TargetTalk

I am hoping to get an informed discussion going, leading to a deeper understanding. So far, little success (see thread: Torque settings (again!)), although I seem to have one disciple! In this forum, there seems to be a lot of repetition of the same old mantras, but little explanation of the fundamental principles. Thus, religion is born: 'truth' cannot be questioned - it is because it is - end of discussion. Unlike religion, the fundamental principles at work are known, but, it seems, not widely understood.

Let's start with optimizing group size by adjusting the king screw torques. The torque applied alters the compression of the interface between action and stock (it also distorts both). This alters the mechanical coupling between the two, and thereby how energy (vibrational) passes between barrel/action and stock. This influences how the barrel vibrates. How the barrel vibrates determines how the bullet is deflected off-line when it exits. The problem is - if the bullet velocity differs from shot to shot, the bullet exits the muzzle when the muzzle position is at a different phase of vibration (ie pointing in a different direction). Barrel tuning therefore works for only one muzzle velocity (or, more explicitly, time of arrival). This is why re-tuning is required for different ammo.

Therefore it is critical that the chosen ammunition has a consistent muzzle velocity. Is this the major parameter which determines which ammunition best performs in a given rifle? Is it pointless to try to tune a barrel for an inconsistent ammunition? How many people directly measure muzzle velocity? I contend that the optimization process should begin by assuring one's self that one's ammunition has consistent muzzle velocity, and that this is determined by direct measurement.

Since the coupling between action and stock is known to be important, the coupling between stock and whatever is holding the rifle is important. Barrel 'harmonics' will not be the same when the rifle is shot from the shoulder as when it is clamped to a test rig. So why tune your rifle for optimum performance in a rig? OK, it takes out the human variable, but I contend that, unless the rig mimics the suspension of the rifle at the shoulder, it will produce sub-optimal results.

I would like to explore other beliefs, but this is enough for now. Please can we have some informed discussion, not just repetition of the same liturgy?

great line of theoretical discussion, but IFtop grade ammunition was developed to give a constant point of impact, not a constant shot-to-shot velocity...?

Regards,
Spencer

I'm sure it was, Spencer, but HOW? What makes ammunition good? I'll bet that good ammo has consistent velocity as well as tight dimensional and mass tolerances.

This might help explain some of it.

http://www.the-long-family.com/shock%20 ... nation.pdf

Old eyeball wrote:
Let's start with optimizing group size by adjusting the king screw torques. The torque applied alters the compression of the interface between action and stock (it also distorts both). This alters the mechanical coupling between the two, and thereby how energy (vibrational) passes between barrel/action and stock. This influences how the barrel vibrates. How the barrel vibrates determines how the bullet is deflected off-line when it exits. The problem is - if the bullet velocity differs from shot to shot, the bullet exits the muzzle when the muzzle position is at a different phase of vibration (ie pointing in a different direction). Barrel tuning therefore works for only one muzzle velocity (or, more explicitly, time of arrival). This is why re-tuning is required for different ammo.

Since the coupling between action and stock is known to be important, the coupling between stock and whatever is holding the rifle is important. Barrel 'harmonics' will not be the same when the rifle is shot from the shoulder as when it is clamped to a test rig. So why tune your rifle for optimum performance in a rig? OK, it takes out the human variable, but I contend that, unless the rig mimics the suspension of the rifle at the shoulder, it will produce sub-optimal results.

The people that I know who shoot at a high international level set the action screws at the factory recommended torque settings or something close to that and leave them there, checking periodically to assure the settings have remained constant. These same people use a machine rest to identify those lots of ammo that perform best in their individual rifles. Occaisionally, some lots of ammo will shoot well enough that additional tuning through the use of a barrel tuner provides no additional benefit. This may just be coincidence in that a tuner "just happens" to adjusted correctly for that lot of ammo. On other occaisions some adjustment becomes necessary. The bottom line is that the proof is on the paper. Ten five-shot groups that are sub 10 ring in size appears to be sufficient proof that the rifle/ammo combination has sufficient inherent accuracy. At that point velocity measurements, with the attendant chronograph inaccuracies, become almost irrelevant. Retuning is not always required for different lots of ammo. Often several different brands/lots of ammo will shoot equally well in a given rifle with no additonal tuning. Again, the proof is on the paper, not the chronograph.

As far as dissimilar harmonics between rifle performance in a machine rest and one that is manuall held, that would depend to some degree on the design of the machine rest. If the rifle is attached to the machine rest using the butt plate attachment and the sighter rail, performance would be likely be fairly similar. At our club we have a young man who has shot many clean prone scores in NRA and international events. His prone groups consistently replicate machine rest groups fired from his rifle.

I understand and somewhat agree with your comments on religion, but in the pursuit of optimal accuracy, performance is always the ultimate deciding factor. If the oft repeated "religious" tenets and somewhat empirical and simplistic approaches result in ultimate performance, then adjusting torque settings and verification by chronograph are mere mere scholastic exercises.

