OzFclass

Regarding the information that David's generously sharing with us through multiple threads about barrel compensation, I'm left with some thoughts...

Is the compensation characteristic of a barrel set in concrete by it's make up and dimension or is it able to be reversed via the load and/or assembly of the rifle?....... so....... with out using an adjustable tuner...............

If I have a negative compensating barrel, can that be changed to compensate positively by changing components such as bullet weight or powder type, or will the compensation always trend the same way?

Could it be changed by changing the way an action is mated to a stock or the torque spec used to tighten the barrel? .... Or can it only be altered by changing the barrel profile and/or length?

Apologies if this is obvious and I've missed the answer.

Denis,

In a basic sense, the barrel whip is commenced under recoil and the projectile travelling forward initially bends the barrel down in the section well before the muzzle inducing in the muzzle a fundamental upward lift, like plucking a guitar string . The forward motion of the bullet also induces further vibrations and the higher frequencies travel back from the muzzle before the bullet exits to create a clash of acoustic vibrations. We use the vibrations mapped by sine waves to tune the barrel by concentrating about the waves' peaks and troughs, big or small, whatever is best in the macro profile of compensation and try to dampen high frequency waves interfering with a desired pattern mechanically where possible.

It is just so important to recognise the distinction between the old skinny barrels doing about 2500 fps that used to compensate at 1000 yards using fundamental whips of the muzzle and the modern gear we are using and there are hugely different profiles even between f standard and f open and they are considerably quicker.

With the general gear we are using currently, the barrels have less fundamental whip, so the natural compensating range is much shorter in distance and there is less opportunity to tune using the fundamental wave or macro wave because the barrel seems to pulse more with smaller sine waves in heavy barrels. Because the barrels are longer, we can still split the barrel into sections of mid waves and micro waves which can exhibit similar characteristics of larger barrel whip patterns. Quite often we can see a positive pattern in a neutral barrel plot in the first half of the plots length if groups are laid out in the horizontal and similarly we can see a neutral pattern in a negative barrel denoted by the general downward trend of smaller waves over the length of the plotted groups. Quite often we can find a neutral section in the first half of plots in a general positive trend.

In the past we would not have worried about finer patterns so much, because the targets were huge compared to now.

Barrels are basically called positive if the angle of the muzzle throws slow bullets up higher than the fast bullets i.e. when the muzzle is in the upswing. Of course a negative barrel throws slow bullets lower in the downswing of the barrel than the fast bullets. A neutral barrel allows the bullets of varying velocity to exit in the middle of the upswing and downswing but essentially there tends to be low amplitude and probably due to Optimum barrel time and optimum charge weight theory.

A positive compensation range is where through gravity the slow bullets tossed high come down to cross the path of the faster bullet travelling underneath them. This needs some distance to occur, that’s why it is generally associated with long range in the past where tighter groups would form. The vertical spread before the compensating range is manageable but not tight, but much worse after the compensating range.

The opposite occurs with a negative compensation, as slow bullets go lower and lower with distance and never intersect the fast bullets on top of them in the flight path to the target. It is only good at short range.

A neutral barrel tends to hold tighter elevation over most ranges if velocity spreads are kept tight, iit is close to an Optimal Charge Weight tune and if there is good velocity to reach the longest range, but lower velocity can lose its legs and drop shots at 6 o’clock with the longs.

So let’s deal with what we have got now. Because the modern gear can compensate around 500 yards, the aim is to induce a small amount of positive compensation with relatively low elevation spread and use the best hand loads with low extreme spread because we don’t want to fall down quickly after the compensating range into negative territory, we want to fall into a nodal zone and because we have sufficient velocity we should maintain excellent elevation to the longs while reducing elevation spreads associated with greater positive compensation at the short and mid ranges. Five hundred yards is a convenient testing distance for elevation as I have said before because wind won't interfere with plots much compared to further out and is closer to an over tuning moment in the bullet flight than out further. If 300 is manageable, then extrapolating on elevation spreads from test results up short, should mean we will maintain elevation out to 1000. But compensation by itself is not enough.

