OzFclass

Hi Denis,
The load I use is the load I developed when I first started on the 6.5, and it just seems to work in all the barrels I have used. This new one is the 4th. and the first parallel that I have used. It must be the M.V. at around the 2850 mark that makes it work. I use the same 52.2 gr. load in both .284 barrels at around the 2860/2870 mark with 175 M/K projs. From what I understand from a sound engineer the Purdy system has something to do with the way sound reacts in a hollow tube. His scientific explanation left me totally confused. So I do not know why or how it works but it does.
Cheers,
Trevor.

Denis this is longwinded, but hopefully I have covered most things. David.

From the outset, I generally agree Purdy’s approach to harmonics but it is not the full picture in shooting long range given the terminal influences of the trajectory with bullet yaw; and with gravity influences on trajectory in the form of positive, neutral and negative compensation; and the mass dampening model utilised. Trade-offs have to be made to optimise all the major influences for score. In the main, a tuner or damper or barrel weight can form an harmonic function but it also influences a launch angle function which determines the compensation profile of the barrel.

Both elements can be group tighteners if there is an overlap or union in their application or they can work against each other like estranged partners sleeping in separate beds. The classic example in short range testing is when you think you have a super node that turns to crap on the range. What has happened is you have a produced a positive compensation point at the test distance around a node but past that distance the projectile launch angle is on a down swing of the muzzle producing negative compensation. Due to velocity variance the slow shots get lower with distance. On the other hand, a rifle that is tuned to positively compensate at long range will have an element of vertical at mid ranges due to natural velocity variance. The goal for both conditions is assisted by load development with low extreme spreads. But of course during a match series, the odd cracker is an outlier in the velocity spread and of course it will go right where the barrel launch angle sends it (providing it is not an imbalanced projectile) because the harmonics are velocity dependent. However if the velocity spread falls within an acceptable positive compensation spread the shot will hold up but your group will still enlarge in the vertical if your barrel has been fitted with its natural bend in the vertical plane. A pure nodal tune outside the velocity required will drop that shot. An acceptable nodal tune working at the compensating distance will have superior groups. So the trade off for an across the course rifle is one that is mildly compensating, has low extreme spread and overlaps with a nodal point as velocities increase due to say the heat of the day.

There is another issue that is present in group analysis and that is the stacked wave concept which I have mentioned before. Purdy has also taken this into consideration. Sometimes vibrations in a tube of less than perfect harmonic length reflect vibrations back against oncoming vibrations causing a collision. This can interfere with crown shape and work for or against you. Ever seen a nodal group that has a big velocity spread? They tend to be tear-dropped in shape. Sure they will perform due to checks and balances but usually reflected vibrations are bad. Like many of us who use a bloop tube at times, we move the reflected vibrations forward of the crown to clear it of unnecessary chatter because the barrel thinks it is longer and the crown is not distorted and you can also tap into a perfect harmonic length by varying the length of the bloop tube without cutting the barrel back and losing valuable velocity for long range. As Ecomeat has found the correct velocity node is important in the yaw factor with terminal approaches to target. His lips are sealed.

I do all my early load development without attachments to evaluate a barrel. If it ain’t broke, I do not fix it with a tuner or bloop tube. Common sense. I always commence testing with four warm up shots. Some time ago I posted graphs on pressure heat and velocity on both issued Palma and British ammo which illustrate the need to do this. For sure it settles a bit of early fouling but the barrel grows into its working harmonic length so I can clearly evaluate group shape and launch angle. Its done at 140 yards to minimise effects of coning, light distortion and wind. There is the tendency for patterns to hold with distance as Plumbs has recognised.

Let’s examine further.

The barrel is a tube and its resonance or vibration when it is excited, like any other tube, bar or rod will in harmonic theory vary depending on its length and width and how it is secured or closed. The physicists often refer to open, or closed fundamental sine wave patterns when describing vibration. Purdy based his ideas on the one end closed tube vibration theory as opposed to closed end theory like a guitar string to examine the harmonics produced. There is nothing new here, but he applied an intelligent analysis of the problem and application of the theory and integrated the bloop tube come tuner/damper into a calculation of overall length of the tube as a harmonic unit.

