Barrel Behaviour Theory

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!Peter!
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Re: Barrel Behaviour Theory

#61 Postby !Peter! » Fri Jun 08, 2018 1:20 pm

williada,
In your posts you've often referred to the "third point of inertia". I'm not familiar with it neither have I been able to find a reference for it. Would you be able to direct me to a reference for it or explain it?

Also I'd be interested in knowing about the work Rinker has done.

Thanks!
!Peter!

williada
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Re: Barrel Behaviour Theory

#62 Postby williada » Sat Jun 09, 2018 1:28 am

Peter Wikipedia describes inertia as:
“Inertia is the resistance of any physical object to any change in its position and state of motion. This includes changes to the object's speed, direction, or state of rest.

Inertia is also defined as the tendency of objects to keep moving in a straight line at a constant velocity. The principle of inertia is one of the fundamental principles in classical physics that are still used to describe the motion of objects and how they are affected by the applied forces on them.”

At the muzzle there are lots of things happening. The bullets speed is changing, so is its direction and it is a critical boundary.

Firstly, the moving bullet is increasing the frequency of vibration and distorting others.

Secondly, as the bullet nose emerges there is escaping gas. This gas is travelling at approximately 4500 fps and its temperature is about 6000 degrees F. The shock wave at this point is conical because of the bullet ogive and there is a tendency to open the bore before the body emerges. On a shadow graph you can see layers of vibration chewing at the edges.

If the bullet is not centre with the bore axis due to stress waves acting on it just before it emerges, aside from regular in bore yaw, then you can add the chatter stress waves of the conical emergence to the dispersion.

As the bullet emerges from the precursor waves and the resultant muzzle blast, there is about 30 fps increase in bullet velocity - this maybe a small push on the muzzle. What is more important to me is the environment that may force a projectile off centre. The problem would be seen as added dispersion from .1 to .5 minutes of angle or more. Vaughn says .2 degree canted bullet angle resulted in a radius of dispersion 0.2 inches. Whatever causes a cant is a cant but it is critical at the muzzle if stress waves cause more. This is why I feel custom made tuners are the go and off the shelf will not necessarily do the trick and may exacerbate problems if not optimised to what is happening.

There is also a net effect of muzzle blast adds about 30 fps to velocity which I suppose is a backward push on the recoil given the expanding gas in all directions once the bullet passes through the muzzle blast shockwaves.

Rinker wrote the book Understanding Firearm Ballistics but my head is full of trivia with this stuff, apologies.
Vaughn wrote Rifle Accuracy Facts.
Anyway, enough from me. David.

!Peter!
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Re: Barrel Behaviour Theory

#63 Postby !Peter! » Sat Jun 09, 2018 2:17 am

Thanks David,
I'm familiar with inertia and moments of inertia but exactly what you were referring to as the "third point of inertia" had me stumped. If I understand correctly from your description you are referring to the inertia imparted on the bullet as it leaves the influence of the barrel.

Also thanks for the reference information for Rinker, I was unaware of it but will read it soon.

I've got a copy of Vaughn, his experiments on barrel vibration are very enlightening.

Thanks again!
!Peter!

!Peter!
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Joined: Sun Oct 30, 2016 6:35 am

Re: Barrel Behaviour Theory

#64 Postby !Peter! » Sat Jun 09, 2018 2:46 am

This will be my last post pushing out information. I’m still eager to engage in discussion but I won’t be introducing new theory on the behaviour of a barrel, in all honesty, because there isn’t really any more that I know of.

This subject matter has been researched in academia so there are academic papers out there if you do a literature search.

