Re: Help me understand why...
<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Jumper</div><div class="ubbcode-body">The second the hammer impacts the firing pin your rifle barrel begins to vibrate. Next the primer and the powder begin to burn and the case expands and slams into the side of your chamber. This causes more vibration. The wave's increase as they travel toward the muzzle and eventually the muzzle is oscillating in an oval pattern. The bullet will leave the muzzle at some point in its oval oscillation. YOur seeing the difference in point of impact because of the difference in velocity is getting the bullets to the muzzle at different times in the oscillation of the muzzle. Your slower bullets are leaving the muzzle at a high point in the oscillation. Your faster bullets are getting to the muzzle before the muzzle has time to rise to the same high point so its pointing at a lower point on the target.
An accuracy node is attained when the bullets velocity exits the muzzle when it is at a high point in its oscillation or, its low point. Its these two points in time where the motion of the muzzle is at its most stationary.
This is saying the same thing as Teggy when he said "harmonics." </div></div>
Vibrations true, muzzle oscillations true....but you have the physics a little confused.
Barrel haromics are the generic term for the physical actions (movement) that take place as the charge rapidly burns creating pressure, and as the bullet passes down the bore. IOW, barrel harmonics are a combination of the response of the barrel metal to the "explosion" of the charge, and also a result of the diametric expansion/contraction of the barrel metal as the bullet/pressure pass down it's length, some of which "leads" ahead of the bullet. Think of a radio antennea whipping around and a snake swallowing an egg.
The "vibrations" begin in the chamber and travel forward towards the muzzle, then return towards the chamber, then slings back towards the muzzle, repeating this cycle until they eventually dissipate to nothing...think tuning fork.
This slinging back and forth of the vibrations happens several times during the shot before the bullet reaches the muzzle. The muzzle of the rifle barrel reacts to these vibrations in an imperfect circle of movement, oval oscillation. The pattern of this oscillation at the muzzle is impossible to predict or control. The good thing though is that at certain points before the bullet reaches the muzzle, the vibration is back at the chamber end, which results in a quieter movement of oscillation at the muzzle. The muzzle goes through a series of low oscillations and high oscillations as a result of the vibrations moving back and forth along the length of the barrel. For arguement's sake let's say in layman's terms the muzzle doesn't move much when the vibrations are back at the chamber end, and it jumps around wildly when the vibrations return.
It is at the low oscillation points in the process that we want the bullet to exit the muzzle. Exiting during one of these quieter periods during the harmonics cycle is where we glean what we all call "accuracy", and generically term it as a "node". In my experience using the true principles of OCW I almost always find two of the most useful points in a load work up where the vibrations are at the chamber end, and the muzzle is in it's least amount of oval oscillation. One "node" is about two thirds the way up, and the one I'm looking for is right there almost at maximum charge weight.
It's been argued before but I'll also add that up until the point in time that the bullet exits the muzzle none of the above is a result or is related to recoil. Felt recoil takes place after the the bullet leaves the barrel. The cause and effect of the momentum of the bullet moving in the barrel compared to the combined weight of the rifle and the shooter is nearly immeasurable within the overall physics of what takes place.
There are other factors that come into play regarding the OP's question such as different velocities and different bullet weights, and to some degree a different level of recoil. If both loads are shooting good repeatable accuracy then I wouldn't answer his question with barrel harmonics alone. Bullets exiting the muzzle during a high oscillation point will only show dispersion in group size and are going to fly to any random point within the "group". It would be impossible to get all of them to fly to "good" group at a point some inches higher, or lower, than another load as a reaction to how the muzzle oscillates.
