Help with Understanding the Chrono

[marcodo]

Asked this on another forum but the thread died out before I got answers...

I reload for many handgun cartridges and a 223 AR. I like gadgets and hear many of the seasoned reloaders speak of their use of a chrono. Some suggest they don't know how they got by without it.

So my question is "How do you use the velocity data that a chrono gives you"

Do you load to a velocity despite the load limit data or are you stuck inside the confines of the load data despite the velocity. (I think I already know the answer to this one).

How about handgun data in particular...is it for interest sake or does it change what you do?

If accuracy is 1 hole on paper...whats the difference what the velocity is?

I suspect hunting is an issue assuring appropriate power...but is this it?

By the way I already ordered the "Competition Electronic Prochrono"...I just couldn't resist...Now I'm sitting back wondering what I'm going to use it for...sort of the story of my life

[Dragon55]

you could use it for energy delivered if you're interested in that. You know mass x velocity. I have thought about getting one myself but also wonder why.
BUT.... no never exceed max charge. That goes double for me since I reload .40 s&w.

[FlyFish]

Most of us are limited to 100-yard ranges. In order to sight a longer-range hunting rifle in properly - whether for a particular distance, or to maximize "point blank" range - you need to know the ballistic coefficient of the bullet and the muzzle velocity. You can then use published ballistics tables or (better) ballistics software to determine where to sight the gun in at 100 yards to produce the desired results. Bullet manufacturers publish the BCs of their bullets, but the only way you can know the real muzzle velocity, as opposed the published figures which are only approximate, is to use a chronograph.

[rwilson452]

Help with Understanding the Chrono
Asked this on another forum but the thread died out before I got answers...

I reload for many handgun cartridges and a 223 AR. I like gadgets and hear many of the seasoned reloaders speak of their use of a chrono. Some suggest they don't know how they got by without it.

So my question is "How do you use the velocity data that a chrono gives you"

For long range shooters you get your velocity and can predict the drop at range better. when your working up near Max load, when an increase in powder doesn't result in a corresponding increase velocity your at max.

If your load isn't giving you a good group but your velocity is showing you should you have another problem. What problem it could be are many. With most of my rifles I find they like to work a a specific velocity for a given bullet weight.

Do you load to a velocity despite the load limit data or are you stuck inside the confines of the load data despite the velocity. (I think I already know the answer to this one).

Yep, don't exceed the book max.

How about handgun data in particular...is it for interest sake or does it change what you do?

I don't shoot that much pistol. Just practice for self defence. 2-3" groups at 25 yards is about all I can manage anyway. My eyes aren't what they were.

If accuracy is 1 hole on paper...whats the difference what the velocity is?

covered in the first part.

I suspect hunting is an issue assuring appropriate power...but is this it?

Power is good, accuracy is final.

By the way I already ordered the "Competition Electronic Prochrono"...I just couldn't resist...Now I'm sitting back wondering what I'm going to use it for...sort of the story of my life

Enjoy! another reason to go shooting!

[dlb435]

I use my chrono to check my handloads. This gives me the velocity to compare with the data listed for that load. I can also see the range of velocities, so I know how consistent my loads are.
This helps a lot with knowing if a load is good or if I need to increase or decrease the powder charge.
It's a lot better than guessing and a good little chrono can be picked up for about $70.00 at WalMart.

[Bart B.]

Chronographs are a lot like the girlfriends a guy has. Your own performance will effect their performance. If your performance is lacking, their's will be lacking, too. Here's a case in point......

Two people were out shooting the same .308 Win. rifle with the same ammo. One got about 2600 fps average and the other got about 2500 fps. Velocity spread with both was about 20 fps.

One held the rifle resting atop bags on a bench top very tight against his shoulder; his shots were in the 2600 fps range. The other held it looser on the same bags. I was the one holding it looser.

Both of us slung up in a solid prone position on the ground got about 2570 fps.

[ForneyRider]

I got the Chrony Beta for much of the reasons already posted. But, I recommend the remote.