As shooters, the affects of vibrations we call harmonics, are seen in group patterns. In other words Purdy calculated where the node would be and the barrel length to achieve that node. With both ends closed like a guitar string is easy to understand the harmonic note varies by which fret you press to close it off. But with only one end closed on a tube in a barrel with the bolt and the other end open and free to vibrate we only have length to play with to find the desired note. This is something many of us have done by trial and error, nipping the barrel back or reducing the length of a bloop tube which I have mentioned some time ago with regard to tuners and fundamental sine wave patterns and barrel weights. Maybe Danny S could chime in.

Vibration patterns also take other forms and have been used to tune rifles. A tube closed or secured at both ends has a fundamental wave dominating the middle like a skipping rope. So those using a muzzle bed like a .303 in the old days (despite with upward pressure on the barrel which had more effect on preloading barrel action threads) represented the curve of a skipping rope in a sense. The British being masters of the tune used chopped cork in the mid section to dampen the fundamental vibration. It worked because it was dampening the anti-node at the maximum amplitude of the vibration which was in the middle. I think it was Reg Dean who later used the rubber nose method of damping vibrations of the muzzle in the .303 which became popular. So he also in effect added a dampening method to control the overtones of other harmonics at play and perhaps reflected vibrations.

If we take a barrel block bed, are we in fact creating an open fundamental pattern where both ends of the tube are free to vibrate? Partly because the bolt face reflects vibrations and the forward section of the barrel block gives a bias towards a left closed tube theory application. Further dampening of the vibration would be maximised by dampening towards each end of the tube where maximum amplitude of the fundamental vibration occurs or moving the barrel fore and aft in the barrel block to establish a desired pattern. By moving the barrel back and forward in the barrel block you can find the perfect harmonic length without the need for a bloop tube to establish an harmonic length. A straight taper barrel is ideal for this and does have an even sine wave pattern that a taper tends to dampen. Generally speaking, it is more important to have a perfect sine wave to tap into for tune than use any method to dampen it.

Purdy refers to the left closed tube model of vibration and has done some neat practical experiments to refine calculations to define the effective length of the vibrating tube and measures from the bolt face to the crown, then makes a subtraction for reflected vibrations at the muzzle inclusive of the tuner. Check out his method, I may have missed detail as it was a quick read. So in essence he calculates the tube length for a nodal bullet exit more precisely. It appears, he applies a musical principle of scale in selecting a suitable node. Think of organ pipes of different length, or bars on a xylophone which give the perfect note. Or if you have a sports muffler on your car which will give a perfect note with the correct exhaust pressure.

He considers the total vibration in relation to velocity through many cycles of the fundamental wave, but his ideal barrel length considers more than the first fundamental vibration and takes into account in some situations, up to the 11th harmonic in his calculations depending on the velocity of the projectile. The different harmonics are divisions of the oscillation length of the fundamental vibration. In other words a 5th harmonic had 5 equal oscillations in length of the fundamental sine wave. It has 5 peaks and 5 troughs. He might use a 5th harmonic to tap into with a slug gun but most of his calculations are based on rimfire barrels from 22 to 25 inches. So yes Dennis, he does take into account velocity but it is also a trial and error process by using different harmonic values. The theory is applicable to longer high powered rifles. He still gets you closer to an ideal barrel length than by trying the ½ inch cutback bit by bit that Perce Pavy used. This knowledge is old school and the important point is that perfect length matters for perfect harmony.

The purists argue these vibrations are regular if the speed of sound does not change and in the case of projectiles the velocity does not vary. But in reality bullets accelerate and some powders lose puff or the ignition is not efficient. Hence the importance of low extreme spreads of velocity from a harmonic perspective in a practical sense. Of course we know the speed of sound varies with atmospheric density. The best electronic targets take this into account when determining the position of your shot through triangulation with the microphone sensors. It is not surprising then, that your nodal tune will vary with changes in atmospheric density in terms of internal ballistics let alone the external ballistics relating to velocity.

