How do you use your chronograph


PDA






Tune_up
September 10, 2009, 04:10 PM
How do you use your chronograph data. Iím not interested in setup and function, but what value do you get from the measurements and how do you use them to work up loads. I realize that with an average velocity you can calculated bullet drop at various ranges, but what else?
How accurate are chronographs, my ProChrono states at least +/- 1%. Measuring a 3300 fps .223 bullet thatís a 66 fps spread on identical velocities. Yesterday I shot two 10 shot strings on different loads. The first averaged 3300 fps, an extreme spread of 40, and a standard deviation of 14. The second averaged 3021, extreme spread of 90 and Standard deviation of 34. Other than the average velocity, can the other measurements be considered significant? Whats the most likely cause of velocity variation? load? powder? case volume? crimp? my loading technique? ect?

If you enjoyed reading about "How do you use your chronograph" here in TheHighRoad.org archive, you'll LOVE our community. Come join TheHighRoad.org today for the full version!
Steve C
September 10, 2009, 07:58 PM
ES gives you the difference between the highest and lowest velocity measured. SD gives you the amount the data varies, in a normal bell curve distribution of data 68% of the numbers will fall within 1 SD of the average or mean and 95% will fall within 2 SD's of the average. If you SD is low then the velocities measured are closer to gether and the ammo has more constancy that can usually be interpreted as being more accurate.

Whats the most likely cause of velocity variation? load? powder? case volume? crimp? my loading technique? ect?

There can be many reasons for differences in velocity. If the rifle and other components are the same the only major variable is the difference in bullet and how consistantly the powder charge combusts at the load used. The less variability you can introduce in testing the better you can evaluate what things are causing the differences.


Other things that can affect velocity in a significant way is any change in component be it different brass or primer, bullet even of the same weight,or heat of the barrel and how dirty it may be. Air temperature, barometric pressure and wind can change velcoity too. Different guns will usually produce quite different velocities even if made by the same manufacturer and with the same barrel length.

ranger335v
September 10, 2009, 07:59 PM
"Whats the most likely cause of velocity variation? load? powder? case volume? crimp? my loading technique?"

Yes.

1911Tuner
September 10, 2009, 08:05 PM
When talking chronographs, the setup must be correctly done or you'll get false readings.

Place it on a solid surface or a good tripod. Level it. Sandbag the rifle, and place a target at some distance behind the chrono so that you can fire across the screens consistently at the same point for every shot.

Firing across the screens at even a tiny angle makes a major difference in the velocity reading simply because the bullet has farther to travel from starting the timer to stopping it. It can be up/down or side-to-side. Makes no difference.

.38 Special
September 10, 2009, 08:12 PM
I use a chronograph to ensure I am getting appropriate velocity for my loads. When I have several loads going at the correct velocity, I'll look at group size and consider that along with ES. If group size at 200 yards is acceptable, and ES is 50 or below, then I have my load.

chbrow10
September 11, 2009, 07:41 AM
Some competition sports require a minimum power factor. This is (velocity x bullet weight) /1000, so I use it to ensure thst I meet power factor. Most people hand load, so you've got to check velocity for each new load.

Tune_up
September 18, 2009, 01:00 PM
Iíd like to thank everyone for your response. I know what the calculated statistics mean, but in my limited experience, oddly (perhaps itís the +/- 1% accuracy) the smallest SD/ES does not always correlate to the smallest group, and I canít see choosing a load that shoots 1/2Ē groups over one that shoots 1Ē because the stats are better. I have only used my chronograph sighting in at 100 yards, will the lower SD/ES correlate better at 200+?

1911Tuner, didnít think about shooting across the chronograph at different angles. Iím probably pretty consistent during the same session, but every time I set up at the range itís a little different; Iíll have to work on that.

.38 Special, could you elaborate on ďI use a chronograph to ensure I am getting appropriate velocity for my loadsĒ. Appropriate as matching estimated velocity in a load book? As in maximum velocity with accuracy? I assumed that the velocity achieved with a certain load ďwas what it wasĒ.

Sorry if Iím being a stickler, but I have only been reloading for a year and since buying my chronograph Iím not sure how much it helps me. Itís nice to know how fast the bullet is traveling but the best load is the one that makes the smallest group regardless of speed. The load book gives me an idea of how a certain load velocities compare to others, so what am I missing? :banghead:

jfh
September 18, 2009, 02:30 PM
Probably has a couple of unique components, vis-a-vis rifle reloading--and, perhaps, Contender reloading (rifle cartridges in a handgun). Bear with me--

I originally used my chronograph to set up 1911 "barely major" loads for club competition. I was lucky, I suspect, because I got to the right combination of components quite quickly, got a good variant a bit later on--and had both the expected low ES and SD results with good accuracy. (Hint: sorted brass--yes, it did make a difference--, WSPs, 231, and an H&G 68 / 200gr. LSWC.) I had a similar experience sorting out the same kind of load in 10mm. That was nominally 15 years ago.

