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Distance from chronograph?


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10' to the first screen would be a good minimum. You need to be far enough away that the muzzle blast doesn't affect the screens; the gas exiting the muzzle is faster than the bullet and can cause false readings if it reaches the screens. With rifles and the higher pressures they run, you need to be further away - 15' is usual.

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In re reading this thread, it appears that the manufacturers say 5 - 10 feet (for pistol).

If you shoot USPSA and IDPA, the rule book says 10 feet.

Say you chrono your ammo using your own chono - forget calibration differences - and you use 5 or 6 feet as the distance plus you play close to the PF line, you may make PF at your distance, then go to a sanctioned match that's choro'ing the ammo and they are at the specified 10 feet, you will be shooting for fun.

Food for thought.

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  • 7 months later...

To get accurate data on actual muzzle veloicity use 10feet for handguns and 20feet for rifles. This is to the centre of the chronograph, and then back calculate. Sierra have theirs at 10feet and back calculate tue muzzle velocity.

If you must make sure that you meet a certain criteria then 10feet is generally used as the base point. That is 10feet to the centre of the chronograph. Most Chronographs are about 2feet (except Oehler) between the screens or less.

I have a calculation for determining true muzzle velocity. Excel format, PM me your email address and I will forward the file.

I have teseted the formula at 1foot, 10feet, 20feet and 40feet, and the calculation holds true to with 2feet per second of what the chronograph gave me.

Most rule books have the rule that you use muzzle velocity x bullet weight etc. They say Muzzle velocity. IDPA certainly has the specific criteria for measuring velocity laid out in it's rules.

USPSA just says official match Chronograph - Rule 5.2.6.3

The competitor's ammunition, when tested in the substitute handgun, attains the minimum power factor for the declared Division using the official match chronograph.

NRA Action Pistol just says muzzle velocity, this may be changing though, to ten feet to centre of chronograph.

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Does your computation for true muzzle velocity require me to know the current air temperature, air pressure, and humidity, or these variables negligible? If these variables are negligible, how do they compare to the velocity at the muzzle vs. velocity 10 or 12 feet downrange over the chronograph?

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There is a very complicated formula in the Sierra manual that needs all the variables you list to perform it's very accurate calculation of true muzzle velocity. The firmula I use (which I stole) assumes (usually a bad idea) that these factors are negligable or amount to less than 1fps.

The problem is that we have inferior measuring equipment that most ballistic laboratories. We are only testing the loads WE intend to use. I don;t have access to a Oehler PBL83, that Sierra have. I don't have two sets of skyscreens in perfect alignment to calculate time of flight.

I have two chronographs available to me. CED M2 and a Prochrono. Both have proved reliable for my purposes. I used to have access to an Oehler 35P, but the guy who owned that moved away and insisted on it going with him. My chronographs are within 3fps of each other so I can use them interchangably. I prefer the CED.

I used the following to test the formula to get true muzzle velocity

BSA Intl Mk2 Rifle cut to 22". mmo used was Eley Black Match and CCI Std, both are nominally 1070-1080fps. In my rifle both went to 1110fps. This is normal for that rifle as it has exhibited the tendency to shoot faster than box velocity. This was tested on the Oehler using 6 different types of std velocity ammo some years ago. So I know what it does.

Custom NRA AP 38Super. 115gr at 1180fps and 125gr at 1030fps.

Bohica 416Barret, 400gr at 3040fps. Only tested at 20ft and 40ft. Muzzle Blast is an issue.

NEW Sako 85 7mm-08 using Federal 140gr Fusion. Nominally 2850fps. This rifle gave 2780fps at 10ft and therefore the formula would be 2803 at muzzle. So that rifle goes slow. Not unusual.

All calculations that I made were within 2 or 3 fps of what the actual chrono readings were.

I assumed each distance further out was the chrono and the next back was muzzle. So when I shot at 40ft I assumed 20ft was the muzzle put in thedata and it was within the 2-3fps I expected of what the chrono reading was at the 20ft, then did it one distance back and checked again, same result.

The 2-3fps can be explained by the likely error in the sample just being a different sample.

The difference in the 38Super and the 416Barret can also be explained by the likely error I introduced intot eh ammo as they are homeloads.

So a 3fps error in a 22lr load doing 1100fps is less than 1/2%

3fps error in a load going 2800fps is less than .2%.

So all in all for my purposes the formula works. The worst I can be is 3fps out in my calculated muzzle velocity. I can't shoot any of thse firearms accurate enough to tell that I am out by that much.

Unless one sample is taken at sea level +50Deg C and another at 10,000ft at -30Deg C I can't be too far out. I have loaded ammo here in New Zeland in May, early Winter 10-15deg C minimal humidity at essentially sea level, and then used that ammo in Missouri in May 30-35Deg C and Humid as all get out Elevation 800 or so ft, and it goes within 10fps of what I got at home. Shoots to the same point of impact.

I tried using the complicated formula and had to assume the variable data you asked about using weather data from the internet and air pressures etc, using the stds were as per speed of sound calculations (1120fps) using 59Deg F, 78% humidity, sea level and air pressure at 990millibar which is the nominal data for here. The difference from the simplified formula was about 3-5fps. So all in all just outside my own SD of data within any given sample. Just changing to another batch of ammo could and most probably would create a larger change in my figures.

Hope this helps, I confused myself twice just typing this.

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