People naturally assume that as the temp drops, so will the impact of the bullet because in external ballistics we all know that relative air density increases, and in internal ballistics, cooler temps mean the primer lights off a little slower, the powder oxidizes a little slower, and the colder barrel acts as a more influential heat sink, countering the pressure slightly that the oxidizing powder can build in a fixed barrel length.

Here is the RUB:


Where is your ammo tuned on the 3rd mode vibration cycle????


At what range are you shooting???

If you are tuned before the very top of your vibration cycle in warm temps, at shorter range it is possible to create a scenario where your bullet hits higher on the target, or in the same place as it will at the higher temps.

The possibilities become even more complicated when the gun is hot and the ammo is cold, or the gun is cold and the ammo is warm, and how all this interacts with barrel vibrations.

Most guys have a Fluke 77 multi-meter or something similar. Snap-On sells a probe that you can touch to the brass that will plug into your multi meter and give you an instant temp reading of the ammo. This is very handy for testing, and I used to use it all the time. One of my employees trashed the probe trying to check the temp of a cast iron differential casing while preheating it before welding. They are very handy, just do not touch them to anything hotter than 800 degrees fht.

What I have learned over the years to get the best results in an unpredictable world.

1) Design a system with a barrel length that tunes just past the top of the vibration cycle when near max pressure. Back off slightly on your load so the ammo is tuned just before the peak of the cycle. Pick your bullets and powder accordingly.

2) Only use temp insensitive powders for hunting/unknown range work. The interaction of larger velocity variances with barrel harmonics becomes too complex and too time consuming, not too mention what it does to barrel life and lead time getting a system ready to kill.

3) Keep your ammo warmed to body temp at all times until just before shooting. Do this while tuning, hunting, practicing....MAKE IT A HABIT. This constant will pay dividends as other variables change on you. Powder and primer temp inside the case is the biggest variable to initial velocity. Make it as consistent as possible. When bear hunting I keep a couple of rounds in the gun for any short range stuff that may pop up, or when walking around. But when I acquire a long range target those rounds get exchanged for two warm ones out of the ammo wallet up against my body just before the shot.

4) Vertical variance caused by relative air density changes is not linear, and the interaction of this with internal factors like muzzle velocity variance and barrel harmonics is too complex to keep in your head...so KEEP A DETAILED SHOOTER'S LOG BOOK and write down what happens under various conditions RELIGOUSLY!

5) For Big Game sized targets, focus your data collection effort past 500 yards. A properly tuned rifle will show insignificant changes for large targets inside 500 yards, Especially if your ammo is a constant temp.

6) Take advantage of any opportunity to tune a load near the place, altitude and time and temp that you will be hunting.


How to really screw things up:

Buy a rifle that tunes up on the low pass in the vibration cycle at max pressure, tune your ammo short of the bottom (safety margin from max pressure), use a ball powder like 872 or 870 or 8700, keep your ammo in the gun all night instead of your sleeping bag, leave your gun outside the tent but uncovered so as much moisture can creep in as possible, and believe things like my rifle always hits a quarter minute low when it is cold. Leave your log book on the fireplace mantle at home when you travel so your wife can use it as starter fuel for next fire she builds once you are out of sight and out of mind.

quote:

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1) Design a system with a barrel length that tunes just past the top of the vibration cycle when near max pressure. Back off slightly on your load so the ammo is tuned just before the peak of the cycle. Pick your bullets and powder accordingly.

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Can you give a few more details here? How do I know, when a barrel is at the top of the vibration cycle?

I hadn't seen you started this discusion here, so I'll repost here what I just did on the other thread.

S1,

You bring up some real good points, and I'll try keeping the ammo stabilized near body temp verses ambient and see how it works out, it would significantly reduce the effect from that variable.

A friend has a laser type temp probe, works very, very, well. I have a multimeter temp probe, never have used it in anything but liquid though. I'll check it out and see how it works to check ammo and barrel temps.