Also, it helps to have someone to position the chrony, as I tend to walk back and forth several times to make sure it is lined up between the shooting bench and target.

I usually try to pace off 10-12 ft, but have seen 6ft to 12ft in the gun mags.

For my H110 loads, I had to move to 12 ft because of muzzle blast. I was 6-8 and was getting lots of errors.

They are a lot of fun, also increases your chance of shooting someone else's gun at the range.

Some of the more expensive ones have timers, so you can practice for competition.

[Magnum Wheel Man]

just a couple things...

since the long range trajectory was sufficiently explained, I'll just add a few other comments

for just all around & target shooting...

a Chrony can tell you if your loads are of consistant velocity... ( lets talk your handgun loads, but just as important for rifle loads )... at 50 ft, you may be happy with a 2" - 3" group, but if the powder you are using is producing 100-200 fps or more differences in velocity in a 10 shot string... it can be signs that going to 50 yards will open up your group disproportionately to the distance increase, the highest velocity loads may actually be going above recommended pressures, even though 90% of your loads may well be easily in the safe zone... now you accidentally get that 1 - 2 granules more powder or you bullet gets seated a couple .001's deeper ( maybe you dropped the cartridge, or what ever ), & now your gun locks up or blows... you are better off choosing a powder that offers the most consistant velocities out of your particular cartridge or firearm... & while normally group size will be a good indicator, it may not at closer handgun distances...

Hunting... now either you are wanting a hotter load, or maybe not ??? but often the premium hunting bullets will give a velocity spread for best performace... the Chrony helps insure that with your load, & in your firearm, that you are niether too fast, nor too slow for best bullet performance on game...

hope that helps some...

[wncchester]

Theory asside, a real world situation: My old Vangard .243 has always shot well but trajectory didn't match what the manuals predicted. After I got a chronograph I found out why. It shoots "hot loads" about 300 fps lower than predicted!

Had a similar problem with a 700 .30-06. Found it shoots about 150 or so fps slow.

After adjusting my loads, the poor trajectories, past about 150 yards anyway, cleared up.

[snuffy]

Without a chrono, you're groping around in the dark. You don't really know what's going on with your loads.

If all you do is load standard loads, with a run-of-the-mill jacketed bullet, you don't rally need a chronograph.

BUT if you load cast boolits, or lighter or heavier bullets, you need to know if you're succeeding in your goals.

Consistent ammo will show consistent velocities. Since I'm not familiar with the chrono you ordered, I don't know what kind of info will be in the read-out. My Pact gives me the following info; Velocity for each shot in the string up to 99 shots. It then reads out the average for however many shots there were in the string. Then it gives the ES, or extreme spread, or the difference between the highest and lowest. Then it gives the highest velocity, then the lowest. Then it gives the SD, or standard deviation. Then it gives AD, which is a better representation of SD, always a lower number.

Standard deviation is a mathematical representation or means of predicting an average velocity. For instance, if a 10 shot string of .308 ammo has a SD of 15, and a velocity of 2600 fps, you can predict that all the velocities of identical ammo will be within a window of 15 FPS above or below the average velocity. The lower the SD, the better the consistency of the ammo. It, however, does NOT mean that the group size will be better! Group size is a function of barrel harmonics, bedding, sight type and quality, and shooter skill.

As far as never going above what a manual says is max, I've done it on several occasions. But only while running groups while shooting over the pact chrono. AND only when the group size was shrinking AND the velocity was increasing the same amount for each incremental increase in powder charge. (Also watching for pressure signs on the brass, and bolt lift).I certainly don't recommend doing it if you're new to reloading, I've been at it for 46 yrs.!

[SKULLANDCROSSBONES65]

G'day. I think most things have been covered already. I think that Lead bullets need to be under a certain velocity.

One important reason has been left out, a Chronograph can be used to get Beer, or any other item of value that people might bet with. A Chronograph should stop a lot of BS talk from those big mouthed know-it-all that you come across from time to time.

[Magnum Wheel Man]

a Chronograph can be used to get Beer

very good, & true to boot...

