Checking accuracy of measuring tools

[HNSB]

I am curious how many of you check your measuring equipment, and if so, how...

My calipers and mics are checked against certified gage blocks. Depth mics are zeroed on a granite plate.

My scale and chronograph are troubling me. It seems that when I chronograph a load it's 100-150 fps faster than what I expect from people shooting similar loads in similar guns.
I have to wonder then if my chronograph is wrong or if I'm dropping more powder than I think.

I'm curious about how you guys calibrate stuff in general, but especially the scales and chronographs.

[220swiftfn]

Scales are easy, use check weights that are available from many vendors. The Chrony?? Someone else will have to chime in on how to verify the calibration, but that being said, 100-150 FPS out of what, 2700 could be just the difference between two rifles of the same type, by the same manufacturer.


Dan

[ElDorado]

I have to agree with 220swiftfn. That's the reason we get chronographs - to check our own loads in our own rifles.


How do you check calibration? You could try comparing results with other chronographs, but even that would just be a ballpark check, not a true calibration. Each shot will vary to some degree. One would need an object that moves at a known speed to travel across the photo eyes, or maybe a system of timed lights that simulate an object. I don't know of such a device; they may have them at the factory.


Aside from the standard advice, such as making sure the chronograph is set at the distance from the muzzle that the manufacturer recommends, consider this: The chronograph's internal calculations assume a specific distance between the photo eyes. If the actual distance between the PEs is too far apart, it will take the bullet longer to travel across them and the chronograph will report a slower than actual speed. Conversely, if they are too close, the reading would be faster than the true speed.


With that in mind, is there any chance that your set-up may be putting the PEs too close to one another, thereby giving false readings? Do you have one of those Chronys that are hinged in the middle, and possibly not fully opened when in use? That could give a faster reading, theoretically.


Otherwise, call the factory and see how they do it, and maybe they can check it for you.


And +1 on the scale weights.

[country gent]

Check wieghtsfor the scales. round head gages for mics at lower end and for every .025 thru +.100 ( tis shows any anvi wear). I check my scales every time I use it at the actual charge wieght to be thrown. Chrono graphs borrrow one and shoot over both setss of screens at the same time this will tell you the variation unfourtnatly not which one is off....

[btroj]

Same load from different guns can easily vary by 100 to 150 fps.

A gun is a larges et of varying tolerances. I wouldn't expect them to shoot the same velocity unless they were set up exactly the same. Revolvers can be the worst.

[Forrest r]

check weights
block gages
2 chronographs

I check my chronographs by using known lots of high grade rimfire match ammo ($15+ a box) & quality rifles that have consistent ignition. The rimfire ammo will have a sd in single digits to low teens. Set the chronographs up @ 5ft from the muzzle, shoot, zero the rifle for 25yds & shoot 10shot groups & 50ft, 25yds & 50yds. Record the speeds & move the chronographs down range & set them up right in front of the 50yd targets. Re-shoot the 10shot groups & record the speeds.

I find the sd's from both chronographs, they should be within a couple ft of each other. The crosscheck is the speeds & bullet drops @ from the 50yd readings/drops.

Is it a perfect way to check/test a chronograph, no. But it's allot better than guessing.

[SyberShooter]

I've compared my 30 year old PACT to my recent production CED, by placing them side by side and tilting the screens so they overlap, one directly behind the other (then repeat with position switched). That way I recorded each shot on both units, then calculated a correction factor based on the distance offset to their center of screens. My numbers showed they were reading within 8 fps of each other. good enough for me.
I also have tested the same loads in multiple "identical" guns. Mainly 30/357, 45acp, 9mm, 223, 7.62x39 and have found that like mentioned before, the same loads can run consistently 100 fps in one gun over another. I usually start each chrono session with 10 rounds from a specific lot of 22LR from the same pistol, to check that the chrono is giving me expected readings. Don't look at the velocities as absolutes as they will always vary.. always. Look instead at the range your in, the SD, ES and accuracy your getting overall. Then go from there.

[Garyshome]

Call the Chrony people and ask. I was having a bit of trouble with mine and called them, Customer support went very well, they were very helpful and got my problem cleared up. It is kind of like checking bhn, if it is not correct it is still a referance, and it seems most reloaders use a chrony so results should be comparable.

[bbqncigars]

I shot my Sharps over my Magnetospeed V2 and Oehler 35 to see what the respective readings were. There was less than 10fps difference between the two. Close enough for me. I'll trust either unit. Each has its strengths and weaknesses.

[mdi]

I would think that when you check a measuring tool you need a "standard". With mics you would use gauge blocks or another precision "standard" (I use my pin gauges) but what would you use for a "standard" for a chronograph? Unless you have access to a high tech electronics lab, I think the best you can get is checking against another chronograph (can you shoot through 2 chronys at the same time, end to end?), but what if the second chrony is off 200 fps the other way?:groner:

[Larry Gibson]

Forrest r's method of using quality 22LR ammo in a quality rifle shooting 10 shot strings to validate the set up is best. I've been chronographing loads since '75 with 3 different Oehler chronogaphs. I've found large variations in velocity with the same gun and others of equal quality and barrel length with the same load are largely due to variation in set up and conditions.

I've placed my 3 Oehlers end to end and tested with numerous loads and cartidges. The velocities, given 3 differet screen spacings was essentially the same. Also have line up numerous other makes and the have essentially been the same also. Screen spacing does make a difference in accuracy.

Larry Gibson

[HNSB]

I am even thinking about mounting the chrony upright and dropping a known weight a given distance past it.
Calculating the speed of a dropped weight is doable...

[dverna]

hnsb

Mass has no effect on the speed of a dropping object. Air resistance will have an influence so a round ball would work well.

Don Verna