Is there a method to check a chronographs accuracy?

B

bluechip

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My chrono readings are a couple of hundred FPS less than the advertised speeds in the reloading manuals for a given powder weight and bullet weight. Possibly I could get with several other people that own different brands of chrono's and line a few up to test for differences, or is there a better way?
 
Chronographs

They are notoriously finicky...... I have owned 6 different units over the years........... now I own 2....... an Ohler 35P and a Magneto Speed.......in order to get a reasonably reliable and accurate one.... figure on spending around $500.00 to begin...... even with that as a beginning..... still figure on throwing out a few wild readings.......
 
200 FPS sounds like a lot. That's darn near the difference between a 3006 and a 308 with the same bullet and same pressure.

I suppose the best way to check it is set up another unit exactly the same distance from the muzzle on the same day, preferably within minutes of each other.

Of course, velocity can be affected by many things. Groove diameter of the barrel, bullet bearing surface, barrel length, powder too slow, or too fast.

But 200 FPS still sounds like a lot.
 
From the time they are triggered to start until the time they are triggered to stop they are very accurate. The errors are the triggering, sunlight, and so on. When triggered a counter starts which is crystal controlled then stops when the next triggering signal appears. Considering the time base used (in excess of 1million counts per second ) the only noticeable error would be on of sensing the bullet arriving and leaving the measured length. Not sure about the new Radar Device being advertised. From what I gleaned in my first reading it actually tracts the bullet for 50 to maybe a couple hundred yard depending on conditions. That being said in my my world of short range benchrest (100-200yards) all I will ever need to know will be on the paper target.
Bill aka The Trout
 
try shooting some 22 rim fire ammo with an advertised velocity.
 
bluechip said:
My chrono readings are a couple of hundred FPS less than the advertised speeds in the reloading manuals for a given powder weight and bullet weight. Possibly I could get with several other people that own different brands of chrono's and line a few up to test for differences, or is there a better way?
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So, if you get different readings, which machine are you going to consider accurate? :confused:

If you can shoot 300 yds or more, you might consider comparing your results with a drop chart; e.g., the drop chart at which I'm looking shows that, with a 100 yd zero, a 150 gr, 30 cal bullet at a MV of 2800 fps drops 19.8 " at 300 yds -- whereas at 2600 fps it drops 24". Such a chart should help you verify your MV.

BTW, advertised speeds in reloading manuals are, at best, specific to the barrel in which the ammo was shot. Furthermore, as for shooting ammo with an advertised velocity, that's not particularly reliable. A major manufacturer replaced over ten boxes of ammo because the advertised velocity was significantly higher than that shown by my speed-o-meter.
 
I always verify my speeds by my drops out to 1000 yards. It is more accurate than a chrono but you must use a good ballistic calculator like APPLIED BALLISTICS as the freebies on the net aren't much good at all.
 
CMaier said:
try shooting some 22 rim fire ammo with an advertised velocity.
Click to expand...

I have tried that with premium match grade ammo with about 40' per second difference.

A comparison of two units could be made with the same bullet by using two different units with one right behind the other as close as possible. I would not expect perfection only uniformity.

Bill
 
I tried checking how well these two tracked each other a few years back. They were IIRC around 10 fps from each other on average with the high or low not always given by the same unit. Which was giving the right reading? don't have a clue..

DSC00911.jpg
 
The further apart the screens are, the more accurate the average is. We put our Ohler screens at 28' and 128'. We also use Weible radar. The same load gives different velocitys and pressures daily based upon conditions. We shoot a standard reference round and then apply correction factors.
 
I tried some target 22 rimfires that were advertised at 1080 and got 1076 and 1085 so I guess all is well at least at lower speeds.
 
bluechip said:
My chrono readings are a couple of hundred FPS less than the advertised speeds in the reloading manuals for a given powder weight and bullet weight. Possibly I could get with several other people that own different brands of chrono's and line a few up to test for differences, or is there a better way?
Click to expand...

I'm kinda like Saltamaccia (sic) except not as good a photographer

What I do


3 chronos.jpg
 
bluechip said:
My chrono readings are a couple of hundred FPS less than the advertised speeds in the reloading manuals for a given powder weight and bullet weight. Possibly I could get with several other people that own different brands of chrono's and line a few up to test for differences, or is there a better way?
Click to expand...

BTW I do all my testing under artificial lighting. IMO lighting is the single largest variable so I've eliminated it.

That said, it's actually rare to find advertised speeds to be correct, 200ft high seems almost to be a norm. And it's also rare that your rifle correlates to a reloading manual.
 
I guess you gotta just figure out what kind of accuracy you really need. If you can get by with a little error go with something cheaper. Like al i use lights on my chrony. If your needs go beyond that type accuracy then you need a setup like mil gunsmith has and a ballistics lab to go with it. Call oehler and tell em your budget and they can fix you right up.
 
I just spake with Oehler last week. (I'm desperately searching for the equivalent of a 43 setup since the one I useta' use is in another state! LOL)

An entry level system starts at $25,000.00
 
bluechip said:
My chrono readings are a couple of hundred FPS less than the advertised speeds in the reloading manuals for a given powder weight and bullet weight. Possibly I could get with several other people that own different brands of chrono's and line a few up to test for differences, or is there a better way?
Click to expand...
Comparing the readings against other chronos is a sound method of checking the calibration of the electronics of the various units. However, having a good reference firearm/ammo system is still important. Having said this, guns firing projectiles at the most uniform velocities are not firearms, but competition air guns. Accuracy for airguns occurs below 600 FPS, and high end guns have a velocity variance range of less than 15 meters/second. (See http://www.feinwerkbau.de/ceasy/modules/core/resources/main.php5?id=1407-3&download=1 for the technical data for Feinwerbau 800 target and sporting airguns.)

Hope this helps.
 
The sensors are the largest source of error.

I have designed and built numerous sensors and making a reliable re4peatable pulse measuring setup is a real PITA.

You are dealing with very high speed (fast and short) pulses and trying to measure the timing between them.
Since the two units are NEVER exactly the same the pulses are NEVER exactly the same shape.
You can spend as much money as you have to try and make then match.
The air force (radar) and navy (sonar) have spent huge amounts.
 
the time bases are vary accurate as that has been mentioned in the thread. so build a couple of screens, bullet breaks wire start then next break stop.
 
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