[FlyFish]

I use statistics every day in my profession and I'd like to correct a bit of misinformation about the statistics that chronographs calculate for you.

First, the AD, or average deviation (statisticians tend to call it the "mean" deviation) is the average of the absolute values of the individual deviations from the mean of the group. If you calculate the average of all the velocities in a string of 10 shots (or whatever - the number of shots doesn't matter), then subtract that average value from each of the individual velocities, you get a group of 10 positive and negative deviations from the average. Make the minus values positive and calculate the average - that's the average deviation.

If you calculate the deviations as above, but then square them before calculating the average, that produces a value known as the variance, which is obviously going to be a larger number (because of the squaring). If you then take the square root of the variance to return to the original scale of measurement, you produce the value known as the standard deviation (SD).

In a so-called normal population (population of measurements, that is, not people - and in this case we're talking about velocities), the SD can be used to describe certain properties of the distribution. (A normal population is what lay persons refer to as a bell-shaped curve.) If the population is truly normal, and few real populations of measurements actually are, then 67% of the measurements (velocities) will fall within plus or minus one standard deviation (sometimes called "1 sigma) of the mean (average), 95% will fall within plus or minus 2 sigma, and 99% will fall within plus or minus 3 sigma.

Generally speaking, if a single measurement is more than 2 sigma away from the mean, you should begin to question if it really is from the same population - in real terms, you might begin to wonder if you dropped a bit too much or too little powder, or crimped it differently, or if the chronograph failed you, etc. If it's more than 3 sigma off, it becomes very likely that something unusual happened, and by the time you get to 4 sigma it's a virtual certainty that there was something different about that round and it's not an accurate representation of the remainder of the group.

And in all honesty, even though I know this stuff like I know the back of my hand, I tend to look at the average deviation, assume smaller is better (which it most likely is) and let it go at that.

[Bart B.]

Mr. FlyFish, thanks for your excellent and and well written info on statistics in chronographs.

Question, please....How many shots in a group are needed to indicate 90% confidence that the size of it is what the firearm + ammo + shooter actually have?

Military arsenals typically shot 200 or more shots in each test group for 30 caliber ammo at 600 yards. That's a bunch, but their statistical level of confidence had to be very high. And they're quite impressive when the 10-inch group has 250 holes in it; very dense in the middle but rather sparce at the outside edge.

[FlyFish]

Bart, thank you for your kind words, sir.

With regard to your question, very generally speaking there is no particular number of measurements that yields 90% (or any other %) confidence. The way statistics works is that as the number of measurements you have to work with increases, the confidence increases, and therefore the confidence interval (defined below) decreases. I'll use an example to illustrate.

Let's assume you have fired four individual five-shot groups and the individual results for extreme spread are, respectively: 1", 1", 3", 3". That gives you an average of 2". To calculate a confidence interval (i.e. the range within which we know the "true" group size is, at a specified level of confidence), we need to calculate a statistic known as the standard error of the mean, which is the standard deviation (as discussed earlier) divided by the square root of the number of measurements. In this example, the standard deviation is 1 (in fact, it's usually calculated to be a bit larger, for reasons we don't need to get into), and the square root of the sample size (4) is 2, so the standard error is 0.5. We then go to a table of so-called "t" values to obtain the value of 2.35 for .9 (i.e., 90% confidence) at 3 degrees of freedom, which is the sample size (4) minus 1. The 90% confidence interval, then, is the mean (2") plus or minus 2.35 times the standard error (0.5). In this example, the confidence interval is therefore 0.825 to 3.18 inches.

In English, what that means is that from the data we have to work with, we can have 90% confidence that the true group size falls in the range of 0.825 to 3.18 inches. (If there are any real statisticians out there, I know that it in fact doesn't really mean that, but that's how it's commonly interpreted).