By conducting an incremental load test you will some notice groups of varying velocities do strike at similar elevation levels and sort of contradicts what I just said. But it is like tuning into a radio to find the super sweet spot as we do in the bush due to poor reception in a broader node about the same elevation level where the stable velocity theory holds; but sometimes other factors distort the model at short range in the form of positive compensation and coning effects. That means careful measurement of charge to .1 grain, then ironing out a suitable seating depth in suitable test zones. This applies to optimum charge weight and nodal tuning. In some cases the barrel length and profile are not of suitable length to extract the best resonance so a change in powder, chambering or projectile may assist.

Let’s assume you have a suitable barrel length etc., then you do not need a harmonic tuner or damper and using an incremental charge over the chronograph where the data is collected and graphed will reveal relative flat spots to test further load development because they represent broad areas of harmony of many vibration sources. You do not need a physical target, just chrono results in the early stages. You then set up a target to test incrementally those prospective load zones discovered by the chrono test.

There is still great merit in Purdy’s thinking because as the competition evolves we will know what chamberings and velocities give the best harmonic signatures for barrel lengths we are supplied with and we will effectively impose our own standards and appropriate barrel lengths which take into account terminal velocities and launch angles for compensation.

You might be asking yourself how do waves travel back down a barrel? The moments of inertia create vibrations in firing from the ignition process, the movement of the bullet, and the pop of gas at the muzzle. In a sense the bullet is encountering vibrations preceding it from moments of inertia and torsional vibration of the rifling and the different environmental conditions as it pops at the muzzle bouncing forward and back from the closed end of the tube. The external atmosphere can’t reflect vibrations to any great extent but it does. There is a short moment after muzzle break before maximum velocity is achieved which reflects vibrations. Those who advocate the Optimal Charge Weight tune rely on this observation. In terms of the Optimal Charge weight tune there are many that advocate pet loads for different chamberings which also finds its roots in Ken Water’s work with his book of Pet Loads. While not optimised they are good starting points because each chambering sends a major vibration donut down the barrel with a peculiar signature with reflected vibrations.

I have mentioned the work of Boots Obermeyer before, with regard to bloop tube length and internal diameter. Purdy also takes the internal diameter into account with his formulation and links this to reflected vibrations. Boots as I am to believe, thought the internal diameter optimised the conditioning of the atmosphere behind the pill and managed the powder flack.

My approach has been trial and error with the length of the tube to get the right harmonic expression in my test groups. Its easier to cut back a bloop tube than a barrel. But I have found the Obermeyer internal bloop tube diameter of .86” for 30 cal to be worthy. So for other calibres, I have maintained the same ratio and they work fine. It was interesting to see that from reading Purdy’s comments on a .22” on one blog that tube internal diameters were .62”. It’s the same ratio as Obermeyer’s. This was the same tube diameter I used on a new tuner for a .223” and it filled up the super centre at 500 yards on a rough day at Moe last week.

We can also move to tune out unwanted vibrations by way of a damper to some degree but it is not a perfect harmonic tune. A harmonic balance on a car engine as Denis would know, dampens vibrations of the crankshaft at a set rpm very well and does not balance the crank which would further limit vibration. “This type of damper reduces the vibration at specific engine speeds when an excitation torque excites the first natural frequency of the crankshaft, but not at other speeds”. The harmonic balance is a mass damper and is placed in the next most convenient place, on the end of the crank shaft. The analogy I make with firearms, is that you can still dampen unwanted vibration in a non-blue printed action and scope setup but not entirely eliminate them with some sort of rubbery compound which in a sense is a spring damper on the muzzle which is a convenient place. Besides it is an area of maximum barrel lift which sometimes needs dampening to refine launch angle. We are seeing the snubber tuner of Jackie Schmidt which has rubber around the body of the metal tuner which operates as a combined spring and mass damper in theory. I feel the rubber dampens fine frequencies necessary for super centre count.

DaveMc would probably be the best bloke to explain the Stockbridge damper used on power cables which dampen vertical oscillation that we as shooters are concerned about. He might explain Aeolian vibration. Although it doesn’t relate to barrels, it reminds me of the flutter induced by lower order vibrations in the heavy profiles of F Open where often the tune we can tap into is on the 2nd and 3rd or 4th harmonic because the fundamental whip is reduced by barrel stiffness as Craig and Tony have learned using the theory of the second best. We see these as short sine waves in group patterns with incremental loads and the slower the powder the more finicky this distance between nodes becomes.

