#42 Postby williada » Tue Jun 07, 2016 6:12 pm
“If you eat All Bran you do not need All Bran”.
I am reluctant to continue making posts of this length and wish to pass the baton as it is very demanding at times to respond to all requests as much as I would like to help at the moment it is difficult.
In other words if the size of your sample is big enough, say over hundreds of shots it is more likely to form a normal distribution pattern to which you can expect future shots to follow. The minimum sample size of significance is 30. This pattern will be in its simplest form, shaped like a bell. If you plot the data of a normal distribution, the shape is symmetrical. It will show dispersion of shots or data, either side of the centre of pattern to be the same. That’s why the arithmetic mean (as we commonly refer to the average on your chronograph) will be in the middle of the bell shape as well as the mode which is the most common result; and the median which is the halfway point in your data, say the mid-point in your extreme spread.
In such as case the SD (standard deviation) will be representative. So loads based on a big sample size will work as predicted.
To answer Denis, the SD’s and ES’s might be higher than you think without big sample testing of your reloading practices even though they may represent a normal distribution. Small sample values might look great one day and terrible another when really they are not reflecting the value from a bigger sample. Sometimes people are mislead by SD’s which are higher or low by chronographs which still spit out SD’s on 3 shots or a few more yet their groups are ok and become confused because of this.
The second best method to improve your diagnosis is to accumulate many small samples as indicators and trends if overlayed in successive tests to display a trend like in your charge test if you do not do large samples. Then you can effectively increase your sample size if you know what to look for. This saves wearing out your barrel. Keeping records of shoots does this too, but how many F class shooters write things down? Three shot groups mean very little unless they are representative of a bigger sample size such as working your variable tuner based on a previously known tight tune. Nodal groups represent harmony of just so many vibrations. If they are round, you would expect the SD figures to hold for that tune condition.
But again patterns you may see as nodal may be a result of variable velocities that have reflected vibrations and overtones influencing them that balance out or maybe stack up as a standing wave to form a small group.
Because there are just so many variables to contend with, we have to look at the magnitude of effect each one has, and then secondly their interaction. This means we have to demonstrate the extremes of effects to understand concepts e.g. Positive compensation compared to negative compensation. Once we get the concept we then refine it to pass analytical judgement. This means we look at the macro effect, then the micro effects, then interactions. Some things cancel each other out, some things cumulate and add to the effect, some things are in harmony and other things are not quite, due to different frequencies and overtones and reflected frequencies. It is very complex for the uninitiated to get their head around. It takes many shots to say confidently this will happen.
The macro concept of barrel lift and fundamental vibration is used at a basic level to explain tune concepts of a nodal tune and compensation. We know this is affected by barrel profile, stiffness and velocity in determining the amplitude and the frequency of the fundamental sine wave and exit timing as Alan says in the vertical. So it’s not surprising the charge weight has the biggest impact on vertical followed by seating depth and neck tension.
The fundamental lift is not as great in thicker barrels which induce higher frequencies as used in FClass. A further consequence of slower burning powders the frequencies of many inputs, are lower in amplitude and shorter in length. They tend to have choppy waves. All it means is the extremes of lift are reduced and more attention to lesser frequencies must be paid in order to tighten groups further. The frequencies still exhibit the strengths and weaknesses of the macro scale. Barrels might appear to be a neutral profile, but in reality exhibit slight positive compensation. I would call that barrel a neutral/positive, or vice versa a neutral/negative. So we get a range of finer tunes and nodal points to select from indicated trends. We have to start somewhere.
In order to examine the micro scale we have to bear in mind the sine wave is a two dimensional concept of forces applied in a barrel we see on the target with 2 degrees of freedom, vertically and horizontally. That’s why I use actual group analysis to see what indicated tune best suits the in terms of its personality. It’s a case of reverse engineering with experience and pointing the tongue the right way I have gained over the years - never too old to learn though and have an open mind as to how improvements can be made.
If I was a miner and had a core from a diamond drill to inspect, we would slice it up the middle, end to end, to get a visual idea of what the core held. A barrel has a helix or twist of rifling end to end; and if we section that too like a core we can see the vertical pattern of the rifling in the same way we visualise a sine wave to get a good idea of what is going on.
