6% Failed Chrono! So remember this…

[jwhittin]

In preparation for several major matches in the area, a club performed a chronograph check at a local match last weekend. About 6% of the competitors failed to meet their declared PF. This included several experienced competitors.

So remember this! The average velocity (and hence PF) you measure at the range will always be better than the official USPSA chronograph results. Yes, it is true! The two measurements are like comparing apples and oranges. It has nothing to do with the small differences between chronographs and of course all the laws of physics apply equally in both cases. So how can I say that?

It is strictly because of the number of rounds used to determine the average velocity in each case. To pass the official PF test the first time, only 3 rounds are used compared to 8 or more rounds we typically use at the range. The bottom line is that velocity is random in nature and the best way to understand and manage it is to use statistics. So statistically speaking, the term “better” means less uncertainty. We can quantify this uncertainty in terms of probability which can easily be determined from statistical tables.

My post "Reloading to Meet PF with Confidence" located here ( http://www.brianenos...opic=229005&hl=

) illustrates the problem and provides a very simple solution. Also in this post, there is an example which compares the probability of being at or above the same average velocity in each case (at the range versus a USPSA chrono). Using 8 or more samples the result is at least 97.3%, but using 3 samples the result is only 85.2%. And it is important to note that the two results can be much farther apart.

A seemingly obvious solution is to just chrono your ammo using 3 rounds. The problem is that using only 3 samples causes wild variations in the results and you will drive yourself crazy trying to get any type of consistent measurement.

The good news is that the link above provides a very simple solution to avoid this problem. For a better explanation and more examples, see page 70 of the Jan/Feb 2015 edition of FrontSight Magazine.

Edited May 23, 2016 by jwhittin

[Sarge]

Single digit SD serves me well!

[DagoRed]

Ok so I'm not a math person, I read your original post too (originally), and still don't really get what number to look at. I have an OLD Chrono chronograph, I measured and had a solid 10' between muzzle and front of it. The load I settled on gave me 178.7 for 8 rounds. 3 slowest gave me 175. I figured since I have an old chrono stick w that. If I go down .1 grains I get 170 from the three slowest. 173.7 for all 8.

I don't have a standard deviation on my chrono. But w an online calf I found I got 16.3 and 13.9, respectively. So am I safe going down .1 or stick w the solid 175pf?

I go to my first bigger shoot in October and don't wanna be below major

Red

[jwhittin]

Hi Red. What's your bullet weight?

[DagoRed]

Sorry you'd think I'd put that. 200g bayou swc

[johnbu]

Ok so I'm not a math person, I read your original post too (originally), and still don't really get what number to look at. I have an OLD Chrono chronograph, I measured and had a solid 10' between muzzle and front of it. The load I settled on gave me 178.7 for 8 rounds. 3 slowest gave me 175. I figured since I have an old chrono stick w that. If I go down .1 grains I get 170 from the three slowest. 173.7 for all 8.

I don't have a standard deviation on my chrono. But w an online calf I found I got 16.3 and 13.9, respectively. So am I safe going down .1 or stick w the solid 175pf?

I go to my first bigger shoot in October and don't wanna be below major

Red

Take the standard deviation of 16.3, multiply by Z (I use 3 as safe is better than sorry!). That gives you 48.9. Subtract from your average speed (I back calculated yours as 893.5), multiply by bullet weight of 200 and that says you have little chance of dropping below a pf of 168. A "safe" load, but trading off with more recoil.

Dropping .1, your average calculates to 868.5. Subtracting 3 x 13.9 gives 826.8 for a minimum pf of 165. So you should have very little chance of dropping below 165.

But ! I'd worry why your deviation is so different between the two loads and for the low speeds, that is troubling. If, the next 8 give the higher debiation, then you would have a higher chance to drop below pf165. Maybe you should work on reducing the variability, then you can be more confident in the speeds you get.

Just my thoughts.

[jmorris]

At nationals one year there were 3 chronographs setup so each shot passed through all 3, none of them read the same velocity.

[okorpheus]

At nationals one year there were 3 chronographs setup so each shot passed through all 3, none of them read the same velocity.

I wouldn't expect them to since the bullet will be continually slowing - and I'm sure the measurement accuracy with $100-200 equipment is less than perfect anyway. It would be interesting to know how much they differed on average and what the SD of the differences between them was. Also if which measured highest vs. lowest was consistent throughout the day.

