POI Shift for FBHP vs BTHP

[Cowsearshooter]

I've come across a little problem and I need some insight on this.

Realizing that the barrel vibrates in a 3D sine wave when fired it seems possible that the POI shift between two distinctly different loads could be off by a lot laterally and vertically.

Is this an accurate assumption?

Here's the situation: I load both 175 SMK BTHP and 125 SMK flat base bullets in 30-06 for the mid-range/long line and short lines respectively. I don't mind a vertical shift because it is relatively easy to account for. On the other hand, I noticed a significant (though repeatable) lateral shift in my loads.

What is this situation a function of? Bullet style, weight, powder selection?

Please advise...constructively.

 

[Fuzzball]

Re: POI Shift for FBHP vs BTHP

<span style="font-style: italic">"Realizing that the barrel vibrates in a 3D sine wave when fired it seems possible that the POI shift between two distinctly different loads could be off by a lot laterally and vertically. --- Is this an accurate assumption?"</span>

Yes. It's a function of the barrel viberations any change in the load might make. And it's only as predictible as that.

 

[nso123]

Re: POI Shift for FBHP vs BTHP

I have had some that were off by a fair amount, and have has some loads that were dead on with the zero of the usual load for the rifle.

 

[MitchAlsup]

Re: POI Shift for FBHP vs BTHP

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Cowsearshooter</div><div class="ubbcode-body">Realizing that the barrel vibrates in a 3D sine wave when fired it seems possible that the POI shift between two distinctly different loads could be off by a lot laterally and vertically.

Is this an accurate assumption?</div></div>

No, not really. The barrel does vibrate in a 3D wave, but the wave is a superposition of at least a dozen sine and cosine waves (although only the first 4 sines and cosines will be important). Thie input to the wave generation is the initial rise of pressure in the chamber--a different shape in the rise curve will produce different barrel vibration patterns.

 

[George63]

Re: POI Shift for FBHP vs BTHP

might be able to cure that by reworking the seating depth on one of the loads - to get the bullet exiting the muzzle at a period of less movement

 

[Fuzzball]

Re: POI Shift for FBHP vs BTHP

"The barrel does vibrate in a 3D wave, but the wave is a superposition of at least a dozen sine and cosine waves"

Correct as stated. However, the single biggest factor in muzzle displacement when a rifle is fired is the fundamental wave whip, not the harmonics - they quickly decrease to irrelivance in magnitude.

Standing harmonic sine waves occur in a regular pattern by time (frequency) and magnitude. But ... "cosine" waves? I have no idea what a cosine wave might be. ??

 

[jephthai]

Re: POI Shift for FBHP vs BTHP

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Fuzzball</div><div class="ubbcode-body">But ... "cosine" waves? I have no idea what a cosine wave might be. ?? </div></div>

Phase-shifted sine waves, of course.

Reminds me of how my dad (a professor) would occasionally throw in the word "cosusoidal" in some of his tests just to see if his students were paying attention.