Originally Posted By: BubbaChickenCat, not to be a stickler, but what about this article is bad science? I have discussed this and read about it for a while, and there seems to be some common agreement out there. Not to say that common agreement is all powerful (there are nuts that tell people that humans somehow are ending the ice age of the Pleiocene (sp?) but I know that such global warming crap is rubbish). I am just curious to know your take on all this so that I am better informed than I am of the detractors from the theory that bullet accuracy group size are possible in theory to remain the same over an extended distance, or even similar to the point that the MOA calculation (as a percentage of spread) actually decreases. I am serious, I really want to know what you mean that it is false (implied) or based in bad science (stated).
You can be a stickler, I don't mind - but it is such a long "type" - I need a secretary!
"I finally had to re-read the section in McCoy's book "Modern External Ballistics"[1] to get to confirm this assertion that it is possible for a group size in terms of MOA to get smaller at longer ranges for some projectiles."
No one has ever documented that a rifle can shoot smaller groups at a longer range. Such a documentation would require a benchrest grade rifle to be mounted in a machine-rest, on a concrete base, so NO movement could occur, not eve 1/10000".
Several bullet companies use such rigs to test the production bullets for group before releasing the lot number - none of these companies have ever seen a bullet that makes groups smaller at long range (in group size or MOA). If you ask them about it, they just giggle.
"This is unquestionably true. I have observed this myself at the range, and Chapter 11, Section 4 discusses this clearly. They even do an analysis for a 168 grain SMK, 0.308 bullet. Here is a quick summary:
If the bullet yaws coming out of the muzzle, you can observe three distinct errors or deviations from the "perfect" trajectory. The first is "Aerodynamic Jump", which is a complicated way of saying that the bullet will go in the direction of the tip as it yaws at the exact moment of exit. This causes the majority of the deviation from the "perfect" path that we see. Just what causes the bullet to yaw at that exact moment is what I am most interested in studying, and is the essence of the research into the Acoustic Shock Wave theories."
"Aerodynamic Jump", which is a complicated way of saying that the bullet will go in the direction of the tip as it yaws at the exact moment of exit.
This is pure BS - the bullet comes out of the muzzle in a straight line - it is impossible for it to exit at an angle, because that would mean that (for a 30 cal) the .308" diameter body would have to be canted in a .308" bore (a physical impossibility).
If the bullet "yaws" after the exit, then it is traveling in a new (non-coaxial) direction, and the center of mass determines the direction - if the center of mass if going in a new direction, then the bullet is now going in a new direction, and there is no force to bring it back, because the bullet doesn't know where "back" is.
There have been millions of high speed photos of bullets leaving the muzzle, and none of them show a bullet going off in a new direction - this is pure speculation on the part of the writer.
The second error is a "Epicyclic Swerve", which is a fancy way of saying that the bullet chases its tip as it wobbles (precesses, like a top) as it flies along. This causes aerodynamic forces to make the bullet travel in a helical path around the now disturbed path (remember the Jump error!). For most small arms bullets, the damping forces on this wobble are very small, and in the case of the 168 grain SMK, are actually positive, meaning that the helix INCREASES in size as it goes down range. In other words, it doesn't go to sleep, it gets more awake! So, if the Swerve component is on the order of 0.25", it stays at this level, or gets slightly bigger as the bullet goes down range. If you keep the Jump error small then this is what you will observe, a 0.5" group at 100, and 0.5" at 200, etc. Here is where you can see that the MOA can actually decrease at longer ranges. The group size never decreases, but the dispersion in terms of MOA does. A subtle but important difference.
"Epicyclic Swerve" is pure fantasy - if the bullet is disturbed after leaving the muzzle, then there are several "givens" that apply to everything in the universe that moves.
1 - When an item moves through space, the center of mass (the center of gravity for you old guys) travels along the "line" of movement - any other projections, like points or appendages can flail around this axis, but the path is defined by the motion of the center of mass.
2 - If a mass is in motion, and the direction is disturbed, and the mass takes a new direction,, the new direction now becomes "it's direction" without any "memory" of the old direction. This means that there is no "memory" of past directions, so the mass cannot find it's way back, since it doesn't know where "back" is.
