April 3, 2010, 05:07 PM | #1 | |
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Chrono at 10' vs MV
When establishing a velocity at 10 feet via a chronograph, how much difference is there between that velocity and MV?
Meaning....how do I tune up a load to a spec published in MV, at the 10' distance? For instance: how to reach 850MV 230 grain ball, .45 auto, and I'm at 810FPS at 10'
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April 3, 2010, 05:37 PM | #2 |
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Pretty easy to determine *IF* you have a drag coefficient (because it WILL differ from bullet to bullet)...
Back of the envelope (I'll work in metric because it really is an easier system): MuzzEnergy = KineticEnergyMeasured + WorkLost WorkLost = Force * Distance Distance = 10' = 3.0 meters Force = Drag = 0.5 * rho * Vel^2 * Cd * RefArea rho = DensityOfAir = 1.225 kg/m^3 (adjust according to local conditions) Vel = 810 fps = 247 m/s Cd = ?? ...Bah, we'll call it 0.5 (feel free to get "real" numbers) RefArea = pi/4*Diam^2 = 3.14/4*0.45^2 = 0.159 in^2 = 102.6e-6 m^2 Drag = 1.92 Newtons. So... WorkLost = 1.92 * 3 = 5.75 Joules Going back to the beginning.... KineticEnergyMeasured = 0.5 * mass * Vel^2 mass = 230 gr = 0.0723 kg KineticEnergyMeasured = 2.205 kJ MuzzEnergy = 2205 J + 5.75 J = 2211 J MuzzEnergy = 0.5 * mass * MuzzVel^2 => sqrt(4422 / 0.0723) = MuzzVel MuzzVel = 247.3 m/s = 811 fps Now, that's a pretty sloppy methodology that could easily be improved with the use of a spreadsheet and integration methods, but it's close enough to give a pretty good idea of what's going on in those 10 feet.... And it looks like the answer is "not much." |
April 3, 2010, 05:46 PM | #3 | |
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Fantastic much appreciated! Really....sorry to make you type all that.
I'll go with one tenth of a grain and call it a day.
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April 3, 2010, 06:12 PM | #4 |
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Actually, the problem that you're likely to have with the chrony 10 feet from the muzzle is a velocity in excess of muzzle velocity.
When the bullet first exits the barrel, it effectively "uncorks" the very high pressure gas behind it. The gas then expands VERY rapidly and can actually push the bullet to beyond muzzle velocity. I have been told that a chrony distance of approximately 25 feet will actually give true muzzle velocity because the bullet will have lost that extra little "boost" at that range.
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April 3, 2010, 06:17 PM | #5 |
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Is velocity REALLY measured at the muzzle?
Wow!, Inigomontoya, that's a lot of arithmetic! And no small amount of math, either.
Would this be easier? After recording velocities at 10', set up the chronograph at 20' and fire one or two more strings of a load previously clocked at 10'. Then determine the velocity loss factor from 10' to 20' and apply that factor (in reverse) to calculate velocity at 0'? 1100 fps @ 10' 1075 fps @ 20' velocity loss factor: (1075 / 1100) = 0.977272727 1100 / 0.977272727 = 1125.6 fps But, to answer the original question: I suspect that published data is not actually measured at the muzzle. Too much turbulence, smoke, unburnt powder at the muzzle. I have never seen anyone at the range set up their chronograph closer than 8-10 feet, and for more powerful cartridges, further out than that. If anyone in the gun business knows for sure the answer to this question, "Is muzzle velocity really measured at the muzzle, or is it measured elsewhere and calculated back, or is it measured a standard distance from the muzzle and considered close enough?" Maybe SAAMI knows? Lost Sheep Last edited by Lost Sheep; April 4, 2010 at 03:36 AM. |
April 3, 2010, 06:23 PM | #6 | |
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Now we're in a ballpark that will let me play.
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April 3, 2010, 06:23 PM | #7 | |
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April 3, 2010, 06:29 PM | #8 | |
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Well then...this one's toast. Thanks all.
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April 3, 2010, 06:42 PM | #9 |
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LostSheep, I'm not in the gun industry per se, but I am involved with tests that involve guns (guns are tools to us, not the end all). We're VERY cognizant of where the measurement was made. If we speak muzzle velocity, we backtrack the velocity as I did in my first post in this thread (albeit using more rigorous methods). We also make measurements at the target. And sometimes we track the projectile with high speed photography and make half a dozen different determinations through the projectile's flight path.
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April 4, 2010, 04:07 AM | #10 | ||
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Thanks, InigoMontoya
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For my own purposes, I don't care what the velocity at the muzzle is. Because I am comparing my loads to others of my loads, usually with the same bullets (same ballistic coefficient). And I use the same distance from the muzzle all the time. So, if all my velocities are off by the same percentage consistently, I could not care less. (The story would be considerably different if I were shooting long distances and velocity, trajectory and velocity loss were significant, but I am not there yet.) The original poster seemed to be trying to precisely duplicate a factory load (and if memory serves, 850fps 230 grain ball is the original military specification in 1906? for the 45 ACP). That said, alloy, if you do succeed in reproducing the exact velocity you seek, it still may not exactly reproduce the factory load. External ballistics would be the same, but internal ballistics may be a little different. A little more of a fast powder or a little less of a slow powder may give the same muzzle velocity, but inside the gun, the pressure curve over time will be different, resulting in more or less residence times in the barrel for one powder over the other. That, in turn will affect how much muzzle rise the gun will experience before the bullet leaves the barrel, which will affect the point of impact at the target. Small differences, to be sure, but I have no idea how precise your needs are. Quote:
Good Luck. Lost Sheep |
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April 4, 2010, 04:34 AM | #11 | |||||
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I'm well under a max load which is 8.0 grains of sr-4756. I've currently been shooting 7.4 grains and now that the range dried out enough for a tripod I thought I would eek the load up a bit to reach the desired original velocity, since I don't think I will be reaching that max to do it. When I looked up the spec, I saw it was(at the first source anyway) published as a muzzle velocity. Quote:
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I figure the chrony is a tool as well as a measurement device, a couple of my loads last fall weren't near what I wanted(most were)....one 38 special plinker in particular. Right out of the book...650 fps A few loads were surprising, and published data is sometimes amazingly accurate, and sometimes amazingly inaccurate.
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April 4, 2010, 05:37 AM | #12 | |
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Published data was published by humans
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You might be interested in the tests conducted by a trio of guys (I think there are three of them) whose results are posted on a web site, "Ballistics by the inch" Happy Easter Lost Sheep Good night |
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April 10, 2010, 09:41 AM | #13 |
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Well, there were three of us initially, then we had a fourth join us for the second round of testing last spring.
And just as a point of reference (explained in our site protocols), we used a pair of chronys, the first at exactly 15' from the gun rest, the second immediately behind that. Cheers! Jim D.
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James Downey http://www.ballisticsbytheinch.com/ - BBTI handgun cartridge research http://www.guns.com/author/jim-downey/ - Guns.com where I'm a features/review writer http://www.communionofdreams.com/ Communion of Dreams - my current novel. |
April 11, 2010, 10:50 PM | #14 |
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Thanks For This Post
Thanks For This Post because it has led me to "Ballistice By The Inch". It is late now but I can tell that it is good stuff and I am going to read and study over the next couple of days. I have been thinking about creating a post about powders, bulk density of powders, cartridge powder capacity and barrell lengths and I think this will make for better questions.
Later, James |
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