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Old November 13, 2012, 08:05 PM   #34
Unclenick
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Join Date: March 4, 2005
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And that's sort of where I am. I don't entirely buy the OBT theory, as I've posted in other threads, but the idea is interesting to investigate and seems to predict results fairly accurately in many instances.

The issue with estimating sustained ringing is about the losses. If you whip a barrel on any axis in a stocked rifle that becomes a tail wagging a dog, and there are lots of opportunities to transfer energy from the barrel to the stock. That makes it hard to say a steel property necessarily had a lot to do with dominating how fast the ringing damped out. Even a metal that's not hard (brass bell) can ring for a while, provided its elastic limits aren't exceeded and it isn't transferring energy out of the system to anything other than air. How much energy is lost as heat due to the flexing the metal itself, I just don't have information on. I’ll try to look for some. I’ve got about half a dozen things backed up now that I need to go the university reference library to find. The web doesn’t have everything yet. I could also make some measurements.

A source for a compression wave is pretty easy to come by. If you look at hoop stress equations you find that any time you apply radial stress from the inside to expand a tube, the expansion pulls on the tube longitudinally. I've gone through the hoop stress equations to estimate muzzle strain from that. It didn't appear to directly affect the bore diameter more than a couple of ten thousandths of an inch in most instances. But that's a static analysis and not a dynamic one. It needs to be born in mind that the OBT theory is geared toward finding only the neutral dead spots in the compression wave.

As Bart suggests, hoop stress due to pressure would create two such waves, with one going forward down the tube and the other making the short trip to the breech. From the breech, the rearward traveling wave reflects back forward to chase the other going forward. The OBT calculator does, indeed, create differently spaced null times resulting in pairs of barrel times that aren’t too far apart, followed by a longer period during which no null spot appears.

As an aside, in an email correspondence with Chris Long several years ago, he said he had one rifle that defied his calculator completely. It was in a custom receiver heavy barrel .308 bench gun. He said he machined the threads as class 3 super tight threads, and they were so snug he couldn’t thread the barrel by hand. The whole length needed a barrel vice and wrench. He concluded that the tight thread had tied the receiver strongly enough to the barrel that it became an extension of the barrel, greatly spacing the vibration period out.

Possibly apropos of that last paragraph, Harold Vaughn, in his book, Rifle Accuracy Facts, had some illustrations of normal thread loading, showing 36% of the load is on the first turn of a barrel thread coming off the shoulder. This is because the further away from the barrel shoulder a thread is, the greater the span of material that can stretch. The end result, he showed, is the rearmost threads can actually slip radially, allowing more barrel swing. In effect the barrel is pivoting around the first turn, then. This indicates the tie to the receiver, from the stand point of transmitting a pressure wave, is not normally great.

Vaughn also shows there are a lot of recoil moments in a real gun. Even just the asymmetry of having a gas vent hole drilled on one side of the receiver and not the other produces an asymmetry that causes a moment he could measure. So it's sensitive and lots of opportunities exist for the system to introduce odd deflection angles and opportunities to dump energy.

As to the OCW, I’ll repeat myself: it doesn't depend on the OBT theory being true to work. The OBT theory is just one possible explanation, but the OCW system itself is dependent only on observation. While everything Bart said about sensitivities of group sizes at different ranges is true, I’m not seeing how that affects the OCW concept. The OCW concept is just to develop loads that mimic the accuracy and broad applicability observed to exist in good quality factory match ammo. It’s not trying to be the equal of hand-tuned loads for a particular gun at a particular range. The OCW development system eschews all benchrest techniques, like case prep, and uses fully resized factory brass and factory COL’s, then sees how far you can get with that toward making a load that works well in a range of rifles. It makes ½ moa groups possible at shorter ranges in many instances, but it’s not geared toward bugholes.

I think you can use a target round robin like Newberry’s to help find bughole loads for a particular rifle at a particular range. In that regard it may be used like a latter day Audette ladder, but having higher resolution and every dispersion axis taken into account. But if you’re going to use this round robin method with prepped brass and to check the effect of changing seating depths on a load, then you’ll want to be firing your tests at the range you intend to shoot the load at, and accept that the final load may only be really good in your particular rifle at that particular range. If that’s the case, by definition, it’s not an OCW load, but a gun-specific load.

In this thread the OP asked how to mimic a factory match load, and that’s what makes the OCW concept so applicable to it.
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Last edited by Unclenick; November 14, 2012 at 11:47 AM. Reason: typo fix
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