Viscous dampers have been around a long time and can work like a shock absorber on a car when fluid can be forced through narrow holes. Heat generated dissipates the vibration too. Ever noticed a quick string is often very tight? As mentioned earlier, the heat of 4 warm up shots prior to load development is a pre-requisite to my system of load development as much as it is fouling a clean barrel because it dissipates unwanted vibrations so I can get a clearer view of the harmonic group shape and the compensation trends. Also as the barrel it warms it grows so this influences its true harmonic length. It’s like turning the shower hot water on in cold Victoria, and you can feel and hear the smooth flow of the water as the shower head heats up. Its the same with a lathe. You must warm it up for smooth cuts i.e. getting things to operating temperature. The logic behind this was posted on graphs of velocity, heat and pressure from some time ago. A common mercury recoil reducer in the butt maybe seen by some a viscous damper but it acts indirectly on the fundamental barrel whip which we tap into for positive compensation tuning at long range because it is really acting like a mass damper and is no different to a moving weight because mercury is heavy and the heat dissipation is bugger all. What is novel with some recoil reducers is that a finned weight or counter rifled weight slid in an oil bath encapsulated by a tube can be used to offset torsional vibration which assists bag handling.

Those that use a 600 gram weight (more or less) on the muzzle but not extending past the muzzle are using a mass damper to reduce the amplitude of the muzzle. Each barrel is different, in profile length and natural curvature and in its hardness and metallic composition and so is the affect on amplitude and length of vibration. One glove does not fit all, so this type of mass weight, mechanical impedance needs to be matched to the fundamental vibration. If this is not matched you tend to be playing with 2nd to 4th order harmonics in load development and they simply won’t be as good as where 1st, 2nd and 3rd order harmonics intersect. This can be revealed by a technique called, a Bode plot of displacements or bastardisation thereof.

The beauty of a bloop tube is that is sucks away at reflected vibrations that can distort the muzzle on bullet exit creating undesirable bullet yaw and launch angle much like a crook crown. On the other hand a muzzle weight of significance as opposed to a bloop tube can stiffen the barrel and be more temperature stable where it counts.

If you add a bloop tube which may only be an inch forward of the muzzle on high power rifles or up to 5 inches etc. depending on the magnitude of the oscillation cycle tapped into there are additional benefits. Moving a variable weight along the muzzle or bloop tube, only really gives you the ability to fine tune the launch angle in practical terms. That means you can vary the thimble to compensate at different ranges or varying mound to target heights. It also means that if you can collect enough data you can adjust your variable tuner when velocity changes due to environmental factors as measured by density altitude or temperature in the main. Beware that the perfect harmony is lost with velocity change and the adjustment really only works on the compensation factor, so groups are still bigger but you can reduce vertical assuming the rifle and ammunition are sound. The only way to keep things the same, is something modern snipers do by adjusting charge for a different temperature range for extreme distance shooting to maintain velocity for harmonic tunes, and something many bench rest shooters do as the day warms. Remember, the variable tuner compensates for velocity by altering launch angle and has greater impact on elevation. It requires only tiny changes to alter launch angle. Or you can set it and forget it maybe for winter or summer shooting and manage larger groups by reading conditions better.

There are many ways to skin a cat. For what its worth, I do my load development without a tuner on in the first instance. Then if required, I add a light bloop tube made of aluminium which I incrementally cut back unlike Purdy who predicts the desirable harmonic length. Mine is more an art than a science and knowing the group patterns I am seeking and the degree of barrel lift. I will have to experiment here with high power rifles in terms of his formula because he varies his harmonic factor based on velocity.

