Bore diameter expansion during the entire process of firearm discharge .

M

Metal god

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This question comes from one of my threads on another forum . I'm currently in the middle of a OCW ( optimal charge weight ) test http://optimalchargeweight.embarqspace.com/#/ocw-instructions/4529817134 and OBT ( optimal barrel time ) just came up . There is quite a bit of thought that went into it and I'll link the paper on it below .
http://www.the-long-family.com/shock wave theory summary explanation.pdf

One of the theory's is that the diameter of the bore it self expands and contracts with the shock wave created by the firing process ( barrel harmonics )

The theory is you need the bullet to leave the barrel when the bore diameter is at it's least at the muzzle when the bullet leaves the barrel . If true it would seem to be reasonable I how ever don't completely understand the bore expanding and contracting part . If the expansion follows along with the shock wave and the shock wave travels up and down the barrel a few times before the bullet exits . What is expanding the diameter of the bore ? I can see how the bore can be expanded do to pressures behind the bullet but how does the expansion get past the bullet and follow the shock wave ?

After reading the whole paper . I see he gave great detailed graphs measurements and other detailed info on his "THEORY" . He then only said he ran the test with a strain gauge but did not give the same detailed info that actually confirms the "THEORY" I was expecting to see his Theory graphs side by side with the actual stain gauge graphs/data showing they were the same . Or did I miss something actually showing the theory was confirmed ?
 
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One of the theory's is that the diameter of the bore it self expands and contracts with the shock wave created by the firing process ( barrel harmonics )
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It does not expand, but it does vibrate. If it expanded you would get work hardening and splitting/cracking, just like you do with a case. We know that doesn’t normally happen even after thousands on rounds.
 
Don't believe everything you read on the Internet. However, barrels do expand in diameter and they twist too, but those modes are less significant that the several bending modes. Take a look here for more information:

http://www.varmintal.com/amode.htm

The Theory of OBT doesn't match my careful testing. The so called "node calculator" doesn't match any of my known good loads for quite a few bullets, many powder types, and several rifles. Often the results are the exact opposite from what shoots best in my rifles. That's because barrel expansion and contraction is much less important than muzzle angle.

It makes sense that if you ask Quickload to suggest several recipes using different bullets and powders which have the same barrel time as your "good stuff", then you might expect similar precision. Sad to say, my tests show that to be untrue as well.

Others disagree of course, but I suggest you do your own testing.
 
In theory I should be able to cut a dog knot on a barrel to prevent the bullet from expanding the barrel diameter, that should give me one of two results. One the bullet could stop or the bullet could be sized down.

Then there is my rifle with the bedded barrel; the barrel is bedded on the top, bottom and sides along with the receiver. It is so bedded the bedding around the barrel is also the recoil lug. It a most accurate rifle, the only improvement I can make to the rifle is to increase the distance the firing pins travels before it hits the primer. There I have choices; I could increase the firing pin spring pressure or increase the distance the firing pin travels but when they say 'short hook-up' I am thinking I should apply the leaver policy. I will leaver the way I found-er.

F. Guffey

Then there are barrel with bosses like the old Model 70.
 
Barrels do expand due to the pressure. This is why a strain gauge is able to measure pressure. It actually measures the expansion and depends on it to be proportional to pressure, which it is. You can do a hoop stress calculation to see how much expansion their is, but it will change inversely as the difference between bore and profile thickness changes and directly as the pressure changes with bullet position in the barrel. The reason this does not have much effect on bullet diameter is that the pressure is behind the bullet and trying keep up with it, and not along its sides. There is some upset of the bullet by acceleration that affects its friction with the bore. This is greatest at the pressure peak, of course, maybe a couple of inches into the bore, which is why copper fouling is greatest there. But the bullet has a little bit of spring to it and it's enough to recover from the amounts of distortion involved (on the order of an average of a couple thousandth of an inch in diameter in most instances; the bore more, the exterior less).

Varmint Al did some sophisticated analysis of barrel harmonic modes on the Lawrence Livermore Natinoal Laboratories fancy FEA analysis software. While he was able to model eight different vibration modes of in a barrel, he concluded these mainly ring after the bullet clears the muzzle, and that recoil moments and pressure distortion are the main things that determine the muzzle swing position at the moment of bullet exit. This fits with Harold Vaughn's measurements of recoil moments and recording of their effects on group size and shape. This swing still has a harmonic component, but it's not tuned to the same "pitch" as the barrel by itself is, and the effect of pressure distortion is why.
 
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