[jwhittin]

Red,

I use the average velocity and I don’t worry about the PF except to know the minimum velocity needed. So for a 200 gn bullet, the minimum velocity to make major PF is 165000/200 or 825 fps. So what we want to do is ensure that our average load velocity is at least 2*SD above 825 fps. This gives you about 90% chance of passing the official chrono the first time. So the math goes like this:

So 825 + 2 X 16.3 = 857.6 or about 858 fps. As long as your 8 round average velocity is at or above 858 fps, AND your SD remains at or below 16, you are good. If you want more margin you can use 2.5 * SD or even 3 * SD. The same calculation with 3*SD is 825 + 3 X 16.3 = 873.9 or about 874 fps.

So as long as you verify your 8 round SD is 16fps or lower AND your average velocity is somewhere in the range of 858 to 874 fps, you are good!

I hope this helps.

[jmorris]

I wouldn't expect them to since the bullet will be continually slowing ...

It was 3 of these.

One in the center and one on either side tilted inward so one shot went through all of the sky screens at the same distance from the muzzle well within an inch or so. Some of the readings were as much as 20 something FPS different, they used the highest reading one.

One of those times the old "a man with one watch always knows what time it is, a man with two is never sure." situations.

[d_striker]

I wouldn't expect them to since the bullet will be continually slowing ...

It was 3 of these.

One in the center and one on either side tilted inward so one shot went through all of the sky screens at the same distance from the muzzle well within an inch or so. Some of the readings were as much as 20 something FPS different, they used the highest reading one.

One of those times the old "a man with one watch always knows what time it is, a man with two is never sure." situations.

Did they have all three chrono's in a coffin with lighting?

[pkm]

If you watch the chrono numbers at majors with the CED M2s in the box they will bounce back and forth on which one is higher... Paul Hyland and I actually had a good conversation about these types of inconsistencies and the variance while waiting for d_striker to play with our guns at the Chrono.

[jwhittin]

I thought 20 fps sounded a bit high, but after checking I found that the CE Pro Chrono accuracy is only +/- 1%. So at pistol velocities of about 1000 fps, that can certainly happen if one chronograph reads high and another low. Some manufacturers don’t even state their accuracy, hmm, I wonder why.

Others that state their accuracy typically are no more than +/- 0.5% such as Shooting Pro Chrony and Oehler.

[BritinUSA]

How are they determining the accuracy of the chronograph ?

[Aceyduce]

NIST calibrated standard would be my guess.

Sent from my Nexus 6P using Tapatalk

[jmorris]

Did they have all three chrono's in a coffin with lighting?

No they were under a white canopy IIRC, I think it was 2006 nationals at CASA and I have had a few beers since then but I do remember the angles on the chronograph bodies and they looked like they were designed for It.

[Dr Mitch]

How are they determining the accuracy of the chronograph ?

That is funny

NIST calibrated standard would be my guess.

Sent from my Nexus 6P using Tapatalk

AHAHAHAHAHAHAHAHA, even funnier. This is better than Chappell's show in it's prime.

[cnote]

..

Edited May 26, 2016 by cnote

[jwhittin]

They designed it to a specific accuracy based on a marketing/business decision. The design, technology, and techniques in large part determine the accuracy. Probably the most important is the clock speed they use to estimate projectile flight time across the sensors. There are other contributors too like the precision of the sensor distance, sensor performance, and even the manufacturing process.

[jmorris]

Also how you use it.

The extra time it takes the bullet to go that extra half inch, the chrono would tell you it was going 2880 fps when it was actually going 3000 fps.

[BeerBaron]

I've often wondered about that jmorris and concluded that if shots are not straight you get different readings.

I imagine closer together sensors would alleviate that but then I imagine they are not precise enough if they had to calculate the speed at say an inch between the 2 sensors.

Interesting.

[DagoRed]

Most of my rounds were very close together I had a couple outliers here and there which could have been due to my old chrono or maybe because I had a lot of problems w this powder sticking. It was sticking everywhere, scale pan, case walls. I suppose maybe the bullet seating might have not pushed it all down the walls

I don't worry so much, tested it through two guns so feel it's pretty solid. Thanks guys for explaining the math to me better. makes a lot more since now

Red