Ballistic Precession???

Brutus

Brutus

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I am not sure if I am spelling this right??

But if I am wrong, based on my explanation, some-one more informed than myself, please feel free to correct me.

Ballistic presession: (?) External ballistics of a selected bullet in flight, initially unstable, (yaw/pitch about it's axis) until it has stabilized at a certain velocity window (IE; Now more stable, than when it exited the rifle muzzle)

Please, someone with more ballistic brains than myself, explain if this is an accurate definition????

Cheers................:)
 
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Do you mean ballistic "Precision"? (The title of the forum can be used to help wth spelling in future) :)

I'm not sure the term you've come up with adequately describes the continuum of bullet stabilization.

Bullets do not strictly require velocity to become stable, they need RPM, which is why heavy bullets do not generally work in loose twist barrels. Bullet stabilization is a byproduct of the velocty for a given twist rate. I truly wish that Bullet companies would publish optimal RPM rather than twist rates because of the huge variation in velocities (and thus RPM) between cartidges that use the same bullets (ie: 300 Whisper vs 300 RUM). That would allow us to choose a twist based on the anticpated velocity for a given projectile.
 
Precession has to to with the stability of a gyro in space. Them bullets spin up to a right quick rate once they've been through the tube- think of this- 2500 ft/s and a 1 in 12 twist means a rotational rate of 2500 rotations a second- hence they are a gyro in space. One of the intrinsic properties of a gyro is that it is rigid in space, which is to say that it becomes its own frame of reference (frame of reference: you're either on the ground watching a train go by, or in the train watching the world go by), which means that it tends to hold an axis perfectly in spite of any forces trying to move it. What this implies is that after the bullet leaves the barrel, it will tend to point along its axis of flight for the duration of air time. The catch is, that when you shoot IIRC to the north or south, the spin of the earth interacts with the spin of the bullet, causing it to wander off a perfectly true to theory line. No worries though, because the deviation due to this wandering is not significant enough to be noticed at any reasonable rifle shooting range. It's more of an issue for naval guns and that, that is if you're into lobbing shells over to the next area code.
 
Gyroscopic precession is indeed the law from which the term "ballistic precession" is derived. The reaction of the gyro to outside force is the critical thing. The gyroscope reacts at right angles to applied force so, if the trajectory is such that drag is not precisely on the point of the bullet, the bullet will deflect and tend to yaw. This has been demonstrated. In addition, pressure form the side of the bullet (wind) will result in a vertical component of drift.
Yaw (wobble) can arise as a result of precession or it can be a function of bullet imbalance.
My understanding (feeble at best) is that a bullet spinning at the ideal speed for it's length, will fly with the axis of spin corresponding with the line of flight. One which spins faster than required will tend to fly with it's axis of spin as it was at the time it left the barrel so that it may not correspond with the line of flight as range increases. Obviously, in the second instance, the ballistic coefficient of the bullet would be adversely affected.
A bullet which is spun too slowly will be unstable at the outset and never stabilize.
Mostly, I don't worry too much about this stuff. I have trouble enough coming to grips with the facts of wind drift and mirage without trying to deal with anything else. As long as the bullet arrives point on and hits where I want it to, I don't much care what laws of physics were employed to get it there. My brain has limitations ( some would say, severe limitations) and I try to live within them! Regards, Bill
 
The explanation about stability is space is excellent. If you are familiar with a gyro, then just consider the bullet a long skinny gyro.

But there is more. The RPM required to keep a given bullet stable is also a function of the medium it is in. Air density changes from day to day with both air pressure and air temperature. A load that is just barely stable in the summer will work great, but tumble in the winter.

Also, the faster a bullet goes, the more dense the air around it. The good news about this, is that if the bullet is not tumbling at 100 yards, you don’t have to worry about it remaining stable at long range. As the bullet slows down the effective air density drops, and the bullet gains stability.

But, if you have shot at long range (over 900 yards) then you know some funny things happen. This relates to that little gyro holding a fixed orientation in space. As the bullets goes down range, the trajectory arcs over and starts to come down at a fairly steep angle. But the gyro (bullet) is still point slightly up, the same as when it left the barrel. So, relative to air flow, it is coming down more and more sideways. This was mentioned in a post above.

As the bullet path arcs over, there is pressure on the bullet to arc over, too. And it will, if it is not too stable. Stability is noted as “s” meaning the rpm needed to make a bullet barely stable as it starts down range. A real high quality bullet can be shot at a stability factor of, say, 1.2 times the minimum. An average bullet can be shot at 1.3 to 1.5. Those of you shooting light bullets at high speed can have S factors over 2, unless you use a special slow twist barrel. (For example, a 147 gr military 7.62 bullet only needs a twist of 1:16 to be stable, so you can imagine how much extra RPM it has in a 1:12 or 1:10 barrel).

At long range, the S factor matters. Over 1.3, the bullet will stay pointed at the sky as the trajectory arcs over. Phrased another way, at a RPM over 1.3S, the trajectory will arc over, but the bullet won’t. Under 1.3, the air pressures will force the bullet to stay lined up with the trajectory, and it will come down, nose first. For long range accuracy, this is better.

But, the gyro will prerecess. It will turn 90 degrees to the force applied to it. So as it is forced to tip over, it will also turn to the right. So, at long range, it still has some sideways aspect to the airflow.

Long range shooters have long noted that funny things happen between 900 and 1000 yards. Loads that work well at 900 sometimes crap out at 1000. This is often ascribed to variables shooters can relate to, such as twist or going sub sonic. It is related to twist only in that more twist makes it worse. It is related to going subsonic only in that the longer the bullet stays super-sonic, the flatter the trajectory and the less the bullet turns sideways to the airflow.

So what does all this mean? It means buy quality bullets, develop an accurate load at 500 yards and then test it to see if it works at 1000 (or more). The last step is the more difficult, but critical.

Makers of nose-fused munitions are very concerned about these issues, because they need their toys to work at long range.
 
When considering terminal ballistics, precession can influence bullet performance in two ways. First if the bullet precesses in flight and impacts the target at an attitude other than 90 degrees, the increased resistance reduces penetration. We have all seen pics of big game solids in which the tail of the recovered bullet was inexplicably flattened, or expanding bullets in which the nose of the recovered bullet is flattened to one side. In either case straight line penetration is compromised. In the case of the solid, this situation can be minimized with the choice of a bullet which is as short as possible, with parallel sides, and a hemispherical or flat nose. This is why it is sometimes observed that a bullet with lower velocity will penetrate deeper than a similar bullet fired from the same barrel at higher velocity. This is because the bullet that exits the muzzle with less precession impacts the target straighter than the bullet fired at higher velocity.

Precession also occurs when the bullet impacts a medium thicker than air, and yaw increases as the bullets attempts to overcome the increased resistance of the target material. If two identical non-expanding bullets are fired at a target material, the bullet with the faster spin will penetrate deeper, because the faster spin more successfully overcomes the precession.
 
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