Two years ago, I got going on the 2" revolver "replica reloads" idea and have now shot variants of 38 and 357 recipes in barrel lengths out to 4". I've gathered LOTS of data as I expanded from "Replica Reloads" into my "Short Barrel Revolver" reloading project. In addition to the Chrono readouts, I have temperature and sky coverage, and related factors. There's been no attempt yet to seek correlations there, however.

Lots of data. However, the information--only selectively and anecdotally studied so far--has nearly always shown wide ESs. In fact, I now consider ANY ES under 60 to be "good." In my attempts to chase this down, I have tried controlling powder position--i.e., tilt-the-revolver-up-before-each-shot. That does help some, it appears. But, the problem simply is not going to go away with our modern propellants. It's the "large-cases-small-charges-and-powder-burn" problem.

I just built some reduced-cowboy loads--4.0 gr. of True Blue under a 125 LTC in a 38 Special case. I cannot believe how little powder volume that is in that case; it makes me want to find a Kapok supplier. I'm about to range test these to see how they work.

No great insights, AFAICT--but, if you want to isolate variables, look at the powder volume / position issues in handgun loads. Personally, I gather the data, and with one powder I seem to get good results (in 38 Special) no matter what--low-to-high: Ramshot's True Blue. On tests to be replicated, I actually have an SD under 3 for at least one recipe.

Jim H.

snuffy
September 18, 2009, 03:34 PM
Low ES is obtained by consistency, PERIOD. A load that's in a manual has the potential to be consistent. By that I mean, the powder listed has been tested with certain components to be balanced, the burn rate, bullet friction and resistance to acceleration fall into what should result in a consistent load.

Now you have to use the same brand/make case, same primer, powder, and seat every bullet to the same depth. Not necessarily the same they used, but NOT mixed brass.

There's a war between two camps in the reloading world. Whether weighing each charge, or measuring by volume is best. I weigh each charge when I want extreme accuracy and consistency in long range match rifle rounds.

Now comes the shooting. For chrono readings, the rifle rest MUST be the same each and every time you pull the trigger. Your body also must be in the same position, so the recoil is the same each time.

Then for small groups, you must be able to SEE the target. That means a good or even a great scope. My personal advice to new shooters is, if the scope doesn't cost what the rifle costs, it isn't good enough.

Finally, if you have large numbers in the ES, you WILL have vertical stringing at long range. Even though the group looks good @ 100 yds, the longer the range, the more strung out it will be. Just a fact of shooting.

mongoose33
September 19, 2009, 08:44 AM
ES is the least useful of all the statistics simply because a single round can cause it to wide considerably. SD is generally much more useful as it is not as sensitive to a single outlier as is ES.

One rule of thumb is that your SD should be less than 1 percent of your mean for a string of shots. In the example you give (mean of 3300, SD of 14), that's very good.

The second string you posted is not as good. What was different when you shot that one? Was your gun as cool (or hot!) as it was with the first one? Different load? Different chrono setup? Did you clean the barrel in between them?

I'm a relatively new reloader (just over a year). I consider my chronograph an essential part of the reloading process. How can you know what you've got unless you're measuring not only accuracy but velocity?

And if you've worked up loads as you should, and run them through the chronograph, you have seen how velocities climb as you add powder, and you can as well determine an optimal point for reloading. I've seen workup strings suddenly become very consistent at a particular load, and hotter loads get worse.

Steve Marshall
September 19, 2009, 10:07 AM
1st off, if your machine talks about 99% accuracy, they are speaking of the inherent accuracy of the chronograph and not that your load will vary +/- 1%
That is a big difference. What they mean by that is your 3000 FPS load may read 2970 fairly consistently or 3030 fairly consistently or someplace in-between fairly consistently.
Secondly we come to the variation on bullets going through the screen. Say you varied YOUR kocation by 5 degrees in relation to the screens. Huge variation, right? Let's see. The hypotenuse of this new triangle would vary by .015 feet. (tangent 5deg.x 4ft = .35 ft) This means that you would have to deflect your shot, due to position, by over 4 inches. Admittedly that would be a huge position variation from shot to shot. But with the screens available, might not even be possible. But assume it is. So with one leg being 4ft. and the other being .35ft. our hypotenuse becomes 4+/- .015ft. This translates to +/- .4% variation. So on a 3000FPS shot, by varying your position by 5 degrees you would read 2988-3011, hardly a huge fluctuation and given the width of skyscreens not even possible.
As to the worth of chronographs. You can use them to see when you are arriving at a good load. Let's say you are seeing 70 FPS increase in velocity on average per additional grain of powder. Suddenly with that 48 grain load you see 125 FPS faster than your 47 grain load. You could look at this and say, why I could increase this charge by 5 grains and see a 625 FPS increase in velocity OR you might say, and probably should say, oops, that was a pressure incursion and that might just indicate my pressure is getting out of hand.
Low ES and SD doesn't tend to matter much at say under 200 yards or so but when you start shooting at e.g. 600 yards it does. Let's take that 3000 FPS load and extrapolate just a bit. We'll assume you have an ES of 150 FPS. with a 100 yard zero and a Sierra 168 with a BC of .450 your bullet will vary about .7 inches due to velocity at 200 yards. Most of us would never be able to detect the difference. But look what happens at 600 yards with a 100 yard zero. There will be a 14" difference due to that velocity variation. That is significant. Now 3000 FPS with a 168 is a lot as is 150FPS spread but you get the idea. That is why many of us tended to pooh pooh large ES and SD as you could still say I have a great load look at my .75 MOA group at 100 yards. But when you extend that you'll see that it isn't all that good a load.