4) Vertical variance caused by relative air density changes is not linear...

Please explain what you mean there. I have just looked at how the various ballistics programs modify bullet path at different temps and pressures, not a whole lot of hard data yet.

Vibration cycle -
We're talking nodes on the vertical, correct?

If I understand this right, if your group moves up on the paper with higher and higher MV as you increase the load, then it moves down with further increases, you've identified a node.

Question I have now is, is this the top of the vibration cycle as you describe it, or the bottom?

When you posted: Buy a rifle that tunes up on the low pass in the vibration cycle at max pressure, tune your ammo short of the bottom (safety margin from max pressure)... in regaurd to a less than desirable state of tune, I had thought this was the desirable point to load for if it could be indentified.

Let me clarify what I mean, I could very well misunderstand you.

If I tune a load in higher than normal anticipated hunting temperature with the anticipation of shooting with possible lower MV on the hunt or further practice, I don't want the barrel vibration to come off the node in a downward direction as the bullet exits. This coupled with the loss of MV would cause much lower POI than would be expected from just the loss of MV with no change in muzzle direction due to vibration.

If I tune it on the node which produces a rise in POI with lower MV's if it swings off of it and shifts high, this cancels out some/all/most of the effects of the lower MV encountered.

This has been my understanding of tuning on the correct node, which the way I understand it is the bottom node is the one to tune on at your highest MV in high temps, a low MV at low temps on the top node.

Is my understanding completely opposite of yours, or are we saying the same thing in a different way, or something in between the two?

Now, I've got some groups I shot the other night that POI was .25 MOA lower than what seems to be the norm, but temp hardly varied at all, BP change was very small, ammo was stable to ambient temps it was initially tested in. Fired some groups at 500-600-700-800 the other night again and the drops were riight back to where the norm seems to be. I wish I had tested the MV with every shot fired throughout this last week testing, as I have not tuned the load to a node and may just have had the MV fall off/increase enough to put me between them. More testing, with the 35P next time...

Brent...We are not saying the same thing....(I think)

A faster bullet exits the barrel earlier in the sin wave, a slower bullet later. If you tune to just before the trough (bottom) a faster bullet should have a slightly higher launch angle. A slower bullet will have a slightly lower launch angle.


Tim...."Can you give a few more details here? How do I know, when a barrel is at the top of the vibration cycle?"

You can see this in the incremental loading process at close range, 50 to 100 yards. In that close, gravity drop does not affect the bullet placement on the target near as much as barrel harmonics. As you increase your loads by .2 percent, the group centers will gradually rise or gradually fall relative to your point of aim on the target.

Okay, my understanding was exactly what you just described, only some how I managed to twist it around in my head and envisioned the opposite. I had the sine wave in my head with the slow bullet leaving sooner, a left to right, low to high thing stuck in my head. Thanks for the clarification, I needed it!

In this case, my other load with Retumbo I'm on the top node at top MV, no way to get to a bottom node without backing down the load less than I'd have liked. Haha, now I see I had it switched around in my head and I'm right were I wanted to be the whole time!

You are the first one I've ever heard advocate 50-100 yard testing for vertical though!

Recoil effect with larger MV changes is another thing to consider. I was out with my Son's 308 Win last month and saw this in the biggest manifestation so far. I had some 150gr loads, at 2850 (20" bbl) all dialed in and grabbed a 165gr load that chronoed at 2400 and printed a group 3.5" higher at 100 yards!

I usually do not advocate tuning at 50 to 100 yards. If it is a system I designed, I already know if it will tune on the high side or the low side of the sine wave and have chosen my bullets and powder accordingly.

Tuning at short range is just an easy way for someone with less experience so see what is happening. I usually tune at 200 yards and then check it at 600 yards for minimum vertical dispersion, and look on the chrono for small velocity deviations if I am considering this combination for Ultra Long Range Work.