With current barrels, I am using a bloop tube about 1 inch forward of the muzzle in 30 cal and 1 ½ inches in .223” with Obermeyer internal ratios. This is based on an average of past results because I am a bit lazy these days to vary bloop tube length with every barrel which I should, to perform at a high level. You soon see the patterns enlarge of shrink with different bloop tube lengths. The next step is to add weights to the threaded bloop tube to capture the desired launch angle that hits on the same elevation previously established without attachments of my most suitable group in terms of size and launch angle. Then I make a new tuner to the final specifications. With that I do a final tuner test in quarter turns commencing with the rear of the tuner thimble in line with the muzzle for about 1 ½ turns and plot the sine waves. I use this to tell me which way to move the tuner and how far when groups dramatically change shape due to poor environmental conditions and significantly varying velocity for compensation purposes if I am to salvage anything because the nodal sweet spot will be less significant as it is more velocity dependant and I rely on the node breadth which is not the sweet spot where possible to compliment the compensation.

I can't help you with apparent aiming mark shifts by way of mechanical device, but I believe the modern technology is using lasers for that. Hmm.

Hi David.

Wow, thank you so much for the time and effort you've put in to that explanation as you always do.

I was hoping you'd give some insight on the PRX

Being a simple minded bloke it will take a few reads of your explanation to try and understand all of the components that you have commented on here.
A lot of it makes sense immediately as you have explained and discussed many of the theories before.

Thank you for explaining the means that MV is accounted for in the PRX and also what a harmonic is with your example of the 5th harmonic.

A few things come to mind immediately. The theory of improving a barrel tune gets more complicated the more you try to understand it. The old sayings are true. "If it ain't broke don't fix it" and "Ignorance is bliss". I wish I had it in me to just shoot this game without question, but being a mechanic I'm a tinkerer and probably shouldn't be with topics like this. I guess the tinkering nature of us all is what's attracted us to F-Open.

After your explanation of tuning harmonics through bloop tube lengths and such I feel quite silly for having thought I could rubber bond a weight on to the end of a barrel and hopefully see an improvement. I probably should have run my idea past you first. That's said, then I would not have learnt and seen for myself the characteristics that I did, Trev would not have made me aware of the Purdy Prescription and described his success with it and you would not have explained it so well.

There is an amazing wealth of knowledge on this forum and there are some contributors that share above and beyond the amount that they need to. It would be so much easier for the individual member not to comment on posts or share experiences, but because some do take their time and aren't afraid to say things on a public forum, dopey buggers like me learn a lot more.

Again, thanks David and Trev for you time and knowledge.

Hi David , my head hurts again being a humble "pluummbber" lol! after reading that it makes me more convinced that I'm on the right track . Funny you mentioned about cork in the middle of the barrel! I'll pm a photo of my rifle as I don't want any overseas competitors seeing what's going on and see what u think? Early test on another rifle proved positive ! It's carrying on from Cam's and Mark's Idea . I think the best thing to say is it makes ur tune a lot more forgiving to differences in conditions or clean barrels and doesn't make ur barrel anymore accurate eg if ur loads are crook this won't help, it's kind of insurance ! What I will also say is moving those rubber things up and down the barrel , has a huge effect on poi and tune !

Regards Graham!

Hi David & Denis,
A most interesting post. I am using just the bloop tube without any additional weights as I am just under the limit of 17lb. for light gun in the 1000yd. B/R. comp. I started out with an I.D. of .5" on the .284 but found it to produce erratic groups larger than bare barrel. I then went to 1" and by the time I cleaned up the drill hole it measured 1.014. This worked, and I believe it did so, because it gav e more room for the gasses to move away quicker from the projectile. That was my Maddco barrel. I purchased a Bartlein and set it up from scratch with the same load and tube i.d. of.977 that is what it finished at, and it shot well.
After reading the section of your post about Obermerer"s theory it has got me thinking again and have done the calculations and came up with .814 for the .284 & .757 for the 6.5. Will make a new one for the Maddco barrel with the new i.d. and see how it goes. That will be some time away as I am in the middle of having cataracts removed and will not be back shooting till April. As for the 6.5 the back bored i.d is .787 which is .030 larger than Obermeyer"s calculation, but as it is shooting very well I will leave it for the time being. One day I may make a sleeve and reduce the i.d back to .757.
Iwas just thinking what a pity it is we live so far apart as I am sure we three could have some long and meaningful discussions on this and other shooting topics, "The tyranny of distance"
Cheers to you both,
Trevor.

