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5 or 3 shot group when determining ES/SD???

 
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  #50  
Old 03-10-2012, 01:08 PM
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Re: 5 or 3 shot group when determining ES/SD???

Quote:
Originally Posted by Bart B View Post
Please, folks, get in touch with a mechanical engineer who's knowledge and skills are what you trust.


A rifle barreled action is exactly like a guitar string when plucked with a pick or finger; it vibrates at the same note (frequency) each time. Only the loudness (how much it swings back and forth making sound waves) changes directly with how hard or soft it's plucked.

All this stuff about "harmonics" is grossly misunderstood by 99.99% of all rifle shooters. Once the rifle's built, its barrel whip frequency and harmonics thereof don't change......ever!!!! Shoot a round or smack it with a ball peen hammer; it vibrates the same for each.
Fair enough. I can't say I dissagree with these statements except that I don't think I need to seek the advice of another engineer or that 99.99% of all shooters grossly misunderstand harmonics and that I believe frequency intensity can be different when different varibles are interjected. In other words, heavier or lighter bullets with a bigger or smaller internal explosion will cause more or less intensity as well as different wave node locations. The point I am trying to make is that the bullet still has to exit the barrel at a given point in the 'frequency' for best results. Also, you can't compare a guitar string to a rifle barrel. One has a fixed boundary and the other has an free boundary.

When I refer to 'harmonics' on this site, I am referring to the frequency of barrel whip you describe. There will always be a dead or neutral point(s) (AKA: 'wave nodes') on the barrel as well as a maximum amplitude. Max amplitude occurs between the wave nodes. Exit the bullet at max amplitude ('anti node') and the results suck. Exit the bullet when the barrel is in line with the 'wave nodes' and life is good.

I think it is you who said that you had experienced high hits at 1K with lower velocities and hit lower with bullets going faster. As you stated, this was due to 'barrel whip' I understand this principal. The bullet is exiting the muzzle while various points of the barrel are in the 'anti node' point in the frequency. The anti node is the point where the amplitude of the sine wave is at its maximum. That said, if you exit the bullet at the right time for that frequency the effects of high hits for lower velocities and vice versa is eliminated. The bullet needs to exit when the whole barrel is on the same sheet of music with the wave nodes for the best possible results. If you can find that sweet spot, it is typically referred to as the 'harmonic node'.

You can illustrate the point I am trying to make by holding an arrow (this works best with no insert or tip) horizontally with one hand by the nock and tapping the arrow in the middle of the shaft. You will feel the vibrations with the hand that holds the arrow. As you tap closer to the open end of the shaft (free boundary) the frequency becomes less intense. When you get to a certain spot, there is a 'dead' spot or 'node'. All you feel is the tap of your finger and NO vibrations (AKA: 'frequency'). Then, when you pass this dead spot itself, you feel the harmonics again. There is also a dead spot between the middle of the shaft and your fingers holding the arrow. When you interject an insert and tip, this dead spot or node moves with different weights on the head. The heavier the weight on the end of the arrow, the closer to the end the dead spot will be. Target archers typically want their arrow rest on or very near this dead spot (AKA: 'wave node') for the most forgiving launch. These dead spots are 'wave' nodes. There is a point during the frequency wave where the whole arrow shaft is straight and in line with these wave nodes. In other words, there is a time during the frequency when the arrow is straight. If you can visualize that, then you can see why you want a bullet to exit the muzzle when the barrel frequency is at 0 amplitude.

I am sorry if I did not use the right 'verbiage' in a previous post to lead you to think I need to consult an engineer.

Are you an engineer?

M
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Long range shooting is a process that ends with a result. Once you start to focus on the result (how bad your last shot was, how big the group is going to be, what your buck will score, what your match score is, what place you are in...) then you loose the capacity to focus on the process.

Last edited by Michael Eichele; 03-10-2012 at 02:46 PM.
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  #51  
Old 03-11-2012, 09:13 AM
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Re: 5 or 3 shot group when determining ES/SD???

Quote:
Originally Posted by Michael Eichele View Post
I think it is you who said that you had experienced high hits at 1K with lower velocities and hit lower with bullets going faster. As you stated, this was due to 'barrel whip' I understand this principal. The bullet is exiting the muzzle while various points of the barrel are in the 'anti node' point in the frequency. The anti node is the point where the amplitude of the sine wave is at its maximum. That said, if you exit the bullet at the right time for that frequency the effects of high hits for lower velocities and vice versa is eliminated. The bullet needs to exit when the whole barrel is on the same sheet of music with the wave nodes for the best possible results. If you can find that sweet spot, it is typically referred to as the 'harmonic node'.

Are you an engineer?
I also stated that some shots at higher velocity struck low; I had put each group into thirds of all shots fired.