Now, let's see how increasing the sample size decreases our confidence interval. Let's assume that instead of 4 groups we fired 16 groups, with eight of them measuring 1" and 8 measuring 3". Obviously, our average group size (2") hasn't changed, and (perhaps less obviously) neither has our standard deviation - it's still 1. But now, the standard error of the mean is 1 divided by the square root of 16, which is 0.25 (i.e., 1 divided by 4), and we have 15 degrees of freedom (16-1), so the t value from the table is 1.753. Now the 90% confidence interval is between 1.56" and 2.44".

If you want to be more confident, say 95% or 99%, using the same data, the t values that come out of the table will be larger (3.2 and 5.8, respectively, for the 4-sample case), and therefore the confidence interval will be larger (which should make sense).

The obvious conclusion, as any good statistician will tell you, is that it's always better to have more data! The corollary, as any good statistician will also tell you, is that you never have enough data.

I hope that made sense - if not, let me know and I'll try again.

[snuffy]

Thanks flyfish,,---er I think thanks is the right word. But you gave me a headache. I failed every math course I ever took, but your explanation made sense to me. At least I now know what the AD number means, um-- at least I think I do.

What I do know is I wouldn't be without my chronograph. My first one was made by a couple of electronics engineers by the name of Schmidt-Weston. It was a huge box by today's standards. The main box was aprox. 5" high X 16" long and 9" wide. The screens were alum boxes about 4" square, they each had a 9V battery in them and a bunch of circuitry. It could be run with it's built-in gell cell, off a 110 volt source, or off your car battery. All that and all it did was give you a velocity for each shot! You had to write down each one, then do your own calculations!

A friend that was an engineer gave me a formula for calculating SD. I managed to mess it up every time I tried to run it on a calculator of 1979 era. He then told me of a calculator that cost over $100.00 that had a built in program for SD, then he fished around in his drawer and said "this is my old one that has that program in it, use this key for that". I simply input the velocities, hit that key and viola, out came the SD number! Simple huh? This is kind of to point out that these kids now a days don't know how good they have it!

[FlyFish]

This is kind of to point out that these kids now a days don't know how good they have it!

Absolutely correct! You can also do a lot of these calculations quite easily in Excel - in fact, I have a spreadsheet I made up that plots the location of each shot in a string, calculates the center of the group, gives you an AD based on location - that is, what is the average per shot deviation from the group center, and then prints out what the group looked like so you don't need to save the original target.

However, some of the built-in statistics in Excel aren't programmed correctly, or make assumptions that aren't necessarily appropriate, so you need to be careful and/or know how to make corrections along the way.

[snuffy]

It seems this discussion has gotten off target,,, no pun intended. I was strictly talking about chronograph readings, targets and groups are an entirely different subject.

Group analysis is something that could/should generate another thread. We all strive for that magic 5 shot-one hole group. In reality, in this universe, it's not done very often. Even custom built rifles, firing special handloaded shells, are hard put to do it. The skill of the shooter is where the problem lies. Did he have an argument with the better half? Too much coffee? Wind? variable light conditions? Then, can you repeat that group with shells loaded at a later time?

Again, I'm certainly NOT a statistician, I simply look at a group for it's symmetry, and measure the size with a calipers. Then, if shot over the screens, I look at the ES and SD numbers for a feel as to did it work??? If I were able and interested to mash numbers, it would probably mean more, but I doubt I would enjoy it as much.

[Bart B.]

Flyfish, thanks for your time and explanations. That explanation cleared up a couple of things I didn't quite understand. Now I do.

Talked with an engineer at Lake City Arsenal in the early 1970s about this when the military team I was on had problems with one lot of LC M118 match ammo. We called that lot our "90% confidence lot" as about 1 round in 10 would strike 2 feet low at 600 yards; the other 9 did just fine. Checking charge weights across several dozen showed all to be correct; maybe it was the primers. He said their test groups usually had 200 or more shots each; they wanted a really high level of confidence about the accuracy of each lot. But couldn't offer any explanation why we were getting bad shots.

He's the one that convinced me to shoot at least 20 shots per test group which would be a lot more meaningful than four 5-shot ones or even seven 3-shot ones