My final advice/ thoughts before I get off my soap box and wait for the discussion is:
    - To understand and apply this you need to segregate and compartmentalise the different things that are going. If you can do this then while it is still complex, it is not complicated.
    - A plastic hose is a better analogy than a guitar string.
    - Start from the source of the wave and critically think about what happens:
      - Is a longitudinal or transverse wave formed, or both?
      - What is the speed and shape of the travelling wave?
      - What will it do when it encounters a boundary?
      - Is the amplitude big enough to cause a material effect on the bullet?
      - If the amplitude is big enough what effect will it have on the bullet?
      - Is a standing wave formed?
      - Remember, just because a standing wave can form doesn’t mean it will.
      - If a standing wave is formed what is its frequency and where are the nodes located?
      - Will the standing wave be formed before the bullet exits?
    - Take your time in digesting this and don’t expect to immediately piece it all together.
    - Try thinking about the work that's out there in terms of the physics outlined here. E.g. OBT/OCW, VarmintAl, Positive Compensation, Purdy's PRX, Geoffrey Kolbe, and of course this site
I’m still deepening my understanding but I hope I have helped the forum’s understanding of the physics behind barrel behaviour and that this leads to more and clearer discussion of tuners and rifle design on this great forum.

Now I can catch up on the "More on Barrel Tuners" thread!!

sungazer
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Re: Barrel Behaviour Theory

#65 Postby sungazer » Sat Jun 09, 2018 9:09 am

Peter I dont understand why you are so fixated with the standing wave. As you have shown any traveling wave will cause a deflection of the barrel. That traveling wave will reflect back and forward until its amplitude has dampened down due to the energy being converted to other forms of energy. Those resulting reflected waves will add or subtract to any other waves in the barrel.
I agree with the process of compartmentalizing each source of wave and looking at its amplitude and frequency also determining how many reflections will occur if any before the bullet leaves. I think the timing should not be overlooked as it can simplify how many reflections need to be considered in forming the total resultant wave at the time of departure also the amplitude at that time. This has to be a very important consideration.
The barrel will continue to oscillate well after the bullet leaves in ever decreasing amplitude. perhaps the time it takes to completely stop is important to the machineguners :D

wsftr
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Re: Barrel Behaviour Theory

#66 Postby wsftr » Sat Jun 09, 2018 11:14 am

!Peter! wrote:All,

I don't have the experience of others on this site in tuning a rifle but I have spent a bit of time digging into the physics to understand what is going on so that I can learn to tune a rifle faster.

I welcome all contributions and discussions.


Hi Peter - thank you for your work. I wonder - do you think you have achieved this? I was hoping to take the theory and have targets posted online to see how any of this influenced decision making in the world of holes in paper.

sungazer
Posts: 113
Joined: Fri Mar 17, 2017 7:58 pm

Re: Barrel Behaviour Theory

#67 Postby sungazer » Sat Jun 09, 2018 12:06 pm

williada wrote:Peter Wikipedia describes inertia as:
“Inertia is the resistance of any physical object to any change in its position and state of motion. This includes changes to the object's speed, direction, or state of rest.

Inertia is also defined as the tendency of objects to keep moving in a straight line at a constant velocity. The principle of inertia is one of the fundamental principles in classical physics that are still used to describe the motion of objects and how they are affected by the applied forces on them.”

At the muzzle there are lots of things happening. The bullets speed is changing, so is its direction and it is a critical boundary.

Firstly, the moving bullet is increasing the frequency of vibration and distorting others.

Secondly, as the bullet nose emerges there is escaping gas. This gas is travelling at approximately 4500 fps and its temperature is about 6000 degrees F. The shock wave at this point is conical because of the bullet ogive and there is a tendency to open the bore before the body emerges. On a shadow graph you can see layers of vibration chewing at the edges.

If the bullet is not centre with the bore axis due to stress waves acting on it just before it emerges, aside from regular in bore yaw, then you can add the chatter stress waves of the conical emergence to the dispersion.

As the bullet emerges from the precursor waves and the resultant muzzle blast, there is about 30 fps increase in bullet velocity - this maybe a small push on the muzzle. What is more important to me is the environment that may force a projectile off centre. The problem would be seen as added dispersion from .1 to .5 minutes of angle or more. Vaughn says .2 degree canted bullet angle resulted in a radius of dispersion 0.2 inches. Whatever causes a cant is a cant but it is critical at the muzzle if stress waves cause more. This is why I feel custom made tuners are the go and off the shelf will not necessarily do the trick and may exacerbate problems if not optimised to what is happening.

