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December 18, 2018, 01:39 PM | #26 | |
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I think I grasp it pretty well, I just think it doesn't apply to cartridge cases the same way it does to a hose. Yes, I understand about fluid dynamics, gas laws, venturis, and other things. The hose nozzle analogy is accurate, for a hose, but not so much for cartridge cases, because there is no changing the "nozzle size". Also, unlike a water hose, where you are using one standard system pressure and varying the nozzle size, cartridges have different pressures, created by the type and amount of powder we put in them. Each one is different. Why does a .357 Magnum run at 35K and a .223 at 55k (or 62k?) not because one is a straight case and the other is bottlenecked, but because of OTHER FACTORS that dictate what the safe and allowable pressures can be. The guns they are being used in is the major factor.
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December 18, 2018, 02:40 PM | #27 | |
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It is one of those for certain things; I know the pressure is going to increase; what I do not know is if the bullet is going to get out of the way before the pressure gets to the point it renders my rifle scarp. I like the ideal of the running start, I want my bullet past the rifling before the bullets knows the riflings are there. F. Guffey |
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December 18, 2018, 02:48 PM | #28 |
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And then there is that thing with 30/06 and 338/06, I wondered and then started on a 280/338 and someone asked 'WHY? And then I mused over the question and the logic behind adding .051" to the length of the case over the 338/06.
And I wondered if it had anything to do with .7854. F. Guffey |
December 18, 2018, 06:06 PM | #29 |
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Thanks MarkCo: I will be looking at choked flows and supersonic flow theory and other equations I have found on the net. Might keep me busy for a bit. LOL
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December 18, 2018, 06:35 PM | #30 |
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No problem dgang. I love the stuff, but most find it to be other than I do.
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December 18, 2018, 06:46 PM | #31 | |
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Sorry the analogy did not help you, but be assured, the mathematical models I have run accounting for all relevant variables, both as a consultant for, and in doing my own product design on gasses from gunpowder and solid rocket propellants have clearly shown that form factor does impact pressures, and constriction of flow most definitely increases pressure. If I had made a mistake on a few of them, it would have made national news. |
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December 18, 2018, 09:22 PM | #32 |
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F. Guffey
F. Guffey: You mentioned necking up a .30/06 to a .338/06. If you were to do so with the same weight projectile to the same velocity, would it take more, less or the same amount of the same powder? Seems to go to the crux of the matter.
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December 18, 2018, 10:41 PM | #33 | |
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You may find a small pressure difference due to case mouth narrowing by the time the bullet nears the muzzle, but the difference is just a few percent. I think QuickLOAD's model does a pretty good job of predicting this. It gives you both muzzle pressure and the pressure in the chamber at the time the bullet base gets to the muzzle. For a 100 grain bullet in the 25-06 fired with Varget to a peak pressure of 60,000 psi, with a 24" barrel the pressure at the muzzle is 21.5% lower than at the chamber. For a 30-06 with a 144-grain bullet (to match sectional density) the difference is 17.7%. So you get about 3.8% lower muzzle pressure due to the narrower bore. At the peak pressure, when the bullet is still moving slowly, the difference is going to be much less. On average you might see a 2% difference in bullet acceleration due to it. Not much.
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December 19, 2018, 12:12 AM | #34 | |
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December 19, 2018, 01:44 AM | #35 | |
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Unclenick: Just out of curiosity, in your comparison of a 25-06 and a 30-06 load what would the optimum barrel length be?
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December 19, 2018, 10:51 AM | #36 |
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5whiskey,
Note that in a larger diameter projectile a same-weight bullet has lower sectional density than its narrower counterpart. That means it takes less pressure to achieve any specific rate of acceleration. Thus, to achieve a specific velocity, less powder is needed with the wider, lower SD bullet. Dgang, I don't know what you mean by optimal. If you read the Houston Warehouse experiments you will conclude that 21¾" empirically seems to be best for rifle accuracy regardless of chambering, but clearly not best for long range shooting. If you mean, what barrel length will extract the maximum amount of energy from the powder? that will be the length at which acceleration just reaches zero. For a .22 LR, that is 16 to 19 inches, depending whose load you fire, the high speed being at the longer end and standard velocity being at the shorter end. After that, the bullet gradually starts to slow down in the bore due to friction, though not enough to matter in a 24" .22 LR barrel. For high power rifle cartridges with their much greater powder capacity, though, a barrel long enough to achieve that is impractically long for many cartridges. With long barrel friction activated, QuickLOAD puts it at 45.6 inches for the 30-06 load, above, dropping to 45.0 inches with a 175-grain bullet and the same powder loaded to the same pressure. Switching to IMR4350, a slower powder loaded to the same 60,000 psi, it grows the barrel to 47.8", so you can see this doesn't change a lot. Maintaining accuracy with a barrel that long whipping around would be difficult, at best. It only gains about 100 fps over a 32" Palma barrel, which is long enough for most folks.
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December 19, 2018, 11:03 AM | #37 |
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Thanks Unclenick, I was referring to max velocity. My guess was around 4 ft. for a .30/06, so I was pretty close. Had just read an article about velocity from shotguns being fastest at 18" to 20", that would put my BT99 at a much slower rate than I had thought. Have never been able to get a good reading from a Chronograph, probably because of shot column, wad, gases, etc. Thanks for your answer.
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