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Muzzle Velocity?
D@D
Member Posts: 4,407
When you read on a box of ammo & they state the muzzle velocity how far away is the end of the barrel from the chronograph? The reason I ask is I bought an F1 chrony & it says I need to be at least 10ft away from the rifle. Do factory ammo makers use something different or do they also test from a distance of 10ft? When I think of MV I think it should be close to the muzzle. I guess what I'm asking is do all ammo makers test MV from the same distance or does it very from ammo maker to ammo maker?
Comments
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10 feet from the muzzle is to protect the chronograph itself from muzzle blast and ejected powder; I don't think it has anything to do with the actual velocity measurement.
I don't think the measurement distance is actually standardized in the industry, but in practice, I doubt it makes any real difference in terms of measured bullet velocity. And remember, ammo makers are usually testing from long "test" barrels, not necessarily actual guns.
The amount of velocity loss of any conventional centerfire rifle bullet in the first ten feet of travel out of the barrel is going to be absolutely negligible, in the amount of a few feet per second.
The exact loss will depend on ballistic coefficient, caliber, initial velocity, humidity, elevation, temperature, etc. The velocity loss also isn't linear (ie the bullet doesn't loss the same amount of velocity for every yard from the muzzle).
But as a first approximation, a typical .223 will leave the muzzle at 3200 fps and will be travelling at about 2900 fps at 50 yards (ie 150 feet). Extrapolate that 300 fps/150 foot loss over the first 10 feet and you're only going to see a loss of about 20 fps in that distance. You'll see roughly the same overall velocity loss from muzzle-10 feet from a .22-250 as well.
A typical .30-06 or .308 (which use heavier bullets with better ballistic coefficients) will only see about a 100-150 fps loss over the same distance. So now you're only at about 10 fps (or less) loss from muzzle to 10 feet. 30/30, 7.62x39 or the like are somewhere in between those two.
Bottom line. . .forget this. Unless you've put your chronograph ridiculously far downrange, or are looking at really low ballistic coefficient rounds moving slowly (eg BBs or air pistol pellets), loss of velocity between the muzzle and chronograph is just not a significant enough factor to worry about.
In reality, you'll probably see more normal shot-to-shot variation in velocity between different rounds out of the same box of ammo than you will between chronograph at muzzle vs at 10 or even 20 feet away.
Usually results in better accuracy. It is really your most important feedback of what is going on. Barrels vibrate, fps is bullet time in the barrel exiting at a constant point of motion.
An exception to calculating MV is the U.S. Military. Their velocity figures were given for 78 feet from the muzzle. Screens were placed at 28' and 128' from the muzzle which gave them a base distance of 100 feet.
Most shooters chronographing their own handloads will put their first screen at a distance that will avoid the muzzle blast. Screen spacing depends on the chronograph being used. The chronographed velocity is what is recorded midway between the two screens, which is usually around 10 to 15 feet. That number can be adjusted to MV with the addition of approx 10 to 15 fps for most rifle cartridges.
Today, ammunition manufacturers have much more sophisticated equipment for measuring velocities. They may use things like dopplar radar or other electronic measuring systems that we can only imagine.
And, contrary to some of the statements above, chronograph readings are really a measurement of internal ballistics. Except for only very small effects, things like air temperature, elevation, humidity, caliber, bullet shape, etc. affect only the external ballistics, not internal.
The sound of a hair splitting. [:D]
The same ammo, fired from different rifles, can vary by 300fps (or a little over 10%)in a 30-06 class cartridge. The only difference being the chamber/throat of the rifle being tested (manufactures use a minimum spec barrel, the further you get away from minimum the lower the pressure and velocity)
Velocity at the muzzle or 10 ft. away?
The sound of a hair splitting. [:D]
You are correct. But, not everyone chronographs at 10'. I chronograph at 12'. Others at 15'. The military at 78'. The old wire chronographs used 2'. So, in order to compare ballistics, it's a good idea to convert the chronographed number to MV.
Nit-picking? Probably. But why bother to chronograph if you are not going to get the most accurate number possible. You might as well estimate the whole thing and spend the money you'd use to buy a chronograph for ammo or beer.[;)]
Ray
Lets break it down here...Chrono-graph. Chrono, meaning time, and graph is the plotting of measures of time on a chart. Each time is a point, and the plotting of more than one point, is a graph.
To go further, a chronograph as we have come to know it, is nothing short of a timer, that has an added math co-processor in it. The timer itself, is started, and stopped, by two switches, called sky-screens. They are opto-electric switches, that when an object passes thru, it registers a voltage, or current difference, indicating that. It passes that information to the co-processor, which also has been supplied with the distance, between the two screen's. The co-processor, then does the work, of dividing the distance by the time, and provides us with a number of feet per second, or meter's per second. You might term that number, as average velocity, between the two screens, but that is not exactly correct, either, as the velocity loss between points A, and B, is not linear, it is logoritmic.
So, yes, I am saying that this...
quote:An exception to calculating MV is the U.S. Military. Their velocity figures were given for 78 feet from the muzzle. Screens were placed at 28' and 128' from the muzzle which gave them a base distance of 100 feet.
is not quite correct. Neither, is this
quote:Most shooters chronographing their own handloads will put their first screen at a distance that will avoid the muzzle blast. Screen spacing depends on the chronograph being used. The chronographed velocity is what is recorded midway between the two screens, which is usually around 10 to 15 feet. That number can be adjusted to MV with the addition of approx 10 to 15 fps for most rifle cartridges.
It is absolutely obsurd to believe that a measurement, or even a calculation, based on it being the average between two points, will be remotely close, given there is no sensor or switch there.
What I do believe that the ammunition manufacturers are doing, and this applies to only the major ones, is that they are using ballistic software to backtrack their MV(muzzle velocity) numbers. Meaning that the laws of averages will take effect. They first get a measured velocity at whatever distance they have. Then they enter the ballistic properties of the projectile into a software that does trajectory calculations. After that they shoot at some distances, and record their bullet path differences, from the calculated trajectory. They then enter those number's into what is called a calibrated velocity, where it adjust the velocity to match the actual trajectory. The velocity, can then be given for any point along that curve, including "ZERO DISTANCE", or the muzzle.
The only thing getting more complicated for the velocity measurement of small projectiles today, are the wavelength of light the more accurate sky screens are using, and the speeding up of the circuitry that is needed to utilize them.
If you need more on calibrated velocity, and where to get the software to do that, please inbox me.
Best
That said, there is absolutely no way that the chronograph can
Somebody has been very busy on Google.