There is also a net effect of muzzle blast adds about 30 fps to velocity which I suppose is a backward push on the recoil given the expanding gas in all directions once the bullet passes through the muzzle blast shockwaves.

Rinker wrote the book Understanding Firearm Ballistics but my head is full of trivia with this stuff, apologies.
Vaughn wrote Rifle Accuracy Facts.
Anyway, enough from me. David.


The effect due to inertia is going to be a very hard one to quantify. The effect will depend greatly on the stock design and the fulcrum point. It is why everybody is trying to design stocks that recoil directly back parallel to the aimed point. Its why benchrest shooters spend so much time prior to taking a shot making sure when the gun recoils the aiming point does not move. what is the timing of this effect? The barrel profile would be a consideration and why another reason why thick barrel profiles are preferred due to their resistance to bending forces.

Would the increase in MV directly after the projectile leaves the muzzle have something to do with less friction forces on the projectile?

williada
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Re: Barrel Behaviour Theory

#68 Postby williada » Sat Jun 09, 2018 9:34 pm

Good questions Sungazer, but the responses are becoming wordy. Practical demonstration is really the way to go. One of the essential elements of accurizing a firearm is to make sure that over pressure waves at bullet emergence and secondary flashes do not disrupt the bullet from its path. Traditionally, the worst case is a muzzle crown or muzzle device that is cut at a non-square angle to the bore, so that one side of the bullet leaves the barrel early. This will cause the gas to escape in an asymmetric pattern, and will push the bullet away from that side, causing shots to form a "string", where the shots cluster along a line rather than forming a normal distribution (Gaussian) pattern. So in the case of a tuner, it may be acting as a counter balance to centralize the bullet exit by controlling reflections and muzzle blast if it comprises a bloop tube component and a thimble.

Yes, we do get a reduction in bore friction on emergence but there is a retardant effect with the outside air mass and declining gas pressure with a re-ignition of escaping gases at 4500 fps often seen as muzzle flash at exceedingly high temperature 6000 F degrees. When the bullet penetrates the muzzle blast shock wave, according to Rinker it increases velocity by 1%.

The area around the muzzle inertia point and transitional ballistic zone really matters and there a few with greater knowledge and experience than I who have quantified this stuff, it’s no secret. For example:

• “Pressure Measurements In The Transitional Ballistics Region of A M-16 Rifle, 1975 by Gion, BRL Report No 1765, USA Ballistic Research Laboratories, Aberdeen Proving Ground, Maryland, USA
• “The Effect of Jet Asymetry On Projectile Motion”, 1975 by Schmidt, BRL Report 1756, USA Ballisitic Reseach Laboratories, Aberdeen Proving Ground, Maryland, USA

It was Vaughn who did further experiments to prove if the bullet was canted or left the muzzle off bore centreline, then dispersion took place. Bench rest shooters would hate to let out a group by .2 minute of .5 of a minute because the last bit of the muzzle was distorted as the bullet left the barrel. It is an area of micro tune at the business end dominated by radial stress, shock waves that go forward, then back then forward with muzzle blast. Check Vaughn’s book he has a whole chapter on it with shadowgraphs demonstrating the order of effects.

The inertia point at the muzzle is likely to be more distorted by higher frequency radial stress waves or chatter from bullet emergence to muzzle blast than longitudinal stress waves induced from the stock design and the fulcrum point which is sourced from higher order harmonics which are largely absent or incomplete such as the first harmonic etc as detected in a barrel.

However, these maybe seen as bending forces largely in the vertical longitudinal aspect in the whole rifle bag setup and the amplitude is dependent on the distance the rifle centre of gravity is away from the bore line as part of a macro profile in the degree of barrel lift which I have in the past described as positive, neutral and negative which have different grouping characteristics at different points in the trajectory.

