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When searching for the proper muzzle exit time you're trying to hit the dwell period when the muzzle movement is slowing down before it hits its peak. A slight variation in velocity will hopefully occur in that stage of the barrel movement.

Sure you can. The method was previously mentioned in this thread.

Ok, Jordan, you just made me skim back through all the posts. I assume it's where you talk about tuning for consistent velocity/pressure (SD) with charge weight, powder, etc. then tuning for a barrel harmonic anti-node with seating depth. Do you do the Audet varying seating depth instead of charge weight? Or, how do you do it? Group size?

Does the cake next to your name mean it's your birthday? If so, Happy Birthday!

I noticed that this entire post is about ES and SD. Only in velocity.......ES and SD in pressure is equally important. And yes you can be having some wild pressure swings and have low ES and SB on velocity. And vise versa. Its a mistake to only look at half the picture.
Charlie

I don't doubt you are correct, Charlie, but that doesn't make sense to me. I look up to you and Jordan, too, for that matter, as being way more knowledgeable and experienced in all things ballistics than me. Whatever bones you want to throw my dumb ass by way of explanation is much appreciated.

This discussion of measurement, nonlinearity, harmonics, and variation have some incorrect assumptions and concepts that are muddying the waters.

In experimental labs, we differentiate between measurement uncertainty and error. Measurement error is the difference between the measured value and the true value of the quantity being measured, and is usually unknown due to limited measurement precision. Systematic error relates to an offset built into the measurement, such as an instrument calibration error. Random error is related to the underlying variation in the quantity being measured as well as the limited precision of the measurement. What is more relevant to this discussion is measurement uncertainty. Type A uncertainty describes variation in a measurement due to underlying variation in the quantity being measured, while Type B uncertainty is the result of everything else (resolution limitations of the instruments, etc.).

In this case, we are discussing Type A uncertainty in speed measurements. Usually, multiple sources of uncertainty are combined by adding in quadrature, and we see that the combined measurement uncertainty is dominated by a single source as that source approaches 3x larger than other sources. I.e., if speed variation results in POI changes that are 3x smaller than those due to the precision of the shooter/rifle system, wind effects, etc., then we can essentially ignore speed variation in estimating POI of a given shot.

I’m really not sure where you’re going with the nonlinearity comments, but for clarity, you’re talking about dependent (output) variables that have some non-unity exponential dependence on independent (input) variables.

Barrel harmonics, in this case, have noting to do with specific wave functions or standing waves, and simply refer to integer multiples of the characteristic frequency of the muzzle oscillations. I.e., timing bullet exit with the anti-node of some integer multiple of the periodic motion of the muzzle. Of course, the characteristic frequency of the barrel is dependent on barrel length, stiffness, termination nature, damping sources like bedding, muzzle weights like tuners, etc. In figuring out how to time bullet exit correctly, we can think of it as tuning for consistent acceleration over the length of the barrel, and therefore, consistent bullet exit speed and barrel time, so that the bullet exit is at a similar point in the muzzle oscillation every time. And then tuning the initial position of the bullet (seating depth), and therefore the total distance traveled by the bullet, to time the bullet exit with the muzzle position being at an anti-node.

An alternate approach is to accept some bullet speed variation, but time bullet exit with the muzzle position approaching the top anti-node so that faster bullets exit when the muzzle is at a slightly lower position, and slower bullets exit when the muzzle is at the top of the anti-node. At some distance downrange, vertical dispersion is minimized by slower bullets having a slightly higher trajectory and faster bullets having a lower one, but for other distances, the speed variation causes an increase in vertical POI dispersion relative to the approach that minimizes both speed variation and POI variation.

Yes, and it’s the same method referred to in other posts and used by Cortina, F-class John, etc. I personally don’t use the Audette method because it relies on too much statistical uncertainty for my tastes. I first tune for small SD and then shoot groups with varying seating depths in 0.003” increments to get to an anti-node.

It does. Thank you!

No matter how carefully one loads ammo, there will always be velocity variations.

This is the entire point of the Audette. It isolates the exit time that you have described and minimizes vertical POI variations at long range

Absoluty

That’s correct, and it’s because we usually only talk about peak pressure. In reality, integration over the entire pressure curve is what ultimately correlates with total acceleration, and therefore, barrel time and bullet exit speed. So it’s possible to have different pressure curves with different peak pressures, that integrate to the same total pressure over the bullet path along the barrel, resulting in identical bullet exit speeds.

True, but less is better.

The principle behind the Audette method minimizes vertical POI variations for a given amount of speed variation and a given distance, but perhaps not compared to a load with significantly less speed variation, which minimizes POI variation at all distances.

The ideal approach would both minimize speed variation, and time bullet exit on the upswing of the top anti-node.

Where the muzzle is in it's cycle when the bullet exits can be changed by means of moving weight....a 'tuner' for lack of a better term. If there's no adjustable weight to change where the muzzle is at the time of the bullets exit, other things are used to 'tune' this. Chiefly powder charge, different powders, seating depth, neck tension and different bullets. In other words...normal things we do to 'tune' a barrel.

I did this testing last season with a tuner. Nothing is changed but the position of the tuner as it moves further out the barrel. You can clearly see when the muzzle is at it's 'dwell' points and when it's moving between those two points. It's apparent that the amplitude changes as the tuner moves out. This 'sine wave' pattern repeats over and over again.

I believe the Audette accomplishes both

That's very similar to the target-test I do on rifles used at longer ranges. Have found it's easier and faster than the classic Audette....

Got it. Thanks

How do you control the 'barrel' variable? Barrel temperature plus the condition of the surface of both the lands and grooves must come into play, right?

The problem is that the method doesn’t provide any way to reliably know or confirm that speed variation nor vertical POI variation are minimized. One shot with each load doesn’t reveal much.

It reveals everything. Hate to have to go over it again because you know how it works

Groups are shot with the loads in the node and invariably one of the loads will shoot great with minimal vertical

My method as well to show the "effect" of a higher ES/SD to folks that don't appreciate what you're trying to achieve.

I want single digit SD if at all possible for the longer range guns. I find it easier to achieve in the longer barreled rifles, whereas I really don't sweat it than much in the 300 yds and under hunt rifles/carbines.

Which doesn’t account for barrel harmonics.

You both are assuming the barrel is “pointing” in the exact same place when the bullet exits to lay that much weight to velocities to calculate drop

In a proper node, higher velocity rounds exit quicker when the muzzle is pointing lower, but drop less

Lower velocity rounds exit later when the muzzle is pointing higher, but drop more

The cumulative effect is minimal vertical dispersion and is the characteristic of the best load