Measuring muzzle velocity precisely

E

Eppie

New member
Hi Guys,
I have a Oehler 35P chronograph and I have been measuring velocity in a bit of a sloppy manner. I generally set up the chronograph up about 15-20 feet in front of the bench. Obviously the recorded velocities are not actual muzzle velocities, some kind of mathematical adjustment needs to be made for that distance that I haven't been taking into account. Can anyone help?

Your help is appreciated,
Joe
 
Set up the screens at double the distance, measure the velocity at that distance.

Use the difference in velocity between 15~20 and 30~40 to calculate what you are getting at the muzzle. It will be a linear approximation, but for the speeds involved will give you the answer you are looking for.

Jimro
 
There's no need to know that number. You're losing MAYBE 20fps in that distance. That means your velocities at any given distance are off by no more than 20fps. Use a ballistics calculator and change the speed by 20 fps, see what happens to the trajectory.

If you must know and are happy with a reasonable estimate, JBM ballistics predicts true muzzle velocity if you give it chrony distance. If you really must know and can't except an estimate, get that new style chrony that mounts on the barrel.
 
There are several "tools" available. I have an inespensive program called "Load from a Disk" that I use to calculate Internal Ballistics when developing a new load. One of the tools included with the program is an instrument correction tool.

Enter the distance from muzzle to first screen, screen spacing, bullet BC, and recorded speed. It gives you a corrected speed.

All this said, I find that It's only a few feet per second. Easier to just set up the chronograph using a piece of string that insures your measurements are equal from setup to setup.

I only use the chrono data for the first shot or two at distance to get me "on paper". From there it's all about developing my own DOPE for a given round and rifle. At that point it's all about what the rifle/round does rather than what the chrono "says". Think of it as "doping" your chronograph. Just tighten up your setup procedure by using a more consistent spacing from muzzle to first sensor.
 
Personally, I am happy just knowing what my loads are doing a few feet beyond the muzzle. If I really wanted to know, as closely as possible, the actual muzzle velocity I would follow the process of measuring it at twice the distance calculating from these on back.

This is America. Need has nothing to do with it.
 
10'-15' from the muzzle is close enough. I doubt if it is even 20fps as Brian suggested. At any rate the accuracy of the chronograph and the consistency of loaded ammo will vary more than the velocity you lost in that 10'.
 
Actually it is NOT useless information in two scenarios that I can think of off the top of my head.

1, you happen to be using custom bullet of unknown BC. A home made bullet mold, lathe turned, or modified commercial bullet would fall into this category.

2, you are trying to measure a very specific velocity for a bullet that slows down incredibly fast (such as a paper blank bullet, or drag stabilized projectile from a smoothbore) to determine lethal range. My method of doubling the distance and then adding the difference to the closer reading would NOT work in this case.

In those cases, a precise measurement of the projectile at the muzzle would be quite useful, in addition to the normal 15~20 foot measurements you can begin to calculate BC and trajectory tables.

Jimro
 
Here's a page from Bryan Litz "Accuracy and Precision For Long Range Shooting. Its all about the cumulative effect of lots of little things.

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Here's a page from Bryan Litz "Accuracy and Precision For Long Range Shooting.
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But, you don't have a 30-40 fps error, you have at most 20-ish and likely less.

Look at his chart. A 15fps error with that bullet still puts 99% of bullets inside a 5" bullseye at 400 yards.

And, you have a constant error. You can call it 3480 when it was really 3500. The absolute value is meaningless, except on paper. If you're actually shooting real bullets you are going to know where you hit and how to adjust.
Just as that page says, it only matters (IF EVER) on the first shot. Not the first shot of the day either, the first shot you EVER FIRE with that load.

In any case, the answer is provided by JBM Ballistics or by the muzzle mounted chrony, if JBM's guesstimate still isn't good enough for you.

Actually it is NOT useless information in two scenarios that I can think of off the top of my head.
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1)Getting the number at any two ranges works. Actual "muzzle" velocity is still irrelevant. You'd be much better off to get the values at 20 yards and 100 yards than at "muzzle" and 15 or 20 feet.

2)Let me know when we need that scenario applies.;)
 
Brian, I understand your point and that's good enough for most people. And most people never buy a chrony.

