Rapid Fire Chronograph

[David M]

A recent competition the officals wanted to test the velocities of our bullets, and one shooter asked if the chronograph was calibrated.
The new rule says "a calibrated velocity testing device - to be provided by the Organising committee".

How does one calibrate a chronograph?

[David Levene]

The ISSF News web site has a fairly non-conclusive discussion started by Spencer.

[TomF]

Most advertise to be self-calibrating. So I guess you dont have to do anything but turn it on.

[dhurt]

Out of curiosity, how fast do the bullets have to exit to make legal velocity? Do most standard velocity rounds pass? thanks loads

[Benjamin]

Bullet chronographs measure the time it takes the bullet to travel between two light gates. For mine at least, each light gate is a photocell looking up from the main box at a white diffusing plastic. Depending on sky conditions, the diffusers are not necessarily required in outdoor daylight conditions, however the intent is that the diffusers will be illuminated by the sunlight. For indoor use, I have a light bulb.

The electronics is crystal controlled and is, I am sure, extremely accurate at measuring the elapsed time. For all practical purposes, it is accurately and permanently calibrated by the crystal manufacturer, the same as my wrist watch.

However, the distance between sky screens is not all that far, and a small difference in illumination or in the mounting of the photocells could easily make a 1% or greater difference in the measurement. Could it vary 10% depending on local illumination? Who knows? As per the original question, how would we know?

And, how accurate does the chronograph need to be? "Calibration" of any instrument is always done to some standard of accuracy; what standard would we need for this test?

If accuracy certification is required, I would try to put two chronographs in series, so the same bullet or pellet goes through both of them. Preferably different brands. If they do not give the same result, then as whatever the match rules require, give the benefit to the shooter. There probably is an actual certification method, but I don't trust the chronograph to consistently give the same reading in different light conditions and with the bullets travelling through different areas of the measurement window. Or rather, I DO trust it to be accurate enough for my needs without bothering to test it; whether or not it is accurate enough to meet international match requirements is a different matter.

Trigger weights, OTOH, Can be calibrated by use of an accurate electronic scale; the scale can be calibrated by instrument calibration facilities that get Their instruments calibrated at NIST. They DO vary quite a bit as received from the various weight manufacturers. At Camp Perry, the US Army gunsmith van had an accurate scale, and was adjusting everyone's trigger weights so they were all the same. My weight, which I had previously adjusted using a scale where I work, passed their test on first try.

[David Levene]

Out of curiosity, how fast do the bullets have to exit to make legal velocity? Do most standard velocity rounds pass? thanks loads

250m/s from the competitor's gun. I have heard that Walther GSPs have trouble hitting the correct velocity with some ammunition types.

[David Levene]

Most advertise to be self-calibrating. So I guess you dont have to do anything but turn it on.

But take Chrony for example. The are advertising their products to be at least 99.5% accurate.

Would you feel happy in disqualifying the highest scorer in a competition because the fastest of his test rounds only recorded 249m/s when the chronograph is only guaranteed to be +/- 1.25m/s at the required velocity.
If you didn't disqualify him then you have to explain why you are using equipment that isn't accurate enough.

If you are going to make a rule, as the ISSF have done, then you must be prepared to tell everyone how to police it.

[Chrony-disbeliever]

Chrony 99,5 % accurate??

I aquired, as many other home reloaders, my first Chrony chortly after 1990. This was the early "green colored" version.
It showed very optimistic speeds, to say the least. Well reputed authors of an arms/reloading magazine warned that early Chronys were systematically about 10% "on the optimistic side", when compared (used in tandem) with expensive industrial chronographs.
I do not know it this an intention from Chrony to promote sales. But 10% off is a very high figure.

Later "red colored" versions have given more resonable results. I have owned one that I have chequed repeatedly, and it is only 1,5 % off, still on the optimistic side.
I simply multiply the readout from Chrony with the factor 0.985. And that is it.
On the positive side Chrony made the first relatively cheap and usable chronographs available to the public.

[TomF]

By what measure are you checking your Chrony?

Back to the original question.

[Fred Mannis]

Bullet chronographs measure the time it takes the bullet to travel between two light gates. For mine at least, each light gate is a photocell looking up from the main box at a white diffusing plastic. Depending on sky conditions, the diffusers are not necessarily required in outdoor daylight conditions, however the intent is that the diffusers will be illuminated by the sunlight. For indoor use, I have a light bulb.

The electronics is crystal controlled and is, I am sure, extremely accurate at measuring the elapsed time. For all practical purposes, it is accurately and permanently calibrated by the crystal manufacturer, the same as my wrist watch.

