Chrono Distance from Muzzle

[Dead Buff]

Just to get a guideline for all; take my proposed "rule change" above (please not, please not, please not - new stuff to learn every day....) adding "....measured from first screen...."

Now everybody knows that the furthest will be 6m (±6.56 yards).

Looking at shred's most informative post above, this would mean roughly worst case 3 PF points down for a 180gr .40 Short & Weak and less for our .38 Stupid, etc guys...

Everybody can thus easily adjust for this known worst cast senario, the chrono can still be moved, within spec, and an acceptable margin of error in our measurments can be expected....please do not argue what an "acceptable margin of error is" since chrono's from the same factory used in parallel can differ by 10-15 fps - yes i've tested it....

One guy at the RSA nats (mentioned above) shot minor with his .40 with 1 PF point low - chrono too far away? low temperature? bad loads? altitude diff?

[Phil Dunlop]

I've had replies from Pact, CED, Pro Tach and the Chrony people, Oehler have yet to get back.

10' is either the minimum distance the chronograph needs to be from the muzzle, or that recommended for rifle.

Several models are fine with pistols at 5'.

I'm temting fate without the Oehler result, but with 3 gun in mind, if we do standardise the distance, 12' sounds reasonable to me.

P.D.

Oehler require 10' as well

[Skywalker]

A 124gr 9mm at 1100 fps loses about 1 fps per 1.1 ft of travel

A 230gr 45 ACP at 850 fps loses about 1 fps per 3 ft, as does a 180gr .40 at 975

So:

- the difference in chronoing a 124grs 9mm (@1100fps) at 1m or say 3m is some 6 fps, equal to 0.744PF points less.

- the difference in chronoing a 180grs .40" or a 230grs .45" at 1m or say 3m is some 2+ fps, equal to 0.36 and 0.46 PF points less respectively.

Does this really mean something to worry about? The above figures are well inside the SD of almost every ammo batch you could chrono, and if somebody chooses to run so close to the edge, he'll better accept the risk of being marginally below the declared threshold.

BTW Shred, if the velocity loss above travelled distance from muzzle is your point, then I must say that the whole principle on which all chronos work is no longer valid, because the average velocity measured by dividing the space separating the first and the second screens by time is not representative of the actual bullet velocity upon crossing the first and the second screens.

[Phil Dunlop]

Hi Luca,

if those figures are correct, they do seem insignificant.

My concern is that there are too many variables in the chronographing process and eliminating one of them would be an improvement.

Last time I shot abroad, SA, my ammo costs were exactly $1NZ per round, (Appx 65US cents)

I loaded them in good faith at sea level and checked them on a sunny day on my F1 Chrony and a mates Pro Chrono Jnr, they made 175pf

After 6 weeks they were tested at 6000' on a different machine and only made 171pf.

I know the worst can happen, but anything to help minimise the risk is worth the trouble IMO

[Skywalker]

Hi Phil,

I just made a numerical example to expand Shred's assertion:

So.. every yard is worth between 1/2 and 1/4 of a PF point, depending on your bullet and velocity.

To me, the difference in measured PF is quite negligible if we assume to have chrono test performed everywhere in the range 1-4m from muzzle (I'm speaking for handguns), at least when compared with effects of temperature and humidity (as far as altitude is concerned, I guess conclusions in this post are that it is not that much influent).

OTOH, being such a mentally and socially challenged guy (i.e. engineer), I can definitely understand someone who likes to have scientific test conditions well defined and written, to ensure equity and repeatability of said test.

I have resorted to loading @ 177 PF in Standard Division to avoid any kind of "unpleasant surprise". My 177 PF loads have been chronoed everywhere between 174 and 179 on a wide range of temperatures (+20 - +32 °C), and I feel quite safe this way.

[Phil Dunlop]

"I can definitely understand someone who likes to have scientific test conditions well defined and written, to ensure equity and repeatability of said test."

Thats all I'm concerned with Luca, would hate something so simple to be the difference for an unsuspecting but well intentioned competitor

[Skywalker]

The same for me.

But I wonder: how many people do worry the same about police measuring car speed with radars and lasers?

We don't ask them to use the same equipment, in the same weather conditions, with the same light and so on.

We accept that their measurement is what stands. It's our responsibility to have (first) a functional speedometer in our car, to tell us (grossly) if we're complying with speed limits or not; then it's again our responsibility to apply a safety factor to the displayed speed, thus maybe slowing down a little more for safety.

If we choose to drive on the threshold, we must be ready to accept a speed ticket.

Doesn't this analogy apply to chrono measurements too?

I guess something as simple as "...measured at the nearest suitable distance from muzzle, as specified in the equipment manual..." could provide fair ground for everyone. Let's have the manufacturers declare what's the minimum distance at which the chrono is able to consistently read, and have the match organizers act accordingly.