But since those that go through the some expense to buy a chrony to get a muzzle velocity on their hand loads, doesn't it make sense to get the most accurate MV possible for a little bit incremental effort?

If one is a competitive shooter it is obvious the big difference that it would make on the score on the second target.
 
Eppie- I have a chroney ( 3 of them ) and I also shoot F Class. It makes no difference,because as Brian states The affects are for the first shot. I know that my rifle zeroed at 300 yards takes me 37 clicks to get to 600 yard zero. Now if my bullet is 20 fps slower that will not matter because after my first shot I see the results and adjust accordingly. Most of the time the adjustment would not be even one more click. It turns out to be a hold over of 1/4 inch or so. With 1/4 MOA scope you need to remember that 1 click will be 6 inches or so depending on your zero. There is a lot to be said for 1/8 or more scope MOA. I see what you are pursuing foe sure ,but I don't think it matters to much. If you are carefully doing your hands loads you MV should not change more than 10 FPS between shots any how.

Now on the flip side- I have always wanted to set one of my chroneys at 300 yards right in front of my target to see how accurate JBL and such charts are.
Now you have me thinking again.
 
Brian,

1)Getting the number at any two ranges works. Actual "muzzle" velocity is still irrelevant. You'd be much better off to get the values at 20 yards and 100 yards than at "muzzle" and 15 or 20 feet.
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That is EXTREMELY lazy thinking.

Explain to me again how a non-linear drag function is better calculated without an exact muzzle velocity? You can't, especially not for high drag projectiles.

Let me put it to you this way, one data point is just one data point. Two data points will give you a line between them, a "linear approximation" for the continuous state change between the two points. Three data points will give you an estimate of a curve. More data points means you better describe the curve, and if you have an actual measurement at the origin you can have the higher confidence in your math than if you don't.

Ballisticians use doppler radar to track velocity changes across the entire range of flight for use in calculating BC for a new projectile. You can only extrapolate a line from two points on the curve, so unless you have some magical formulas that defy physics as I know it that can define a curve from a line....

Heck, maybe you do, but then again maybe you are suffering from the Dunning-Kruger Effect here...

2)Let me know when we need that scenario applies.
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http://www.defense-technology.com/products.aspx?pid=3027

Tell me again how getting a velocity reading on that bad boy at 100 yards would do you any good? Answer, it won't, max listed effective range is only a fifth of that distance.

Or if you want something from a rifle: http://always-on-target.com/cartridges/short-stop-srta-cartridges/ or http://www.setcan.com/simunition_ammo.php

Now I get what you are saying, that you don't need an exact muzzle velocity for most ballistics applications, and I do agree with that, for commercial bullets launched within normal velocity ranges. Want to launch a 750gr hard cast .510 7 radius bullet at 650 fps? Actual muzzle velocity might be important...

Jimro
 
Geo_Erudite said: Buy a Magnetospeed V3, they come out this month.
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I agree the magnetospeed are very nice. Easy and no hassle to setup. Made in Austin, Texas just like the Oehler. I don't know how accurate they are, but, I already have a Oehler so that is not an option for me.
 
As muzzle blast will mess with readings, I imagine a cardboard target set up right in front of the barrel will prevent the issue.
 
Eppie said:
I agree the magnetospeed are very nice. Easy and no hassle to setup. Made in Austin, Texas just like the Oehler. I don't know how accurate they are, but, I already have a Oehler so that is not an option for me.
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From the Magnetospeed website

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MagnetoSpeed LLC worked with Southwest Research Institute(SwRI) to quantify the ballistic accuracy of the MagnetoSpeed sensor package. SwRI uses a bench top range setup to do ballistic tests on ammunition and uses a double Oehler sky screen setup to accurately capture bullet velocity. In the table above, seven shots were taken all using the same ammunition (7.62x51mm M80) with the MagnetoSpeed sensor as well as the double skyscreen Oehler setup. Velocity 1 above is the velocity measured at the first set of skyscreens and Velocity 2 is read at the second set. When the muzzle velocity is back calculated based on the distance between skyscreen sets and the muzzle we obtain the "Calculated Muzzle" velocity in feet per second (fps). Comparing this to the Magnetospeed data we get percent differences of less than half a percent. This data series shows that the MagnetoSpeed is at least 99.6% accurate to the Oehler system used at SwRI for this test.
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