3 - If a spinning object encounters a force that causes pressure to be applied to disturb the axis of spin, the object will move 90° from the direction of the force - id est, if you push the nose of a bullet down, or lift it's tail, it will turn 90° left, NOT nose down.
This is not my opinion, this is the law of precession and yo cannot change the laws of of physics - no matter who you are!
When the force is removed, the bullet will NOT return to it's original attitude. The new (90° left direction) is now the new "true" direction.
Following is a very important video taken at a research center - the gyroscope was placed in a box at the top of an 85 foot tower - along with a video camera. When the box is dropped, the camera travels with it.
When the box is dropped, the gyroscope is in "0" gravity and you can see how it and you can see how a spinning object behaves "in air".
Note in the first video, the gyroscope is vertical and stable, and when the box is released, the gyroscope stays pointing in the same direction (the bullet is shot and flying true).
In the second part of the video, the gyroscope is off axis to the force (gravity) and is showing the effect of procession. Note that when the box is released, and the support is removed (the force causing precession) the gyroscope STOPS and stays pointing in the last direction is was, when under the force of precession.
There are two important things to note here - IF, a bullet is launched, and for whatever reason, starting to process or yaw, 1 - since the bullet keps on moving through the air, the force of the air on one side of the bullet nose will continue for the complete flight of the bullet, so it will NEVER stop processing - in fact, the processing will get worse as the bullet travels down range, and I am sure that some of you have seen bulet holes that are sideways in down range targets. And @, IF it was possible to remove the wind force on the aff axis nose of a precessing bullet, the second part of the vider shows that the bullet will stop it's yaw and then keep pointing OFF AXIS nd never return to the axis of the bore (whew, I need a sandwich
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"The third error aptly called "Drift", is a steady (and increasing with range) drift of the flight path to the left or the right as a result of gyroscopic precession from the aerodynamic force applied as the bullet drops. Even though the bullet is moving forward at over 2000 feet per second, it drops (accelerates downward) in exactly the same manner as if it were dropped off the loading bench. As the bullet's downward velocity component increases, a small aerodynamic force is applied under the tip of the bullet, which tries to push the tip up. Since the bullet is spinning like a gyroscope (right hand spin for this example), this upward twist force or torque will result in the nose of the bullet yawing to the right. This yaw is called the "yaw of repose". This yaw in turn causes the bullet to steer a bit to the right (following the nose). The longer the bullet is in the air, the more the downward velocity increases, which causes a continually increasing the yaw of repose, which makes the bullet drift to the right even faster. For our class of projectiles this drift to the right is quite small, on the order of 15 inches for a 0.308 caliber bullet at 1000 yards. It is also fairly predictable, and can be seen as a predictable bias to our long range POI.
Also, BS... "drift" is caused because bullets do not travel nose down - they fly the whole length of their travel, pointing at the same angle that they were launched - so if you are shooting at 1000 yds, and the barrel is pointing up by 5°, then down range, it may be falling at 30° down, but it will be pointing 5° up - in other words, it will be traveling sideways.
When traveling sideways, the air under it will come in contact with the "under belly" of the bullet, and the rotation will cause air to be pulled from the right side over to the left side (looking from the rear) - (it is the same as the forces that cause curve balls in baseball) - the result is that the bullet is pulled to the right - not much at first, but an increasing amount as it travels down range - some "theorists" say 20" to 40" at 1,000 yds, but these green children have never shot at 1,000 yards. The real amount is a few inches... most of the time, an error that is lost in the "noise" of shooting errors.
"So, we are left with an initial Jump that does most of the damage, and a helical Swerve component that comes along with an off-angle departure. The helix stays about the same as the bullet travels downrange, so the Jump is what we see as the major factor for group size dispersion. If we get the Jump down (excellent bullet balance, neck/case/bullet/throat alignment, excellent barrel, excellent crown), then we start to see the helical component. Some say that the "bullet goes to sleep" at longer ranges. It is actually that the Jump error accumulates as range increases, and swamps out the helical Swerve. The Swerve error is still there, you just can't see it as it is dominated by the initial Jump error.
To paraphrase, "So it is true, that this article is a P.O.S. and loaded with...
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Pardon any typos, I need a nap!
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