I combine Optimum Charge Weight and OBT theory with slight positive compensation because the returning waves clash with outgoing waves before the bullet exits and changes bore and muzzle size with both fundamental frequencies and higher frequencies and this too can affect the exit angle of the projectile. Therefore I look for a positive peak close to a node where several groups lie at the same general elevation level which is the basis of OCW theory or people can sometimes see a long node. This takes better account of environmental conditions and assists in maintaining elevation and group size.

I use load development techniques to achieve it. The use of a barrel weight (which is still a tuner) on the muzzle, is only used to compliment the load development to tweak things a bit. A variable tuner is set and not altered unless conditions put you on the negative slope, the same as if you need to change the powder for the conditions. A tuner cannot magically change your major compensation profile much, but it can dampen smaller vibrations and alter the length of their sine waves. All harmonics depend on the length and the fixed point of the object. So a long tuner would lengthen the major wavelength giving greater access to waves to tap in load development and its weight will influence the amplitude of those waves. Better still if the fundamental weight has a variable component to tweak those lateral groups often associated with a selected point near a node.

It’s all a question of degree whether you are on major or minor waves. I still stick by my original contention that in the future we may need a short range barrel, because 300 is just manageable with a positively compensating barrel with current gear and we desire a high supercentre count. We need a mid range barrel like we currently use; and long skinny barrels might come back in for specialist long distance.

The easiest barrel to tap into for tuning is a straight taper barrel. We can still induce greater positive compensation in our modern gear by altering the fulcrum point on the stock lower. We can reduce barrel lift by getting the bore line closer to the stock’s centre of gravity or by raising your butt plate. We can reduce barrel lift by shooting free recoil. We can induce minor lift to fine tune by jamming bullets or making the leed angle steeper. The latter requires a fresh throat every 800 rounds.

Also, any factor that affects bore time or launch angle will affect the compensation profile of the barrel. The length of the barrel, will effect bore time as will powder, bullet weight or where your takedown screws are located in the action, because this changes the harmonic length. What affects the launch angle is the stiffness of the barrel or the angle from the mound to the target and any muzzle weight. Because compensation is based on the vertical plane, having the barrel muzzle indexed in the vertical plane (allowing for barrel torque if there is some) has a significant impact on the bore angle and therefore trajectory.

Torque specs do alter reflected vibration patterns, particularly where wood is involved. Because aluminium is a bastard for upsetting vibrations and not dampening them, it needs a tight torque setting to reduce the amplitude of reflections and requires a glue job, gasket or holes drilled to dampen vibrations. OBT identifies 7 vibrational influences, others have said there are 9. It’s where they clash to form the harmonic frequency we tap into as a node that has to be sorted for extreme accuracy.

I spoke about compensation on the sticky post, “Recoil and Stock Design” page 1 and Page 5. and also mentioned in a throwaway line how the bore changed shape which is directly related to Optimum Charge Weight and Optimum Barrel Time theories which I have introduced in successive threads. I have tried to tweak each thread up a bit. The problem we have with so much information is digesting it all, because it is new again and sorting an order of importance and whether you want to spend the time doing it. It may be trash to some and treasure to others. I just put it out there to level the playing field and to stimulate interest in shooting on the forum for better or worse. Yes I will draw the conversation to a close, but will commit to completing Tony's development so he can share with others.

Regards, David.

Dave, thank you for your in depth reply and I'm sorry that I've caused you to re-write a lot of information that you've already shared. I know your busy and I didn't mean to take too much of, or waste your time.
I feel a little foolish as I think it comes across that I haven't previously taken in information that you've shared, though I have and understand a lot of the info that you've re-iterated thanks to your past threads.

I've read through your reply a few times and will a few more.

My understanding of your reply so far in an extremely summarised version, is that the overall compensation trend of a given barrel can be increased positively a little by altering stock setup and technique, but it sounds like this may be unlikely, with a straight profile heavy barrel, to change a negative barrel to a positive barrel.
Also, tuning a load, using different components will vary the muzzle sine wave and bullet exit correlation but will not alter the barrels natural compensation trend.
A muzzle weight will alter the amplitude of the muzzle sine wave, lengthening the time of nodes but will not alter the barrels natural compensation trend.

Again forgive me if I'm not completely clear yet, I will re-read your post a few times.