You can change powders and bullet weights, and often get a gun to tune on the high side or the low side at max pressure depending on the combination chosen. This is very easy to do in calibers where there is great variance in available weights and powder speeds.

If your gun is wanting to tune on the low side, and you would prefer it not, just change bullet weight and possibly powder also.

Good stuff.

I've also seen primer changes often alter pressure around 3,000 psi, sometimes more or less but, them and OAL changes may not affect accuracy but may effectively move POI to where you want it on the sine wave.

I wonder if this is one of the main reasons why many people advocate tuning a rifle with a given bullet weight to a certain MV range. I assume weight-length-contour of barrel, condition of threads-stock composition-muzzle brake etc. probably plays a role in this decision also? Wouldn't/couldn't any of these changes alter the sine wave alone?

Interesting stuff.

I think it is time for a discussion about what variables affect barrel harmonics in a prioritized way. Here is a list of things in descending order from most influential to least influential, that my testing experience has told me affect the 3rd mode vibration cycle that a free floated barrel enters when fired.


1) Pressure curve

2) Bullet Weight

3) Shoulder pressure

4) Position of buttstock on shoulder

5) Neck Tension

6) Torque

7) Barrel taper

8) Support (front and rear)

9) Standard deviation of Velocity

10) Seating depth.


I am sure there are many more things to list, some of them like 'Magnum Primers' are very influential, but for me fall under the category of Pressure curve. My list assumes that you have a quality device to shoot with, and have exceptional control of ammo quality, I think things like bedding are fundamental, and if not done perfectly, the discussion of influencing barrel harmonics in a useful and consistent way would be meaningless. Stl has brought up this same issue as it related to the shooter, fundamental have to be excellent to realize any benefits from what we are discussing. This is why shoulder pressure and buttstock position are so high on my list. There are fixed elements like action asymmetry that play a big role in barrel harmonics, but the average shooter does not have the resources to change this outside of choosing an action of a different design, so for practical purposes this was not placed in my list.

A little tangent to the subject I think but some clarification if you don't mind. Regarding the tuning of loads relative to highs and lows of the sine wave, my sense of the previous posts is that it is done in the context that the wave are vertically oriented. If that is the case, why? I've seen discussion about tuning this way but...are barrel vibrations always vertical in a floated installation?

Not trying to sharp shoot here, just don't get it.

Good question....



Because of the position of the recoil lug, the stock being below the bore centerline, and action assymetry and the moments created from the interaction of the above conditions.....


The typical bolt action rifle with a free floated 7 taper barrel will vibrate about 4 times as far in the vertical axis as the horizontal axis.

S1,
Great observation on more vert vibration.
And as the barrel warms up and expands, this vert dispersion gets greater than horiz.

One of the correlations I've seen with QL and very good loads is matching "Time Elapsed Since 10% Max" with "Barrel Time" nodes.
If both times in ms fall on a node for the barrel length.
The TES10%Max will be a node faster than BT. It could be 2 be 4 nodes faster depending on powder.
I've noticed the further apart the nodes the less accurate the round. 1 node gave me best results.
IE: a 26" barrel Nodes5 and 6.

quote:

3) Keep your ammo warmed to body temp at all times until just before shooting. Do this while tuning, hunting, practicing....MAKE IT A HABIT. This constant will pay dividends as other variables change on you. Powder and primer temp inside the case is the biggest variable to initial velocity. Make it as consistent as possible. When bear hunting I keep a couple of rounds in the gun for any short range stuff that may pop up, or when walking around. But when I acquire a long range target those rounds get exchanged for two warm ones out of the ammo wallet up against my body just before the shot.

Sorry for my ignorance,
but if the ammo is warm and if I put it in a cold rifle that could cause condensation of water (in the cartridge). Right?

Slip

Not if your ammo is sealed up properly, no worries. I am going to do an article on how to properly load ammo for field conditions in the next week or so.