Reply To: Why does the POI change with a Moderator on?

Forums Field Target Field Target Talk Why does the POI change with a Moderator on? Reply To: Why does the POI change with a Moderator on?

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I_Like_Irons
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“fe7565”

“Kim”

One would make it a bit longer or shorter to dampen the harmonics so that the bullet would exit when the muzzle was either at the top or bottom of the vibration, thus increasing it’s precision.


I have heard this before.  But one would think it doesn’t matter where the muzzle is when the pellet leaves, as long as it is in the same place every shot (and ES is small).

If this is true then POI wander should be from inconsistent harmonic patterns,


I think this is very true. As long as the barrel is in the same position every time it vibrates during a shot when the pellet leaves the barrel the POI should be the same. But if you use a different pellet weight, or type, or air pressure left in the tank (unregulated) varies, the harmonics probably changes with each pellet weight. This may be purely academic, and may be the harmonics does not need to be dialed in with each different pellet weight/velocity combo because the effects are negligible. I suspect that the powder burner guys would know if they need to calibrate barrel harmonics to each and every bullet type they fire.


The reason you want the bullet or pellet to leave when it is at the end of the vibration (top or bottom) is that it is not moving at that point in time.  A little bit earlier or later, it is not moving very fast.  When the muzzle is in the middle of the “swing”  it is now moving quite rapidly.  The projectile leaving the same little bit early or late will have a much greater change in where the muzzle actually is.  Look at how a pendulum swings.  At the point where it is changing direction, the speed is quite slow.  In the bottom of the swing, it is moving quite rapidly. 

Your last sentence about needing to calibrate the barrel (or load for the barrel—generally) is spot on.  I have a particular 6.5 x 55 Swedish Mauser that has unusual vibrations.  The vertical is about 90 degrees out of phase with the horizontal.  I played with the same bullet, powder, primer, and case but only changed the charge weight in order to map what the barrel was doing.  I could get it to group with a vertical spread of six to seven MOA, with a horizontal of less than one MOA, or the reverse where the vertical was about one MOA but the horizontal was now about six.  In the middle, the group is round and about 2.5 to three MOA.  (Note:  The vertical and horizontal are actually about 5 degrees clockwise from true vertical and horizontal.  That barrel is interesting.)