Trev, may I suggest the Obermeyer measurements are minimum measurements and anything slightly bigger is fine. Just a couple of things to consider due to the tyranny of distance. Obermeyer was looking at the boat tail bullet used in Palma competition. I think the flack part of his work is relevant but there is also the upsetting of the projectile by the gases.

I was not sure I would elaborate on Shane's throw away line that the tube is tapered. Some of mine are and others are not. I just stop when I know the group does its thing. The wife's is tapered but it has a short tube.

But lets take this a little further right back to the crown then back to the breech. Some barrels have a 90 degree crown, others have with an 11 degree crown. Some have an inner champher or symmetric bevelof 45 degrees on the 11 degree crown to protect the muzzle from the cleaning rod. Others have a ground spherical crown. With all the trade offs its hard to say which is superior.

Some bright sparks have done shadowgraphs of bullets exiting the muzzle and you can see the gas and flack. If we narrow this down further, there is a different profile that Corbon talks about which links to what a few of us have played with. He comments on the differences in flat based bullets, boat tail bullets and rebated boat tail bullets and how the gas flow differs and explains why his rebated boat tail is better, naturally. I do not judge the merit of his bullet but it does highlight the crown/gas relationship if you are a purist could be a better match to the projectile in use.

To cut to the chase, a flat based bullet may be better suited to a 90 degree crown, and a boattail to an 11 degree crown. Shooting to 300 yards is probably better suited to flat based bullets, but with extreme distance it is the boat tail angle and length that manage the aerodynamics better past the overturning point where so little has been written about because the vast majority of shooting around the world doesn't get past 600 yards.

It is those tiny nuances that can magnify into big spreads at long range. So subtle angle differences do matter. We see projectile boat tails for extreme range moving to 9 degree angles. Remember once a path is altered it can not be recovered, so the idea is to optimise the crown angle to the boat tail angle to minimise the yaw factors.

Now if you add a bloop tube, you want sufficient diameter to sieve out the flack so it does not upset the boat tail because it actually presents a greater surface area compared a flat base and you want the gas to apply even pressure and not to one side of the boat tail which would create yaw. This is also why it is important to also start your bullet straight at the breach because the bullet is malleable its deformation allows for in bore yaw and when it pops at the muzzle the blow off gases can yaw the bullet further because they will work on the side of the boat tail that exposes more surface area.

If the diameter of the bloop tube is sufficient, then the flack in bouncing off the internal walls can't rebound in time to catch the angle presented on a boat tail and so yaw it if the set up was not perfect because the bullet has gone. This is not as critical on a flat base bullet because the thrust of flack and gas is mainly straight behind it relatively speaking.

The taper on a bloop tube works the same way as a gas torch heating nozzle which is cone shaped internally. If you look the flame on a gas nozzle it comes out in a curve to a point. What this does on a tapered bloop tube is sieve out most of the flack as its trajectory follows the angle of the taper as it bounces off the inside of the tube. The minimum diameter is very important here. The other function of the cone is that it allows the gas to flow as a smooth curved shape which focusses on the centre of the boat tail at a critical time when the projectile gains speed past the muzzle for that very short distance and so it minimises yaw. It is also a factor in managing reflected vibrations. It is interesting to note the magic angle of a gas torch is 12 degrees and a good crown is said to be 11 degrees.

The wife's tuner is shorter and so we are faced with another trade off which is the true harmonic length and the cone length necessary to focus the gases past the muzzle. David.

Hi David,
Thanks for that post. As the counter bored barrel tube dia. was a little over size I was not to concerned, as it shot very well. I have shot both 90deg. and 11 deg. crowns and found that it did not appear to make any difference, I do all my crowns at 90 deg. Saves changing angles on tool post. The only chamfering my crowns get after cutting is the clean up with 600 w&d. As a matter my counter bored crown is straight off the cutter. After checking the cut with a cotton tip and getting no residue left behind, job done.
Am looking fore ward to getting back to playing around again after cataract operations. Another 3 or 4 weeks.
Cheers,
Trevor.