Thank you so much, diopter, for this reference. It accords with my view of the rifle as an acoustic instrument. I don't know enough transmission-line theory, and don't have access to (or understanding of) the modelling software used, to challenge the theory.

Theory accords with observation. Excellent! Unfortunately, this only lends supportive evidence to the theory, but a theory can only be tested by attempting refutation. At the turn of the last century, FW Mann (author of The Bullet's Flight) showed empirically that gross butchery to the muzzle of a rifle didn't materially affect group size, although point of impact was changed. So, do we have good evidence that tiny changes in muzzle diameter, at the time of exit, affect group size? Is there another effect caused by vibration? Does any of it matter, so long as it's consistent?

j.edwards - thank you for your comment, but I hear the mantra being repeated.

The vibration theory expounded in the reference provided by diopter should not be overlooked, but extended. The vibrations are not totally confined to the barrel, as we know. I suspect the modelling had to be simplified for practicality. Vibrations will be conducted (to some extent) into stock and bench / shoulder, being reflected (to some extent) by discontinuities in the media (actually changes in the acoustic impedance, and thus changes in the speed of sound in the respective media). So the reality will be more complex than in the elegant model.

No evidence has yet been presented to refute my assertion that tuning a rifle to shoot well on the bench only tells you it shoots well on the bench. There is no reason to suspect that it will also shoot well from the shoulder UNLESS the vibrational energy transferring between rifle and bench is similar to that between rifle and shoulder. Think of the analogy of a tuning fork. It may be that even alloy stocks almost completely damp the vibrations, or, perhaps, the coupling between action and stock is so (acoustically) reflective that little energy is transferred to the stock. Is this the case? Does anyone know?

Please keep the ideas coming!

Hi Old Eyball

You have raised an interesting subject concrning accuracy, bench testing etc.

In Germany and here in the UK we first of all vice/clamp rest for batch testing, when a certain batch of ammo gives a really tight group we then bench test by resting the rifle on a kneeling roll and again test it to see how the rifle/ammo performs un clamped. There are often differences in group size but some times you hit lucky and get just as tight a group.
When this happens you then go on to shoot 50 metres prone to see how it all works out. If it stays tight, touch nothing.

Have you tried asking Eley or other manufacturers for their take on this subject?

Regards

Peepsight

My experience and other shooters:
Accuracy shooting .22 from a bench is worse than from the shoulder in prone.

Hi Juan

I see the point you are making and i understand that a rifle at the shoulder will perform differently than when on a rest etc.

The only trouble is, how do you know if that wide open group that you have just shot is not your fault rather than the ammo or gun's fault?

There are too many variables that come into the equasion when testing ammunition from the shoulder.

Best to establish a tight grouping ammo from a rest then see how it performs from the shoulder. Perhaps one of the batch you tested but was not satified with from the rest comes together from the shoulder.

Its all trial and error in this most demanding of sports which i might add is not an exact science yet!

Regards

Peepsight

Hi peepsight:
I agree with you.
I only wanted to say the rifle accuracy improvement when shooting from the shoulder.
Regards

Well, lots of reads, but few contributions to this debate: why so few?

From the small sample, there seems to be evidence that the rifle shoots DIFFERENTLY from the shoulder, compared to from the bench / test rig. Therefore, optimising the rifle using a test rig can't be assumed to optimise it for real shooting.

When shooting to test ammunition, is one testing:

1. intrinsic properties of ammunition? If so, which?

2. intrinsic properties of rifle? If so, which?

3. A combination of 1 & 2? If so, in what proportion?

If you can't answer this little quiz, how can you optimise the system, except by blind guesswork?

I'm being provocative - hoping only to provoke intelligent discussion.

OK, here's a suggestion:

if you have the facility, try live firing with Noptel and comparing the point of impact on the target for each shot with the predicted point of impact from the Noptel.
That should give an indication of how predictable your ammo/rifle combination is from the shoulder.

Choose a calm day, of course.

My experience of bench testing is that batches that shoot well from a test rig will generally shoot better from the shoulder than batches that don't shoot well from a test rig....

As to why this is, my theory is - that the muzzle of the rifle will deflect during the shot in a roughly sinusoidal manner, so if the bullet exits when the muzzle is at its maximum deflection then at that point the muzzle will also be stationary and so impart NO sideways velocity to the bullet as it exits.
Conversely, if the bullet exits when the muzzle is at zero deflection the muzzle will impart the MAXIMUM sideways velocity to the bullet, and it is this sideways velocity component which increases your group size.

Where in this cycle the bullet exits is dependant on the "barrel time" of the bullet, which may roughly correspond to the muzzle velocity, so what you're looking for is ammunition that has a velocity that causes the bullet to exit at a time when the muzzle is stationary (max deflection) and that has a low variation in its velocity.