Thanks again for your time and info.

Denis,

You are not foolish, there is just so much to digest in writing, rather than hands on. It’s a bit like trying to find the counter rotations in a gearbox when you see it for the first time.

Yes you can alter the natural trend in a barrel, but we don’t always have to in a heavy long barrel as I will explain shortly. Many people new to the game, go for the most velocity with a relatively tight group without properly exploring all the barrel’s characteristics. So they tend to be stuck with the last section of barrel characteristics which are also displaying the barrel’s macro trend.

If you want to, you can for instance, if the barrel is fitted to the action with the muzzle pointing down, in the vertical plane, and is exhibiting negative compensation, re-index the barrel so that the muzzle is pointing up at 12 o’clock. This generally reverses the launch angle and the compensation if torque is not a factor.

Or you can break out the hacksaw and keep cutting the muzzle back to find a point of positive compensation if it is exhibiting negative characteristics like they did in the old days.

Or we can add a long barrel extension like a bloop tube to induce a positive peak on bullet exit, because we have effectively lengthened the barrel in harmonic terms or changed the wavelength pattern that will ripple through the barrel and give you another chance to find a major positive peak. This is mainly used for tackling fundamental vibrations.

The fundamental vibration has greater effect than higher frequencies and advantage should be taken of them if they can be identified. That’s why I try to identify the fundamental characteristic first in very short range testing at 25 yards with 3 shots loaded a grain apart to make mechanical changes before fine tuning. It’s amazing how barrels at long range perform and are dictated by their natural pattern when atmospheric density changes.

Yes a long and heavy weight can change the fundamental compensation characteristic but more often it is impractical because the muzzle weight will tip you over allowed rifle weight or destroys bag handling and therefore precision. So that’s why I use lighter tuners to play with the micro frequencies and they won’t necessarily change the fundamental tune, only dampen it and manipulate group shape if it is a variable tuner.

Yes sometimes different components will alter bore time and change the fundamental pattern particularly if you go from heavy to light projectiles.

Because the target size is on the limits of most people’s grouping capacity with a heavy barrel, we turn to the secondary and tertiary vibrations (micro sine waves) to take advantage of them in the same way a fundamental or primary whip is taken advantage of and try to isolate mechanically further vibrations which jeopardize precision through dampers or glue jobs etc.

A heavy barrel has less fundamental amplitude and it compensates a lot closer because of that and with a negative fundamental swing up short it would be bad out long as slow shots would keep getting lower. Conversely, a positive or a neutral barrel is more easily reconfigured.

The beauty of the long heavy barrel as opposed to a short heavy barrel, is in fine tuning it. A section of minor wavelengths can be tapped into which goes against the major trend. It sounds counterintuitive, but if you slip off that point selected, the major trend will dictate terms.

So we ask ourselves firstly, in terms of load development, can we tune around a fundamental node? If not, is there a section of minor nodes we can work with? Is the barrel sensitive where clashes of higher frequencies could lead to heart breaker shots? Can we isolate those heartbreaker shots in an OCW tune about a positive peak? It’s like three steps forward and one step back, then round and round we go.

Remember you can move forward or back to capture minor peaks and troughs or you can take big jumps by moving from peak to peak skipping the trough in between. So when shooters say they moved to the next node, I ask is it a peak or trough or did you skip the trough? Is it a major peak or are you close to a major trough? All have entirely different consequences in different atmospheric conditions. Shooters tend to look for flat spots in their shooting and don’t differentiate as to whether they are dealing with a node or antinode. The truth is that sweet spot is just before that positive peak.

A different situation might be, if you don’t want a positive trend at 300 yards, you can shoot free recoil, or raise your fulcrum point (butt plate) to reduce the fundamental barrel lift. Or you can induce fundamental barrel lift with firm shoulder contact and a lower fulcrum point on the butt to shoot one thousand yards.

But the most sensible approach is to have mild positive compensation, have it manageable at 300 yards and cover all distances by working on a suitable section of your barrel’s plots from load development.

Cheers Denis.

PM me if you want more help, or someone else may be better at explaining what to look for; and for sure, nothing beats the hands on. David.