire’ trajectories. For projectiles fired at high angles (above ~10-20 degrees above the line of sight), it is possible for the bullet to not track, or trace with the trajectory. This is a common design challenge for artillery shells that are often fired on high angle trajectories. The axis of the spinning shell may be too rigid to bend with the exaggerated trajectory. In that case the shell can ‘belly flop’, or fall base first. However for small arms projectiles on flat-fire trajectories, this isn’t a problem.
Another consideration with spinning a bullet too fast is related to bullet failure. This discussion assumes the bullet remains structurally in tact.
Dynamic instability during transonic flight is also a different problem, not related to the above discussion.
You have me a little confused Mr Litz.
Given your contribution to, and use of the Miller formula for Sg stability calculations, where the suggestion is made that better accuracy might be achieved with an Sg below 2.0, I am struggling with the apparent dichotomy. If a bullet can’t be overstabilised, what is the basis of this recommendation?
Forensic ballistic expert Ruprecht Nennstiel tells us that if a bullet is gyroscopically stable at the muzzle, it will be gyroscopically stable for the rest of its flight.
That’s good enough for me.
Steve,
You’re right that there are potential precision advantages of keeping SG under 2.0.
The conversation above wasn’t addressing dispersion, only the flight mechanics of overly stable bullets.
In other words, additional drag is not a concern for SG over 2.0, elevated dispersion might be. Two different things.
Take care,
-Bryan
Ok. Here is my question. I No it’s on a 222 Remington guys. CZ American in 222 Remington very nicely weighted gun for my crew. Please help answer if possible. I am going to start my 22 yr old girls & teenage boys together. Story goes 222 has a ballistics issue already. Let me guess projectile prob & a good fast burning powder & we are looking good. (Start closer. But thinking we can get 250-300 yards) am. On the right plan. Get them comfy & eyes in on targets. Then progress.
All bout 50-60 kgs.(slightly underpowered. But I will teach them to enter in up under lung & neck.
. Question- I think 1 easy jump to an AR sound feasible to everyone here ? Any info may help. Daryl. Thank you
I vote that we don’t get to vote on whether facts are facts. It is either backed up by scientific evidence or it isn’t. The problem with this issue is that there are two many variables for simple opinion to matter especially when we are talking about super sonic flight. I am not come down on either side, but I am guessing the topic is very likely not on that lay people’s opinion are going to matter much on. The one thing I do know myself is that some bullets if spun too fast literally come apart in the air. This may not be strictly speaking on topic, but it is a much more likely to be an issue that can be tested by most shooters assuming you have one the barrels with the higher twist rates.
All I can say is a 7 twist .223 barrel will shoot 52 grain bullets as well as 80`s. So id say false.
True if laws of physics apply. Either otherwise.
Steve Hurt and Brian Litz answers make most sense to me.
Actually, Bryan’s picture illustrating coning motion is all wrong. The spin-stabilized rifle bullet cones around the mean trajectory with its nose angled INWARD toward that trajectory. I am a physicist. My Coning Theory of Bullet Motions is fully supported by physics. The coning motion of the bullet’s CG about the mean trajectory is driven by the aerodynamic forces of lift and drag.
I have also recently received a U.S. patent for a new monolithic copper Ultra-Low-Drag rifle bullet design. Dan Warner made a first production run of these bullets just before Christmas. David Tubb just test-fired them for BC measurement over his 995.7-yard range instrumented with an Oehler System 88 Chronograph/Acoustic Sensor set-up. These 225-grain 338-caliber bullets measured 0.794 for BC(G1) at 65 degrees F. Average MV was 3378 fps and time-of-flight was 1.068 msec. The average airspeed over the 995.7-yards was 2766 fps (Mach 2.463), allowing for the 30.8 fps tailwind. He used a 35-inch Schneider barrel with a 7.5-inch twist rate. McDRAG estimated a BC(G1) of just 0.703 for this Mach 2.5 airspeed. The extra 12.9 percent drag reduction is attributable to “over-stabilizing” these bullets with an initial Sg of 2.75 (McGYRO and Miller). The coning angle was absolute minimum, and the bullets were flying essentially straight nose forward. There was no yaw-drag over 1,000 yards. McDRAG estimates are calculated to replicate outdoor test firing results for initial Sg=1.5 and about 2 to 5 degree average coning angles. The Oehler system calculated target impact speed at 2340 fps (Mach 2.1). David did not report any “nose-high” bullet impact prints. He did mention firing a 5-inch 5-shot group in early load development.
At 2,000 yards these bullets would be transonic and possibly be flying “nose high” at impact with a significant “Pitch-of-Repose” attitude angle.
I am going to try a 6.6-inch twist (20 calibers) 338 barrel on my 338 Lapua Magnum test rifle.
EDITOR: The illustration used in the original article is NOT from Applied Ballistics. As noted, this is a diagram from the University of Utah Health Sciences Library Firearm Ballistics Tutorial. It is deliberately very basic and is designed to illustrate in very gross terms, basic terminology such as yaw.
What is the answer Mr Litz?
FALSE. What makes you think that bullet would nosedive in a first place?! If it did that we would not even need spin-stabilization. Bullet is aerodynamically unstable, if you drop it from height it will fall tail forward. And yes, it will fly with higher angle of attack at the end of trajectory, and it is cause for yaw of repose.
I have shot 40gr VMAX .224 from a 16″ 1:7 at 3400fps and had moa results at 100y. I am now trying to load for coyote killing with 224 Valkyrie. I will again push the envelope with Sierra 55gr GameKings going 3400fps from a 20″ or 22″ Wilson Combat or Bison Armory 1:6.5 twist barrel. This should be fun. If the 55gr doesn’t work well, I’ll try 69gr SMK and then 77gr SMK.