Yes, thicker barrels have higher frequencies and the interaction of dominant waves of lower order (higher frequency) in the barrel cause stress waves which we still have to be mindful of in bullet timing to minimise distortions affecting the bullet path in the barrel. This is the basis of papers/books written on Optimum Charge Weight and Optimal Barrel Time written by the likes of Long, Newbury and our own Mincham. This relates to micro tune and precision.

Like short range benchrest, long range shooting is all about learning and preparation but it may take up to 10 times longer for a projectile to reach the target. We shoot on curved mounds, not from the flat plane of a bench. We need longer barrels up to 33 inches compared to approximately 21 2/3 inches to obtain sufficient velocity to reach 1000 yards. As such the handling largely due to torque and balance point of rifles is different and as such stock design for a different purpose and position.

Our barrels have to endure longer strings and we do not get the luxury of warm up shots on a sighter board before we shoot for score. The projectiles we use are boat tails which are more prone to in bore yaw than flat based projectiles dominant in short range benchrest.

Of significance is the short range benchrest do not shoot past the overturning point in the trajectory. They don’t worry so much about weighing charges but throw them because vertical is not so much an issue. They are more concerned with ease of use and tracking of the stock for faster strings of 5 shots. Torque in our game is not so much an issue due to longer barrels and where the weight is placed. In fact torque can be reduced by scope height and heel depth in relation to the bore centreline.

Yes, thicker barrels resist twisting forces but often the bullet has gone for that to be a real consideration in accuracy in our game.

It’s horses for courses.

Today, I witnessed a great TR shoot at long range with 9 centres by Mr. Brewster. His barrel was a 35 inch Light Palma i.e. long and skinny and frankly put all the F classes to shame with his group size.

At the other end of the spectrum comments were made on Benchrest Central this year about super thick rimfire barrels that don’t work because they can’t be tuned.

There are a couple of main ways to reduce group size. One is with a heavy barrel where it is so stiff the muzzle hardly moves. There other is on an harmonic tune dependant on understanding standing waves. Both are dependent on controlling that third inertia point for hummer accuracy.

A long F class barrel has to move. Jim Boatright did the stiffness calculations but still had to go to 26 inches for acceptable velocity.

Controlled experiments by me from a machine rest have demonstrated that thick barrels are suspect at long range with six o’clock shots when components are not meticulously weighed and sorted. Fouling also enters the equation with longer strings.

Whatever people choose, it always comes back to the nut behind the butt and what you feel most comfortable shooting.

!Peter!
Posts: 88
Joined: Sun Oct 30, 2016 6:35 am

Re: Barrel Behaviour Theory

#69 Postby !Peter! » Sun Jun 10, 2018 1:00 am

sungazer wrote:Peter I dont understand why you are so fixated with the standing wave.

sungazer, I think you may have misinterpreted my last post. I included some things to think about if a standing wave is thought to occur.

I completely agree with the rest of your post.

I think a major cause of confusion in understanding barrel behaviour is the common use of the term "harmonics" and the analogy of a guitar string as this leads people to think that a harmonic wave occurs.

wsftr wrote:
!Peter! wrote:All,

I don't have the experience of others on this site in tuning a rifle but I have spent a bit of time digging into the physics to understand what is going on so that I can learn to tune a rifle faster.

I welcome all contributions and discussions.


Hi Peter - thank you for your work. I wonder - do you think you have achieved this? I was hoping to take the theory and have targets posted online to see how any of this influenced decision making in the world of holes in paper.


Your welcome wsftr, and good question.

I do believe my understanding has improved my tuning. At the moment the biggest impact is my methodology. For instance, I shoot the horizontal ladder at 200yds and choose the charge weight based on the impact height change rather than looking for "nodes" (i.e. charge weights with the same impact height).

I also know that when I engage in the unnatural act of sticking a scope on my rifle :lol: (this is intended as good natured banter), that I'll see less positive compensation because the rifle's center of gravity is higher.

I'm still building on my own understanding as well. I have test program in mind but don't have time at the moment to do it.