Even if every bullet left at exactly the same speed at exactly the same point in the vertical whip of the barrel, they'll still have vertical dispersion. That's due to the bullets small spread in BC; they all ain't the same 'cause they ain't all perfectly balanced. The more unbalanced ones will have more drag and therefore strike lower. There' typically no more than 1.5% spreads in BC, but that's enough to cause 1/4 MOA vertical spread at 1000 yards.

I think you mean the barrel, when at it's node, it's straight and at the mid point in its whip cycle. I don't think anybody can guarantee that'll ever happen as the bullet exits and nobody's ever proved it as far as I know. Spark photograph tests decades ago showed an M1903 Springfield's bore axis at the muzzle was moving upward and close to the top of its whip when the bullet exited. With most rifle barrels bore axis at the muzzle whipping at their fundamental frequencies of 60 to 100 Hz, they go through one cycle in about 17 to 10 milliseconds (other harmonic frequencies are higher and have much, much smaller magnitudes). Barrel time from case mouth to out the muzzle for most rifle bullets is only about 1 to 1.5 milliseconds. And the angular speed of the muzzle axis' whip at this node or zero point is the fastest; it's the slowest at the peak and valley of the wave.

Here's a link that'll help explain how a barrel whips. Check out all the site's links to barrel movements when they're whipping about.

Barrel Harmonics Mode Shape Movies

No, I'm not an engineer any more. But that doesn't matter.
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  #52  
Old 03-11-2012, 02:23 PM
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Re: 5 or 3 shot group when determining ES/SD???

Well put Bart. That link was excellent as well. Thank you.

I think between what you have been saying and what that link illustrates is that we share the same understanding of this subject more than you might think. At least principally. We can argue about when the best time for the bullet to exit the barrel but that is really a mute point. I think we can both agree (as well as most of us here) that there are definate 'sweet' spots as to where the bullet is in the barrel (or exiting) at a given time during the harmonic cycle.

In any event, I think we share the same basic understanding and principals here.

M
__________________
__________________
Long range shooting is a process that ends with a result. Once you start to focus on the result (how bad your last shot was, how big the group is going to be, what your buck will score, what your match score is, what place you are in...) then you loose the capacity to focus on the process.
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  #53  
Old 03-11-2012, 06:28 PM
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Re: 5 or 3 shot group when determining ES/SD???

Quote:
Originally Posted by Michael Eichele View Post
Well put Bart. That link was excellent as well. Thank you.

I think between what you have been saying and what that link illustrates is that we share the same understanding of this subject more than you might think. At least principally. We can argue about when the best time for the bullet to exit the barrel but that is really a mute point. I think we can both agree (as well as most of us here) that there are definate 'sweet' spots as to where the bullet is in the barrel (or exiting) at a given time during the harmonic cycle.

In any event, I think we share the same basic understanding and principals here.

M
It's going to take me a while to digest all of this and put it into perspective.

But the good news is that we don't have to fully comprehend everything that's going on. We just need to methodically go about finding a "node," "sweet spot," or whatever you want to call it.

By the same token, you don't have to have a chrony, and it doesn't have to be precise to be useful. But, it can be a useful tool when used appropriately.

Ultimately, it's accurate and repeatable performance within the context of your shooting discipline that determines whether you have a good load.

Thanks for the info everyone!
richard
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  #54  
Old 03-11-2012, 09:12 PM
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Re: 5 or 3 shot group when determining ES/SD???

BINGO Richard!

I'm going to continue to strive to gain as much knowlege about long range hunting because it has become an obsession for me. I'm going to continue to improve my equipment to the best I can afford. With good equipment and brainstorm of knowledge I will continue to try and better my abilities as a shooter and hunter. I will never be an engineer and never understand half of what they know. If I can't control it I really don't care but that's just me. If I can improve it I'm all ears. I have nothing but respect however for those who do understand these things. It is from them that we learn the most. When it comes to knowlege these guys are pushing the limits always just like the rest of us are doing with our shooting.

Call it node, sweet spot, or whatever I'm going to try and find it and then load the ammo as consistently as possible and hope for the best. Without all you guys I wouldn't even understand that much so thankyou! I will use a chrono as another tool to help track consistency knowing it by itself is not perfect. As long as you all will let me I'll keep asking questions. Who knows maybe someday I'll be making hits on real long range targets and not just steel or paper.

Ok I'm done.
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  #55  
Old 03-11-2012, 10:51 PM
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Re: 5 or 3 shot group when determining ES/SD???

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Originally Posted by shortpants View Post
Call it node, sweet spot, or whatever I'm going to try and find it and then load the ammo as consistently as possible and hope for the best.
Good idea. Just don't spend too much time getting cases and bullets "perfect" to the nth degree. A farily decent rifle will shoot sub 3/4 MOA all day long at 1000 with brand spankin' new cases and good bullets right out of the box. Use medium speed powders and stay away from magnum primers. Use Redding or RCBS full bushing dies in a decent press and you'll do just fine.
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  #56  
Old 03-11-2012, 10:57 PM
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Re: 5 or 3 shot group when determining ES/SD???

Thanks for the advice Bart.
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