However, the distance between sky screens is not all that far, and a small difference in illumination or in the mounting of the photocells could easily make a 1% or greater difference in the measurement. Could it vary 10% depending on local illumination? Who knows? As per the original question, how would we know?

The biggest source of error in a chronograph is the uncertainty in the distance between the two 'light gates'. Let's say that the boxes holding the sensors are 1000 mm apart and that each solid state sensor is looking out of a slit in its box 3mm wide. And let's say that the manufacturing process is such that boxes can be mounted 1000 +/- 1 mm apart and that the sensors are located somewhere within the slit opening. So the sensors can be from 996 mm to as much as 1004 mm apart. This would lead to a measurement error of 0.8%. The greater the separation between the sensors, the smaller the % error. One of the reasons the Chrony is/can be inaccurate is because the unit is compact - the sensors are close together.
Measurement of the time it takes the bullet to traverse the gates can be very accurate, if the manufacturer spends the money on a high frequency crystal of good quality and on having enough counters.

I have questions on the procedures the ISSF is to use: how many rounds are to be fired from the competitor's gun in computing the average? Second, how will the referee ensure that the shooter is using the same ammo as that chronographed?

[David Levene]

I have questions on the procedures the ISSF is to use: how many rounds are to be fired from the competitor's gun in computing the average? Second, how will the referee ensure that the shooter is using the same ammo as that chronographed?

The ISSF Technical Committee have issued the following procedure as published on the ISSF News web site:-

In accordance with the new 2005 Rules for 25 m Rapid Fire Pistol Men (8.4.6), the Jury may take samples from the shooter’s ammunition for checking.
The minimum bullet weight is 2.53 g = 39 gr; the minimum velocity allowed is 250 m/sec.

For the World Cups 2005, the Jury will take samples as follows, and the Organising Committee must provide Testing Officers, to conduct tests according to the following procedure. One to two shooters per relay in each 30 shot stage will be selected by the drawing of lots by the Jury.

Immediately after the command “UNLOAD”, after the last series has been fired in a 30 shot stage, the Jury must arrange for 5 cartridges to be taken from each of the selected shooter(s). These 5 cartridges must be put in an envelope, and sealed in the presence of the shooter. Then the selected shooter(s) must go with their pistol to the designated testing area.

One cartridge will be taken apart and the bullet weighed. If this fails, a second attempt will be made. If this fails the shooter must be disqualified.

The Minimum Velocity Test will be made using the shooter’s own pistol from the competition with a calibrated velocity measurement device – to be provided by the Organising Committee.The Testing Officer will fire one shot.If the requirement of the Minimum Velocity of 250 m/sec is fulfilled the shooter will have passed the test. If not, a second or a third attempt will be made.If the third attempt fails, the shooter must be disqualified.
Unfired cartridges must be returned to the shooter.Care must be taken with the unfired case from which the bullet has been removed.

[Guest]

I spotted the details of the ISSF letter a few weeks ago. A few things arose in discussion.

What happens if the is a malfunction with one or more rounds during the testing. This is unlikely as the successful competitor will have thrived thus far in the competition with the same batch of ammo.

In terms of gamesmanship, we have seen dud rounds being loaded intentionally in the 10S series of standard as the heavens opened for a downpour. Similarly, a box of ammunition can contain more than one brand of ammunition, the first 35 for shooting the remaining 15 for testing! The is no provision for inspection of ammunition prior to or during the event. Lapua heads look consistent across the range as do their cases, other brands have similar similarities.

[David Levene]

Similarly, a box of ammunition can contain more than one brand of ammunition, the first 35 for shooting the remaining 15 for testing! The is no provision for inspection of ammunition prior to or during the event.

What a very good point, one I hadn't thought of. In fact there is nothing to stop a competitor from starting his 30 shot half with just 35 rounds on the bench. If called for ammunition testing he could then just provide any ammunition from his shooting bag for testing.

[sparky]

I know in some IPSC matches I've been to, a match official has walked up to me at a random stage and asked for a few rounds from the magazine I was about to insert into the gun.
It seems like it wouldn't be difficult to do the same thing in Rapid Fire. After the command to "Load" is given, just have an official walk up to each competitor, have them unload their guns and take the ammo from the gun. Then just have the competitor load again. Just make random which string this is done on.

But how many folks are actually going to go to such lengths to cheat in order to gain such an incredibly small advantage?

[David Levene]

I know in some IPSC matches I've been to, a match official has walked up to me at a random stage and asked for a few rounds from the magazine I was about to insert into the gun.

That would make perfect sense, but it is not as specified by the ISSF.

They say that the sample rounds must be taken "Immediately after the command “UNLOAD”, after the last series has been fired in a 30 shot stage"

Perhaps the ISSF can learn something from other Governing Bodies (shock horror).