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Old November 7, 2012, 05:46 PM   #27
Unclenick
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Join Date: March 4, 2005
Location: Ohio
Posts: 10,188
Quote:
Originally Posted by jimro
"Free Space Loss" of a traveling wave comes into play (the authors epiphany of guy wires transmitting a wave is stupid, there is no fixed point at the muzzle to reflect energy back towards the action) . . .
Jimro,

The place the guy wire epiphany fails by analogy is not the anchoring of its opposite ends, but rather that the observable reflection is a transverse wave, like those in Varmint Al's FEA work, rather than a compression wave as the OBT concept describes. But still, I can see how the visual would encourage someone to contemplate all modes of wave travel through steel.

I assume you get this, but for anyone not following I’ll add that you don't need an anchored end to get compressive wave reflection. All that is required for a pressure wave to make a return trip is for the stress packed into the dead end of the travel to relieve itself by expanding back in the direction from whence it came. Think of the muzzle steel compressing locally as that stress arrives, then, because the steel has good elasticity, it snaps back to shape, sending the compression zone back the other way.

The elasticity of steel isn't perfect, and you do lose a little of the energy as sound and heat with each cycle, but steel is pretty efficient in this regard. Consider that a steel tuning fork rings audibly for many thousands of cycles before gradually fading to silence. Obviously that efficiency is affected by the hardness of the steel, which is spring hard in a tuning fork, and barrel steel can't be that brittle. So a barrel will have more loss per cycle. But even so, if you're talking hundreds of cycles in barrel steel rather than thousands as in the tuning fork, you're still sustaining barrel distortion that is little changed during the few cycles it takes a bullet to clear the muzzle.


Bart,

Dan Newberry can probably defend himself, but I don't see how his OCW system is in any way dependent on the OBT theory. Newberry and Long know each other, but I think that's a case of the theory (OBT) being embraced after the OCW system concept originated because it seemed to predict observed results, and not the other way around. Correlation is not causation, however, so I expect Mr. Newberry would entertain alternative theoretical explanations for the observations, too.

If you read Newberry's pages, you find he began by observing the GMM .308 168 grain SMK load worked very well in a large variety of rifles with various barrel lengths. Maybe it wasn't the very best load in all those rifles, but was consistently pretty darn good. He calls it a "Chocolate Ice Cream load"; maybe not everybody's favorite flavor, but liked at least pretty well by almost everybody. He reasoned that there would likely be other combinations of components that would be similarly pretty darn good for a large range of rifles and set about figuring out how to identify these. He settled on the best indicator of a candidate OCW load being a generous span of powder charge maintaining the same point of impact at moderate ranges (up to 300 yards). That candidate then needs to be verified by a number of people testing it in different guns before being accepted as an OCW load.

That's what the OCW system does that I can see. No theory is required to make it work; just empirical testing.

Even if you don’t buy into Chris Long’s idea about the pressure wave and stick strictly to Varmint Al’s transverse barrel deflections, you still want to hit an optimum bullet exit time, as otherwise Audette ladders would never show a predictive result, even with thin, whippy barrels that exaggerate them. Nor would barrel tuners work to synchronize muzzle deflection with bullet exit. I would distinguish between a calculated optimum barrel time (theory dependent) and the empirically determined barrel time of any apparent sweet spot load, OCW or otherwise. I would expect any such established best barrel time to work with different component combinations. Muzzle velocity alone does not determine barrel, which is why your observation that copying a velocity does not necessarily get you to a sweet spot is right.


Dan Newberry,

Welcome to the forum.

There are two reasons I recommended the Remington and Lapua cases to the OP rather than Winchester. One goes to Bart's point about velocity not being a reliable stand-alone performance predictor, and I had recommended a chronograph as a load finder in this particular instance. The Remington and Lapua cases are closer in capacity to that of the Federal cases than Winchester cases are. For that reason the same powder charge will tend to produce similar velocities and barrel times in his particular rifle. If he were to use the Winchester case, as you say, it would take more powder to get to the same velocity. Because that larger charge weight makes more total gas, it will achieve a matching velocity with a lower ratio of peak pressure to average pressure. That means a smaller portion of the total acceleration happens in early bore travel, so barrel time is longer. That could move him off a sweet spot’s barrel time and force him to go for a slightly higher muzzle velocity to get back to an optimum charge weight.

The other reason is that I've noted a number of complaints about Winchester brass recently, particularly on the CMP forum; primer pockets getting loose in just a few reloads and unusual numbers of split necks, in particular. All the kinds of complaints you more often see about Federal brass. I have to wonder if the move of Winchester’s center-fire ammunition manufacturing from Alton, IL to Oxford, MS, announced in 2010, is having an adverse impact. You lose skilled labor when you do something like that, and I don’t doubt there’s a learning curve for new equipment operators.


For anyone interested in temperature sensitivity:

Board member Denton Bramwell published some experiments in which he found the correlation between pressure and barrel temperature to be much more significant than powder temperature. The temperature stability of the powder burn rate alone would be more important for first shot consistency in a cold barrel, but once the barrel warms up, that apparently ceases to be the dominant term. The only theoretical hypothesis that comes to mind is that the coefficient of friction is increasing between the bullet and the bore as the barrel temperature goes up. It’s normal for friction coefficients between pairs of solid materials to increase with temperature, though I don’t have numbers for steel and gilding metal specifically.

So, to minimize sensitivity to the barrel being hot, I think what’s needed most is a powder whose burn rate changes least as pressure increases. Burn rate charts just show how different powder burn rates compare under a standard set of test conditions. As soon as you start changing chamber pressures, their burn rates change and which is faster or slower can change places.

In the Precision Shooting Reloading Guide, Dave Milosovich describes loading a .308 to a series of fixed velocities using IMR 4895 and IMR 4064 under a 180 grain bullet. At 2200 fps it took a heavier charge of 4895 than 4064. At about 2400 fps they were about the same charge weight. At 2500 fps and higher a heavier charge of 4064 was. As pressure and velocity increased, 4064 had its burn rate increase less. Because it tended to burn more consistently under changing pressure, it should be less affected not only by temperature, but also by barrel temperature, charge dispensing errors, or any other factor that changes pressure. The results amount to 4895 producing 67 fps/grain and 4064 producing just 47 fps/grain.



Unfortunately, there does not seem to be universal corroboration of the above. Hornady’s data (for their 178 and 180 grain bullets) shows no significant difference between the two powders in fps/grain over their span of listed velocities. Hodgdon gives IMR 4064 under the 175 grain SMK an advantage, but only about half as much as Milosovich reported. But then, we know nothing about the barrel temperatures they allowed during testing or how much was actually tested and how much was calculated. About the only thing all three sources seem to agree on is that Varget has the lowest fps/grain in .308 loads with bullets in that weight range, ranging from 36 fps/grain (Hodgdon) to 45 fps/grain (Hornady). IMR 4064 and Varget both have earned outstanding reputations as match load powders.

The disagreement suggests you need to run a test for yourself under your typical shooting conditions at your or maybe even under your range of shooting conditions. You don’t need to go through all of Milosovich’s steps. Just average a good load’s velocity (not too near maximum pressure) and knock the load down maybe 5% then measure enough rounds at that level to get a good average. Divide the difference in the average velocities by the number of grains the charge difference was, and there’s your comparative number. Unless your upper load is causing a problem like forcing uneven bolt lug contact to iron out or is starting to exceed maximums and is stretching steel, that should be all you need to predict velocities from other charge weights.
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