6% Failed Chrono. Remember this...

[jwhittin]

Cross post. This seems like a better fit.

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 make that claim?

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.

[acpie360]

Here is the link to Reloading to Meet PF with Confidence Good writing. Thanks.

[Ultimo-Hombre]

Chrono is average of your best 3 of 7.

[Garmil]

Not sure I agree. Like ultimo is saying The uspsa chrono rules make it easier to pass then just doing a normal average that I would do for load development.

Picking your fastest 3 out of 7 is pretty forgiving.

[mrpredictable]

Best 3 of 7 does give you a lot of wiggle room. However, at area 1 a large number of shooters saw a 5-10 PF drop. So having some extra may not be a bad idea.

[Brooke]

It is simply false that 3 rounds will always yield a lower result than more rounds. The average of any number of rounds is associated with a standard deviation. The more measurements the lower the standard deviation becomes (or alternately the better number for the average becomes). No matter how many rounds, 67% of the results will be within +/_ 1 standard deviation (SD), 92% will be within +/-2 SD, and 96% will be within +/-3 SD.

So if you only measure 3 rounds you can have a wide variation with a high SD either lower or higher than the average. One excellent recommendation is to fire 20 rounds and get a result with an SD of 10 or less. Then you can be pretty sure that the 3 round measurement will be tight or at lease you will pass a retest of more rounds. BE aware on your 20 round measurement you cannot throw out data...for example fire 30 rounds and keep the best 20...that is not how you do it and statistically invalid.

Carefully make your test rounds and carefully make the chrono measurements....use all the data generated. As an example, I recently made and tested some ammo for a match using the 20 round technique. My measured results were 31.9 PF with a standard deviation of 8. Three rounds measured at the match using my gun yielded 31.4 PF. You can be pretty confident if you do this according the proper technique and respect for the math involved.

[zzt]

One of the guys at my club runs ammo so near the limit it is silly. At two matches his load chrono'd 165.4. He brags about how close he can come. At a third match he didn't make major and it ruined his day.

My original Limited major load was a reliable 167.x PF. SDs ranged from 6.05 to 6.50 across multiple 20 shot strings. The slowest round in any string made major, so I was happy.

Even so, I bumped it up to 172PF, because I wanted to be absolutely certain I would make major even with bad chrono technique. With SDs in the 4.8 to 6.9 range (depending on bullet used) I'm very comfortably above minimum on even the slowest round. There is so little difference in recoil between 167 and 172 I cannot see why anyone would risk getting dropped to minor by cutting it so closely.

[GForceLizard]

Chrono 10 rounds and all 10 should be at or above your desired power factor.

If you dig math then do all that SD stuff to see how far above PF you want to be to safely pass chrono. I like my average to be 2 SD above required PF.

[bret]

You need to chrono your ammo you are shooting.

Reason some people fail is they either are bad at reloading, I know a guy that had multiple squids and then at another match he failed chrono, he even has a chrono.

Or people are using ammo that doesn't meet PF, and they get caught at matches with chrono.

Elevation is a concern too, in Florida we are at sea level, I think loads that chrono here will go up some in higher elevations.

So if you are close to PF in a higher elevation and come to Florida with its lower elevation it can drop your velocity enough to fail chrono.

Area 6 some guys bought commercial ammo, it went sub minor.

[Bkreutz]

Personally, I believe that the reason shooters fail chrono is that they're running too close to the minimum. No margin for error in the reloading process or ambient conditions. I haven't changed my load in years and the closest I've ever gotten to the minimum PF is 171. (I load minor to 136 but don't minor anymore). My target PF is 174. At Area 1 Chrono was the strangest I've experienced, My velocity was down 30 fps, but the bullet weighed in at 127.5 (124 bullet). Still made my 174 PF but I thought it was odd until I heard a lot of other shooters comment about strange bullet weights. I'm thinking the strong winds had an effect on the scale.

[motosapiens]

Chrono is average of your best 3 of 7.

yep. i always chrono at least as high as my averages at home, even tho i've never had to use the other 4 rounds.

I think people fail chrono because they are bad at math and bad at reloading. Not just a little bad at math either.

[DocMedic]

I think what also happens is once they find a load that passes mustard, most will run with that load for years without ever needing to find the urgency to check it again. I've notice I need to check my powder throw on my 1050 every few thousand rounds. If I don't pay attention my 3.6 grains of tight group will drop to 3.4 which could put me in sub-minor depending on the gun I'm shooting at the time.

[pmt]

...........or, you could just use more powder.

[jwhittin]

Brooke,

I didn’t claim fewer samples yields a “lower result”. I said more samples is statistically “better”. My point was that more samples always yields less uncertainty (i.e., less variation around the average which is the STD). On that we agree.

Your statement “no matter how many rounds” is false. The number of samples does in fact matter. We use samples to estimate sample statistics and make inferences about a much larger population (in this case, the total population of all rounds of the same recipe, the same process, components, etc.). If the Normal distribution applies (assumption #1), it is well known that for most applications, 8 to 10 samples (assumption #2) yields very good approximations for the statistics of the Normal distribution. IF both assumptions are met, THEN use of the Normal statistics is correct. However, once the number of samples used to calculate your sample statistics goes below about 8 to 10 samples, the results diverge from the Normal distribution and the T-distribution should be used. My example of 95.3% versus 85.2% illustrates the correct results using 8 sample versus 3 samples (at 2STD as I recall).

“One excellent recommendation is to fire 20 rounds and get a result with an SD of 10 or less.”

The SD is a result of the overall process, you can’t “get a result” by using more samples. You only remove uncertainty about your ESTIMATE of the sample statistic by using more samples. That is all. Your recommendation to use 20 samples is unnecessary as the minuscule accuracy gained in going from 10 samples to 20 samples is irrelevant for our needs.

The difficult part is in relating our reload chrono results to the USPSA chrono results which is the point of the Z value and the corresponding chance of failure.

Edited May 24, 2016 by jwhittin

[jwhittin]

Good comments!

Yes, the rules allow for best 3 of 7. I just wanted to pass using the first 3 rounds and not have to worry about it. It was just the approach I chose which made sense to me. I guess I could update my analysis to use the best 3 of 7. But as several people point out, this result is more conservative.