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Handloading & The Chronograph.

The ballistic data provided by this simple electronic device is indispensable to serious hunters, shooters and reloaders.

There is an aura of mystery associated with the modern chronograph. In spite of the myths and misconceptions surrounding this simple electronic device, the number of serious handloaders who don't have one is surprising.

There is really nothing mysterious about a chronograph. Picture a really fast stopwatch and you've got it. Of course, bullets really hustle so the stopwatch needs to be able to break time down to millionths of a second. If you know how long it takes for a bullet to travel a known distance you have everything you need to solve a relatively simple math equation.

The most common form of this equation is D=RT (distance = rate x time). This equation determines how far an object could travel at a fixed rate and in a known period of time. Rewritten to fit the needs of handloaders, the equation becomes R=D/T (rate equals distance divided by time).

The really hard part of the operation is figuring out how to turn the stopwatch on and off. You normally can't see a bullet in flight and even if you could, it would be virtually impossible to push a button that quickly. The earliest chronographs used vacuum tubes for timing and a thin copper wire to start and stop. The bullet had to hit both wires to record a shot.

A major advance was the printed screen which gave a much larger target area, but it took the transistor, quartz clock and integrated circuit for chronographs to move out of the ballistic laboratory to the range. When Ken Oehler developed his "skyscreens," the hobby chronograph became a reality.

Seeing The Light

The skyscreen is a wonderful invention, based on a simple premise. A photo-sensitive diode triggers the timing circuit when it detects a small, fast change in the light falling on it. Perhaps the best analogy is that it detects the shadow of the bullet.

In the early days, skyscreens were easily fooled -- the shadow of a cloud passing over could skew the data. Electronics have been refined and Oehler came up with his "glint shields" that put a diffuser over the skyscreen.

The net result was a more even distribution of light. Direct sunlight falling on the photodiode is not a good thing. In fact, the biggest problem with chronographs these days is probably those stray rays of light that bounce around inside the detector's housing. You'll notice the molded plastic that holds the detector is almost always black and there may also be molded ridges inside to break up reflected light.

The latest improvement to hobby chronographs is Oehler's Skyscreen III that places the detector much deeper within the housing and also provides a much wider glint shield. Yet even those screens can be tricked now and then. A small piece of transparent tape on the slit over the detector will combat those stray light rays. The tape doesn't reduce light transmission too much but serves to diffuse the direct rays that get by the shields.

Elementary Arithmetic

Going back to our simple math for a minute we need to look at the distance part of the equation. The earliest chronographs needed the bullet to travel a fairly long distance to get accurate measurements because their clock speed was slow. Laboratory chronographs of the time often used a 20 foot spacing between the wires.

With the arrival of the quartz crystal clock, speeds of a million cycles per second or more reduced the distance requirement. Today, spacings of a foot or two yield perfectly accurate results.

Speaking of accuracy: Chronographs have limitations, and sometimes they'll lie to us. Every electronic component has a tiny amount of variability The higher the quality of the part. the smaller the tolerance will be, but there is no such thing as a perfect part with no variability.

PACT, one of the largest makers of hobby chronographs, states that the accuracy of their chronographs has a tolerance of plus or minus 0.05 percent. That's a very nice, small number, but it tells us we need to pay careful attention to numbers. Remember that chemical reactions of burning (as in powder) or explosion (as in primers) never give exactly the same amount of energy every time.

As has been the case with every major electronic development, prices of chronographs have generally come down. Sometimes the price will remain stable but new features will be offered. A perfectly serviceable basic chronograph is readily available for well under $100. The more technologically sophisticated the chronograph, however, the more expensive it will be.

Chronographs today can store a large number of shots, download the data to your computer, calculate all sorts of statistical information and print it out for you.

Almost every chronograph above the bare bones level will do some number crunching for you. It's nice to have the average velocity and we rely on extreme spread for information about the consistency of a load. Most will also compute a standard deviation.

The Ins And Outs Of Chronographs

So what can -- and should -- a chronograph do for the average shooter? Someone who reloads just to have ammo to shoot probably doesn't need one because it really doesn't make a difference whether the load is going exactly as fast as it says in the manual. It simply isn't worth losing sleep.

The person who likes to experiment and doesn't have a chronograph is living in fantasyland. It is so rare as to be remarkable for a handload to ever do what the book says it will in your real-world gun.

I use the chronograph almost every time I shoot because of the things it can tell me. If a load is too fast, I reduce it -- too slow, perhaps there is room for a bit more powder. The single thing I like to see most is the relationship between speed and accuracy. As a general rule handguns tend to be more accurate with lower velocity loads. Rifles are often just the opposite.

With three velocity data points, you can construct a graph of powder charge vs. velocity and come very close to the powder charge needed to achieve a particular velocity. Obviously this has to be limited by the maximum pressure for the cartridge.

The relationship between powder charge and velocity plots a straight line and as long as we don't go over the pressure limit we can learn a lot. There comes a point where the wheels fall off and things really go awry, however. At a point that is usually pretty far above the listed maximum powder charge, that nice straight line begins to flatten out as a specific powder increase produces a smaller and smaller velocity increase.

The graph has a plateau where there is no increase in velocity with an increase in powder charge. Then the pressure goes straight up with a small increase in charge weight. This is somewhere around the point of spontaneous disassembly of the firearm. It is a distinctly bad place to be.

If the specific velocity of a given load is really important, the only solution is to buy a chronograph and get an answer. Data generated in my gun is only roughly comparable to yours. The modern chronograph is so easy to set up and use there really aren't too many reasons for not having one if velocity matters to you.
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Author:Petty, Charles E.
Publication:Guns Magazine
Date:Oct 1, 2000
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