If people care to look at simulations on Varmint Al’s site, you will see barrel’s doing all sorts of things. Suffice it to say, that the barrel can move in all directions in reality with two degrees of freedom. So the sine wave application has to be qualified. The patterns we see on the target in both vertical and horizontal represent different frequencies interacting.
If we get a nice round group, to keep the discussion simple, then two frequencies one horizontal and one vertical are acting 1 to 1 on a group and we get a harmonious pattern.
If we get a vertical frequencies two times greater than the horizontal frequencies we can get an elliptical pattern.
The combination of frequencies can produce many patterns such as the ABC logo. All of these patterns are stable. They do the same thing if velocities remain the same. Which one you pick depends on what you want from your group size.
One further advance in tuning is to recognise the vibrations not only go up and down and sideways but a shockwave donut travels up and down the barrel about 7 times as some say, before the bullet exits while the barrel is subject to fundamental to lift and sideways movement. The bullet hits all these hurdles of transverse, vertical, reflected and overtones of the mix. The bullet is retarded by theses pulses and this will influence the SD of its velocity.
Now I come to the bit that has Denis scratching his head. The Optimum Charge Weight practitioners feel the crown is distorted by unfortunate mixes of frequencies. We all know what a bad crown does or if the muzzle is loose. So they tune in an area where groups are tightest with least vibration excitement and that does not necessarily represent a node where frequencies are in harmony and velocity dependent. They get insurance having the lowest SD they can possibly have.
To achieve the smallest group is also to take into account the third inertia force on the barrel when the bullet exits. The vibrations speed up as the nose exits and change a little more as its body and tail emerge. These vibrations have less amplitude and have greater frequency. It’s like looking at the wiring element of an old light bulb. It’s like visualising the flatspots on a chrono incremental load test where there is a long lift up before the next flat spot. If you look carefully at that flatspot, it has small wiggles like the top of a light bulb element. The chatter from it can be dampened by a tuner or you can add a false muzzle to take the chatter away from the crown but that should be based on harmonic length or it could influence a micro launch angle. In any case when you know you are around a good tune area the movements have to be small.
Larger weights have already been considered for damping properties, heat sinking ability to stabilize velocity and launch angle. If anything is not broke and your groups are sound, no need to fix them.
To sum up Denis, anything that prevents flow is impedance. Any variation in the impedance will affect your velocity SD. If there are more wiggles there is more impedance. By how much? Depends on the size of your anti-wiggler. Look at the target to see if it is significant. Over time you may have a large sample size to prove it. The OCW method implies there is least impedance when the barrel is in its straightest position on bullet exit. That implies a low angular tune. They look for an area of least barrel excitement in the fine tune. I have found the patterns to represent a neutral tune. I do not think it is superior to a nodal tune on a neutral barrel. But it does seem to perform well with environmental changes as it tends to embody velocity changes in its structure because you look for tunes on a similar elevation either side of it. I think compensation tunes can supplement these tunes as insurance but always like low SD’s included in my load preparation as separate from knowing the extreme spread range the compensation tune copes with preparing for a specific distance like 1000 yards because a compensation tune is distance specific and not for across the course unless it is slight IMO.
If you believe in weighing your primers, then you would think it wise to check chatter too because that is impedance. If you had a loose fanbelt with an alternator that had suspect bearings, would you run with a loose fanbelt or a tight one? Could you measure the change on the motor which is the main driver like lift in the change of its RPM?
The modelling of the trajectories has its limitations but they are an important tool. The point I make is that the barrel muzzle goes in many directions large and small and they both affect the accuracy of the group. Beware of ballistic models where SD analysis to see what potential group size could be assumes a fixed launch angle. We know in reality this is not so. What we can do with a green barrel is get the best out of it by examining its personality with indicators.
A way to improve the confidence of the test indicators I advocate maybe have many people pool their test tunes at short range with long range performance.
David
, but I'm really looking forward to and answer to this question if anyone has tested it.
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