!Peter!
Posts: 88
Joined: Sun Oct 30, 2016 6:35 am

Re: Barrel Behaviour Theory

#70 Postby !Peter! » Sun Jun 10, 2018 1:13 am

sungazer wrote:.... It is why everybody is trying to design stocks that recoil directly back parallel to the aimed point. ... The barrel profile would be a consideration and why another reason why thick barrel profiles are preferred due to their resistance to bending forces. ...


Depends what your goal and strategy is. If your trying to get positive compensation then the action needs to rotate.

Vaughn's Rifle Accuracy Facts has some good experiments where he significantly reduced the action rotation.

!Peter!
Posts: 88
Joined: Sun Oct 30, 2016 6:35 am

Re: Barrel Behaviour Theory

#71 Postby !Peter! » Sun Jun 10, 2018 4:08 am

One more thing I should add, in the physics classroom they make an over simplification when they say the energy in a wave is proportional to it's amplitude. The energy in a wave is proportional to it's amplitude and frequency. So a higher frequency wave will have more energy than a lower frequency wave with the same amplitude.

sungazer
Posts: 113
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Re: Barrel Behaviour Theory

#72 Postby sungazer » Sun Jun 10, 2018 10:03 am

Peter sorry if my posts seem argumentative they are not intended to be. My written communication leaves lots of room for improvement. I have an electrical / electronic background and have dealt a lot with harmonics on the power grid. I understand the higher frequency has more energy for a given amplitude. for others its about the area under the wave just as DC has a lot more energy as the area under the wave is greater than a sinusoidal with the same peak amplitude.
A question though is all the energy in these waves converted to mechanical energy? Are some frequencies of energy converted to different forms of energy in different ratios? Ie some may be to sound some may be to heat some will be to mechanical then even within the mechanical they may convert to different dimensions?
williada I agree its hard to convey just in text thats why I was going to bow out a few pages back. However its interesting even if we cant reach a scientific conclusion.

sungazer
Posts: 113
Joined: Fri Mar 17, 2017 7:58 pm

Re: Barrel Behaviour Theory

#73 Postby sungazer » Sun Jun 10, 2018 10:08 am

williada slightly on a tangent to this topic but related is I often wondered if a barrel tuner was in fact helping more with the job of acting like a crown than anything else. Is a larger crown face going to help set up that pressure wave behind the projectile and improve its accuracy?
I wonder when device such as the magneto speed is attached to the muzzle we end up with different poi if its just due to the weight or if it is interference with the pressure wave.

williada
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Joined: Wed Jul 22, 2009 12:37 am

Re: Barrel Behaviour Theory

#74 Postby williada » Sun Jun 10, 2018 10:43 am

Hey Alan F, when do I get a pay rise? :D

Sungazer, now you are talking. When I get a chance I will move this discussion back to the tuner thread.

!Peter!
Posts: 88
Joined: Sun Oct 30, 2016 6:35 am

Re: Barrel Behaviour Theory

#75 Postby !Peter! » Sun Jun 10, 2018 11:59 pm

sungazer wrote:Peter sorry if my posts seem argumentative they are not intended to be. My written communication leaves lots of room for improvement. I have an electrical / electronic background and have dealt a lot with harmonics on the power grid. I understand the higher frequency has more energy for a given amplitude. for others its about the area under the wave just as DC has a lot more energy as the area under the wave is greater than a sinusoidal with the same peak amplitude.
A question though is all the energy in these waves converted to mechanical energy? Are some frequencies of energy converted to different forms of energy in different ratios? Ie some may be to sound some may be to heat some will be to mechanical then even within the mechanical they may convert to different dimensions?
williada I agree its hard to convey just in text thats why I was going to bow out a few pages back. However its interesting even if we cant reach a scientific conclusion.

No problems at all sungazer.

If you understand Maxwell's equations and filter design you'll have more than enough maths to read some undergraduate books on vibration.

Conservation of energy law in physics states that energy can only be converted or transferred. Energy can be lost from a wave by being converted to heat or transferred to another medium like air in the form of sound. I suspect the amount of energy converted or transferred is related to frequency but I haven't specifically pursued this aspect as I currently don't see this aspect as being material to understanding the barrel behaviour.


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