BTW, Phil, if you're still interested in shooting matches abroad, I can provide a very "competitor-friendly" chrono stage here in Italy...

[shred]

The worst-case velocity loss is for poor BC bullets (JHP/SWC designs) at high velocity. They lose more than 1 fps per foot. Given chrono rounding errors, a difference of six inches could drop 1 fps from the screen. Remember we don't count decimal fps over 1000. It's 1000 or 1001, no in-between.

You want to tell somebody that missed 125 PF by 1 fps "so sorry, too bad the chrono wasn't 10 cm closer?" or 'too bad we used a four-foot skyscreen, it would have made it on a shorter chrono"

They shouldn't be that close anyway? What if they always loaded to 130 PF at home? Shouldn't 5 PF over enough? Obviously not, given Luca's observations.

Remember, this person's bullets exceeded the required PF when they were fired. It's only where they were measured that hurt them.

We've got a huge gray blob around chronos and PF, yet we insist on drawing a line through it and saying "This side is white, that side is black, end of story, go home"

What I would do: Add one fps per foot (3 fps/m) of distance to the chrono to the measured velocites. That way it matters not how far away the MD places the chrono, and any bullets that make PF coming out of the barrel of the gun makes PF at the chrono. Of course the "125-no-matter-what" folks will complain, but I'd rather let a few guilty parties go free than kill any innocent ones.

[Vince Pinto]

...... yet we insist on drawing a line through it and saying "This side is white, that side is black, end of story, go home"

Yes, that's precisely what we do, and we do exactly the same thing when scoring targets, determing match DQs for an AD and/or breaking 90 degrees, determing whether or not your calibre qualifies for Major and so on. Moreover, from where I sit, there are essentially two ways we can deal with these issues.

One way is by taking the "Einstein" approach, where we need to consider significant numbers, rounding, truncating, variances, tolerances, decimal places, the amount of sun light, cloud cover, relative humidity, altitude, wind speed, the distance from the chrono to the first screen, the distances between the screens, global warming etc. The other way is by taking the "Wal-Mart" approach. If something costs $125 but you only have $124.99 in your wallet, you don't get to buy it.

After much consideration, the Rules Committee has just concluded dealing with a number of rule interpretations, and these will be posted to the IPSC website shortly (and will come into effect 7 days thereafter - see Rule 11.8) but, to save the unbridled suspense I'm sure you're all feeling, here are details of the two changes (in bold below) made in respect of the chrono procedure:

5.6.3.3 From the 8 sample rounds drawn by Match Officials, 1 bullet is weighed to determine the actual bullet weight, and 3 are fired over the chronograph. In the absence of a bullet puller and scales, the competitor's declared bullet weight will be used. Digits displayed on the official match bullet scales and chronograph will be used at face value, irrespective of the number of decimal places indicated on the particular model of measuring device used at the match.

5.6.3.4 Power factor is calculated using the bullet weight and the average velocity of the 3 rounds fired, according to the following formula:

Power Factor = bullet weight (grains) x average velocity (feet per second) / 1000

The final result will ignore all decimal places (e.g. for IPSC purposes, a result of 124.9999 is not 125).

Love or hate the way we did it, the above clarifies two aspects of the chrono procedure.

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When it comes to the new issue at hand, namely screen distances, in view of the comments and suggestions made herein, I think the best recommendation I can make to the Rules Committee is:

5.6.2 The official match chronograph(s) must be properly set-up in accordance with the manufacturer's recommendations, and verified each day by Match Officials in the following manner:

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And, once again, I thank you all for your input.

[shred]

Great. Hang the innocent! No guilty people will go free! The law is perfect!

I will note that in EVERY OTHER CASE you cite (target scoring, DQ's and AD's, caliber, etc) the error goes in the shooter's favor. In the case of the chrono, benefit goes against the shooter.

Example: When was the last time you saw a perfect 9mm hole in an IPSC target? Yet we use a scoring overlay with a 9.5mm circle to judge 9mm hits. The above chrono proceedure is like using a 5mm overlay to score targets.

All to make some mythical poor simpleton happy. And they wonder why we're getting dumber every year.

I just don't get why some people secretly get their jollies seeing other people barely fail chrono. It's not like a load that reads 127 PF over one chrono and 124 over another shoots any differently or knocks down steel any differently.

[Vince Pinto]

I just don't get why some people secretly get their jollies seeing other people barely fail chrono.

Your comments noted.

Anyway, the decision has been made, and I see no point in starting yet another "Thread From Hell" on the issue of chrono, so I'll close this thread, with thanks again to all who contributed positively to the discussions.