.38 Special
September 19, 2009, 04:07 PM
.38 Special, could you elaborate on “I use a chronograph to ensure I am getting appropriate velocity for my loads”. Appropriate as matching estimated velocity in a load book? As in maximum velocity with accuracy? I assumed that the velocity achieved with a certain load “was what it was”.

Nearly all cartridges have a specified velocity with a particular bullet, ie .30-'06 180 grain bullet at 2700 FPS, .416 Rigby 410 grain bullet at 2370 FPS, etc. So when I'm shopping for a particular rifle/cartridge, it is with those numbers in mind. I think loading a cartridge to a much higher velocity than it was designed for is foolish and occasionally dangerous, so I use the chrono to ensure that my handloads are not significantly faster than expected, which in turn tends to ensure that my pressures are not significantly higher than they ought to be.

Generally speaking, I will use several different powders, loaded up in increments, and fired across the chrono until expected velocity is achieved with each. Then I will pick the powder which, at expected velocity, produced the best groups at 100 or 200 yards -- depending upon the cartridge and purpose of the rifle -- and then call it good. For long range rifles I will take both ES and SD into mind, as significantly differences in velocity, from round to round, will lead to vertical stringing at longer ranges. So if I see a load with decent 100 yard accuracy but a lousy ES/SD, I will probably not even try it at long range because it will be a waste of time. A somewhat less accurate 100 yard group that is produced by a load with a low ES/SD is much more likely to be a winner at long range.

Seedtick
September 20, 2009, 12:26 AM
Following up on what .38 said - we didn't have a chronograph but we worked up a load with a Nosler Ballistic Tip for my 7 mag Remington. We worked up toward the max until the accuracy went south and then dropped back down some. I was disappointed with the bullet performance on the first 2 deer I killed with these loads because the bullets blew up on entry. We thought the first one was a fluke.

We dropped the powder charge down some which helped but it was still a little fast. We dropped down a little more and now it works like a champ. I've killed several with it now and I tickled with it. I believe we were just pushing the bullet faster than what it was designed for.

I've recently picked up a chronograph and I plan on loading a few rounds back like the first ones and compare them to what we are loading to now, just so I'll know.

ST

Added - I didn't give particulars on load data because I don't have it here with me. Best I can remember we dropped the charge a total of 1.4 grains. I think? :o

Rugg_Ed
September 20, 2009, 02:30 AM
Hi Tune_up
Normally I work up a load looking for the smallest group possible. When things come together, I will set up the chronograph and see what the results look like. Most of the time they fall in or vary near published data. Also gives me a good idea on my quality at the work bench. Looking low numbers at ES, SD % and compare average Vel to each shot in the string to see if there is a odd ball. Other than that I just use it to check power factor (Major / Minor) for competition. Then Log the information into the computer for reference, and calculate Ft/Lbs of energy, Velocity for power factor with used bullet, and the resulting power factor. Also feed the info into a ballistic program giving me bullet drop, flight time, energy, Velocity at different ranges, wind drift ETC.

Seedtick
September 20, 2009, 04:17 PM
Added - I didn't give particulars on load data because I don't have it here with me. Best I can remember we dropped the charge a total of 1.4 grains. I think? :o

^ I was wrong. ^ :uhoh:

I looked it up and we dropped the powder charge 3 full grains. The powder is IMR 4350 under a 120 grain Ballistic Tip with a Federal 215 to get it all lit. This set up is working great for me.

ST

Tune_up
September 20, 2009, 10:48 PM
Thanks again everyone. I have learned alot from this thread, not only about how to use the data but a couple of things I hadn't thought about when using the chronograph itself.

Bart B.
September 22, 2009, 06:55 AM
If you set a chronograph up so you can shoot through it from standing and prone as well as from a bench, you'll note the average velocities will be different. Highest from prone, lowest from standing; typically what happens.

And two people shooting the same rifle-ammo combination will seldom get the same muzzle velocity. Off a bench, the same rifle-ammo may be near 100 fps difference between them.

These velocity differences are casused by how hard the shooter holds the rifle against their shoulder and how rigid their upper body is, both to resist recoil. Newton's law applies here; the old equal but opposite issues with forces.

If you enjoyed reading about "How do you use your chronograph" here in TheHighRoad.org archive, you'll LOVE our community. Come join TheHighRoad.org today for the full version!