Home Forums Hunting & Shooting Long Range Hunting Why do we ignore the horizontal?

Maybe a suppressor mostly just dampens the amplitude (strength) of the vibration. This seems to makes sense because the frequency of the vibration should really only change if the barrel material was changed. Then with a can you also get the benefit of the way it handles the pressure wave after the bullet exits the muzzle.
I'm going to have to re-read Rifle Accuracy Facts now. It has a lot of big brain stuff on barrel vibration. Good thread everyone.

My lightweight Creedmoor is in harmonic tune today.

Jordan,

I started a new job about 9-months ago that requires me to work on vibration problems. I am by no means an expert but have a very basic understanding of vibrations and mode shapes. Vibration analysis is a very specialized field within mechanical engineering that requires a lot of advanced training/coursework and practical experience... so I am just a hack

In our lab, I have done a few "experimental modal tests". We do these tests to understand the natural frequencies and mode shapes of a structure when it gets excited at these frequencies. Remember the Tacoma Narrows bridge!

We basically use a hammer with loadcell in the tip or a shaker to excite a structure. I've only done the hammer test. We suspend the structure with very soft isolators, excite the structure, and collect accelerometer data all over the structure. This data can then be used to create 3D models showing the mode shapes, like the bridge for example.

These different natural frequencies and "modes" are properties of the structure, and it will behave in this way if the frequency is correct for that mode shape. In our tests, the rigid body mode is the lowest frequency where the whole thing moves in space, via the soft isolators. For a rifle barrel, the first vibration mode is generally believed to be a simple bend, with the muzzle projecting up and down... at a certain frequency.

This first mode "shape" is basically the same for most barrels, but the frequency required to create this shape can be changed by altering the stiffness. We can alter the stiffness by chopping the barrel (stiffer) or adding mass at the muzzle (less stiff). Decreasing the stiffness makes the mode frequency lower.

Some BR dudes talk about "slowing" a barrel down. What they are saying is that they want to lower the mode frequencies using the tuners. Their hypothesis is that the bullet exit needs to be during the upswing, and not during the downswing as Varmint Al illustrates at his website.

I'm going out on a limb here, but I think everyone is still thinking about a barrel "whipping" wildly before the bullet exits the muzzle... or "oscillates". Per Varmint Al, this doesn't happen until AFTER the bullet leaves the barrel. The barrel is getting bent (deformed)... as shown in his animations, then the bullet exits.

At Al's site, he listed ~135 Hz as the first mode frequency. This would be 1/135 = 0.007 seconds for one period. But if you assume that the barrel time is 0.001 seconds (also from Al's site), then the bullet exits well before we have one complete vibration period. The bullet exits at 14% of the periodic cycle. This could be during the upswing, but if the natural frequency and excitation creates a higher frequency vibration the barrel could be past the highest muzzle projection and on its way down. Hence, the BR dudes wanting to "slow" the barrel down.

All that to say that I'm not sure that their goal is to reduce the amplitude, as the different modes can be pretty wild looking. Now the actual mode shape and frequency responsible for the muzzle projection, or excitation frequency, is a different story.

Jason

Dog,

If you search for "traveling wave theory", Chris Long describes what he believes is a pressure ring that runs up and down the barrel. I think he claims that bullet exit while the ring is at the muzzle is bad juju.

I'm not sure anyone has confirmed this or not, but remembered it after reading your post above.

Jaosn

4th Point, have you read Rifle Accuracy Facts by Vaughn? I don't have the education to understand a lot of what he talks about, but you seem to.

No, but I'll check it out. Thanks!

Jason

It's probably a good read for someone with an engineering background although dated. I don't have the education to get the most out of it.

For you guys that shoot ladder tests, do you shoot several ladders and look for a trend? I would think that just shooting it once would introduce a the probability of human error influencing bullet impact points, especially at 600 yards.

If you can't shoot, you can't shoot.

Factory ammo is perfect for those who fit the bill

Gee I'm sorry, I thought I asked a legitimate question about the ladder test and repeatibility. Apparently not. Now if you guys will excuse me I'm going to go buy some more factory ammo while rcamuglia walks on water out to his 600 yard target.

I know when I saw you here your factory loads were doing a serviceable job for you!!!!!!!

Yep, that was those factory loaded 7mm STW 168 gr JLKs and that speed goat was only 456 yards. Not even far enough for a decent ladder test!

Probably enuff gun for antelope, I believe..........

I'm calling BS. Everyone knows you shouldn't shoot across water.

It's a good question but indicates your inexperience with long range shooting. If you can't hold on a 2" dot at 600 with a solid bench rest and squeeze the trigger without moving, then the long range Audette won't work for you. Stick to 100 yard development

I usually drive to the target at 600. Another thing you'll learn with some more experience.

LOL.

Now that we have 3 pages of shared knowledge, Lets start again:

If you have a 20 shot ladder at .1 gr one shot steps and you find a 3 shot point (+/- .1) that is very good vertically but not horizontally what are you going to do about it?


...

That's a good question, but only shows your inexperience with long-range shooting........

.1 grain increments are too fine. Generally for .308 size cases .3 grains is good. For magnum cases .5 grains

When you find consecutive shots that have nearly the same POI, you then shoot those individual loads to test. Pick the best and make seating depth adjustments if needed

This was an entire 560 yard Audette. The impacts were all so close together that I didn't bother to mark them. I chose the highest velocity load under pressure