Now, if the barrel is clamped in a vice the vibrations will be accentuated as the energy has nowhere to go - like a ruler held over the edge of a table and "twanged".
Shot from the shoulder it's more like trying to "twang" a ruler that you're holding in your hand... It's loses energy much more quickly and the amplitude of the barrel vibrations may be much smaller, so producing smaller groups.
(Aluminium stocks do seem to "ring" more than wooden stocks though.)

As for tuning by adjusting bedding bolts - my rifle is bedded on compound and seems pretty insensitive to varying tensions. Actions bedded on wood may well be different.

I have more experience with rimfire benchrest where tuners are the norm. But even so, I have found many lots of the same brand such as Lapua Midas shoot best with the same tuner setting. However they may require different action screw torque. For instance one lot may shoot best with 37 inch pounds and another with 39 inch pounds. Another identical rifle may require 43 and 45 inch pounds. This is all indoors with a return to battery rest used in unlimited benchrest. Outdoors I find to many variables to do this accurately. I'm not talking big changes. Maybe five shot groups are running .21 CTC and an increase of 1 or two inch pounds may reduce it to .19 CTC. But I am at a loss as to the specifics as to how and why. It can take several 2 hour sessions at an indoor range to get the optimum from a rifle. I use a chronograph outdoors when I check out my settings to see how the rifle shoots in a breeze. Its interesting that if I shoot at a set pace I get lower SD than if I shoot randomly such as when shooting a certain wind condition. In benchrest It can be beneficial to shoot a shot off the target if you have waited over two minutes. I never worry with that in 3P. Too many other issues to worry about.

Thanks, KennyB, we are thinking along similar lines.

I believe the vibrations set up in the rifle are more complex, however, with innumerable resonances set up, reinforcing and cancelling in a very complex manner. If it's all repeatable, of course, it doesn't matter - all shots will be identical.

Has anyone investigated what the spectrum of vibrations looks like, how it changes with time and whether it changes from shot to shot?

Can anyone enlighten us as to which parameters make one batch of ammunition better than another? Is it the characteristics of the projectile, charge, or what?

Apparently, when you select a batch of ELEY Tenex they can suggest a suitable replacement batch which will come from the same machine (they have four) and have a similar Muzzle velocity - or so I've been told.

I'd love to understand what produces the optimum combination of barrel and ammunition but, from a practical point of view, we have to work with what we're given. You can't load your own rimfire ammo, you can only select from what's available to you.

After thinking about what I wrote, I was over simplifying - complex vibrations in three dimensions....

Ammunition parameters...

From the work of FW Mann in the early 1900s, and knowing a little about production engineering tolerances (although not in the field of ammunition), I suspect production variations in the projectile do not significantly contribute to group size. Does anyone have any other views?

Charge mass variation and rate of burning seem much more likely candidates. One might expect a different spectrum of vibrations would be set up by a charge which went 'crack', compared to one which went 'whoosh' or 'thud', just like the difference between hitting a bell with a metal hammer versus a rubber mallet. Different charge characteristics might also result in different rates of acceleration, albeit with similar muzzle velocities, causing different barrel times. Therefore the bullet may exit the muzzle when it is 'quiet' with one charge, although a different charge may cause exit at a time vhen muzzle vibration is chaotic. Anyone got any ideas how to measure 'barrel time' with the precision necessary to be useful?

Please can we have some comments? The silence recently has become deafening...

Anything is white or black. Everything is gray.
The French shooter Jeann Pierre Amat (600 in prone) made some of their best performances with a rifle with bedding until half of the barrel. What happen with the floating barrel theory?
The bench is designed for not transmitting vibrations or deviations. They have springs with simple harmonic frequency. The system rifle - human body doesn't have an unique frequency. Therefore the vibrations are annulled mutually, and the groups are better.

Thank you!

I suspect the barrel was well damped by the extensive bedding. I agree that a rifle shoots DIFFERENTLY from the shoulder, compared to the bench.

Surely we should be trying to damp resonances, not simply shift their frequencies.

More comments please!

Old Eye ball,

Let me add another deminsion to the thread. Will a small bore rifle shoot differently in prone, where the rifle(thus the barrel) is securely connected at the sling, shoot differently in standing, where the rifle is not securely "fixed" to the shooter by a sling.

The discussion of harmonics in a barrel would lead us to say that yes the gun will "sing" differently according to where it is held.

Well, disappointing number of participants... However, there does seem to be a suspicion, shared by several, that rifles behave differently when shot from the shoulder versus the test bench or vice. Having recently shot my rifle from a rigid, un-damped bench clamp, it's blindingly obvious that the two conditions are wildly different.

The strong vibrations set up with the rifle fired in the clamp can be felt to decay over about 1.5 seconds. No sustained vibration can be felt when shot from the shoulder. Group sizes significantly larger from the clamp.

So, the question remains - what can you tell by shooting from a bench clamp, if the rifle behaves differently when shot from the shoulder?