Chronograph Accuracy

[heli_av8tor]

I have the Competition Electronics pro chrono. The manual states accuracy at +/- 1% of reading (or better). They don't give a repeatability figure.

So for an actual velocity of 3000 fps the chrono could read as low as 2970 and as high as 3030 for a spread of 60 fps even if the ammo was perfect.

With possible errors this large is the chrono of any value in searching for the ultimate load?

 

[MikeMcCasland]

You don't use the chronograph to "search for a load". You use paper targets for that.

You use the Chronograph to confirm you're at your target velocity, to obtain speeds necessary to build out a drop chart, diagnose problems, and to test performance of various components against themselves (across the same chrony/conditions etc.).

Using a .308 as an example, even if the chronograph is reading 60fps off, that's still only a ~1.7 MOA difference at 1k. You'll still be on paper, and can adjust fire.

Edit: In summary, the 'Satterlee method' (i.e. shooting ascending charge weights and looking for velocity flat spots) es no bueno.

 

[garandman]

I also use my chrony for bullet drop over distance, calculated on Strelok Pro. If my speed is off by 1%, my "drops" are gonna be off too. May be correctable at 600, but 1000 will get dicey (I'm guessing) .

I wonder about the variances *any* chrono will have. Thus far, I've been able to get consistent hits / accuracy at 400 and 600, but not sure about 1000.

 

[Nature Boy]

My guess is that it’s better than 1% or else that variation (60 fps ES in your example) would show up in all the data collected, and it doesn’t

 

[David Christian]

You don't use the chronograph to "search for a load". You use paper targets for that.

You use the Chronograph to confirm you're at your target velocity, to obtain speeds necessary to build out a drop chart, diagnose problems, and to test performance of various components against themselves (across the same chrony/conditions etc.).

Using a .308 as an example, even if the chronograph is reading 60fps off, that's still only a ~1.7 MOA difference at 1k. You'll still be on paper, and can adjust fire.

Edit: In summary, the 'Satterlee method' (i.e. shooting ascending charge weights and looking for velocity flat spots) es no bueno.

@MikeMcCasland, have you ever tried testing for velocity flat spots? I have found this type of testing to be very beneficial in quickly finding my node and then verifying at 1,000 yards on paper.

I borrowed the spreadsheet from the 6.5guys website. Here is the data and results from a recent barrel, all shot at 100 yards using a modified Satterlee method over a Magnetospeed v3. I've highlighted the speed flat spot by mean speeds. 7 FPS spread over a .4 grain powder range.

or if you wanted to graph it:

Then I took this narrow range out to 1,000 yards to measure the vertical spread of each powder charge. 3 shots of each charge, fired in round robin order. 50.6 measured 2.51" vertical, 50.7=1.63", 50.8=2.20", 50.9=4.91".
Speed test got me pretty darn close to 50.7 being in the center of the speed flat spot. Actual 1,000 yard paper target confirmed the same result.

 

[MikeMcCasland]

@David Christian - I have tried it quite a few times in years past. I've seen it work, but I've also seen it not show any kind of flat spot in the velocity curve...just keeps climbing until you're over pressure.

I think it's a bit misleading in how it was marketed by the 6.5 guys initially, and how I see a lot of folks do it online. i.e. actually try and identify the node in ~10 shots.

For those reading I'd take David's word over mine; that said, I haven't had the best luck with it.

 

[Depart123]

@MikeMcCasland, have you ever tried testing for velocity flat spots? I have found this type of testing to be very beneficial in quickly finding my node and then verifying at 1,000 yards on paper.

I borrowed the spreadsheet from the 6.5guys website. Here is the data and results from a recent barrel, all shot at 100 yards using a modified Satterlee method over a Magnetospeed v3. I've highlighted the speed flat spot by mean speeds. 7 FPS spread over a .4 grain powder range.
View attachment 1139132
or if you wanted to graph it:
View attachment 1139133
Then I took this narrow range out to 1,000 yards to measure the vertical spread of each powder charge. 3 shots of each charge, fired in round robin order. 50.6 measured 2.51" vertical, 50.7=1.63", 50.8=2.20", 50.9=4.91".
Speed test got me pretty darn close to 50.7 being in the center of the speed flat spot. Actual 1,000 yard paper target confirmed the same result.

I talked to Scott about this via PM last year. He pointed out that he was using heavy for caliber bullets (147's) and slow for cartridge powders (RL19 IIRC). He was also at 100+% load density and at or over max book pressure.

 

[heli_av8tor]

Thanks guys for weighing in on my question. The topic has drifted somewhat and I'm glad it has.

After starting to write a spreadsheet I found the one from 6.5Guys. I've expanded it to do 10 rounds, but haven't figured out the graph portion yet.

The reason for my original question is that I'm getting ES and SD numbers so high that it's hard to make sense or have confidence in the results. Of course I'd rather blame that on a piece of equipment (my chrony) than my reloading technique. I've reloaded pistol calibers for years but am new to rifle calibers (.223).

 

[chkunz]

The chronograph is an electronic instrument that we have no way of calibrating.

Instrumentation has bias and precision errors that are normally combined in and an estimate of uncertainty. Some people use the term accuracy interchangeably with precision. If you Google "uncertainty" you can find references that give you the technical description and a method of calculation of uncertainty.

If the manufacturer states +/- 1 % accuracy then that is a good place to start. You can "believe" that it is better than that, most people do but most people are wrong.

"For every complicated problem there is a solution that is simple, direct, understandable and wrong."
H.L. Mencken

 

[David Christian]

Thanks guys for weighing in on my question. The topic has drifted somewhat and I'm glad it has.

After starting to write a spreadsheet I found the one from 6.5Guys. I've expanded it to do 10 rounds, but haven't figured out the graph portion yet.

The reason for my original question is that I'm getting ES and SD numbers so high that it's hard to make sense or have confidence in the results. Of course I'd rather blame that on a piece of equipment (my chrony) than my reloading technique. I've reloaded pistol calibers for years but am new to rifle calibers (.223).

Borrow a buddies chrono and see if you get different results.

 

[Barlow]

I lined up my Pro Chrono behind my buddies Oehler a few years back and the numbers were identical. Barlow

 

[STOMP442]

Get a magneto speed. What you see is what you get. Every time I have charted out a trajectory the velocity the magneto has given me is spot on. Great product and easy to use.

 

[T-shooter]

I have the same chrono. It seems pretty consistent. Still if I get a reading too high or too low I'll throw it out and average the rest. I've had trouble with it sometimes getting more than 1 Dup in a row. I've just turned it off and back on to clear it.Usually it will read all the shots within 10 fps. And having the velocities close does improve the accuracy when shooting groups.

 

[Bindi2]

You use the chronny to find what the barrel likes eg a flat in velocities before the big pressure spike. If you have a wide ES there is something wrong with the load combination eg primer to powder. A good combo will have single digit ES for a series of loads on a gradual increase of powder eg .2gns.

 

[jb]

i have ced cronograph in a box with led light strips. with random lc 20-223 cases loaded +or- .03 gr. 15 rounds gave 14 fps es. was told crono with that set up would +or- 3/4 fps. my cronny set just behind the ced read 150 to 240 fps more.