At times, I’m somewhat surprised that most pistol shooters don’t use or even own a chronograph. Life is better with one than without. Unfortunately most pistol shooters rely on published load data and word-of-mouth pet loads, and then never look back.
There are lots of chronographs offered and their price range can be fairly broad. Although, Shooting Chrony, Competitive Edge Dynamics, Oehler and Caldwell all make sufficient products well within the budgets of most pistol shooters.
Most of the chronographs on the market, regardless of the manufacturer, have the same basic design. A set of triangular screens, set up with sensors at the bottom of an inverted triangle. You simply fire the pistol round through the two screens. The distance between the screens is fixed and known. The two sensors measure and read the passage of the bullet between the screens and compares the time when each sensor detected the projectile and then produces a velocity reading.
|Graphic by PRB|
Generally the price differential between different units is directly related to the quality of the sensors, the clock speed and other elements of the electronics. All will come with a set of screens and their purpose is to block sun light and hopefully provide a consistent lighting condition for the sensors. Even modest changes in the lighting can lead to inaccurate velocity readings.
Alignment of the screens is very important. If the screens are not level, or the bullet passes through them at an angle, it can affect the accuracy of the readings. Like most things we do, preparation is far more important than just getting a reading. If the chronograph isn’t level and the bullet’s path isn’t level relative to the sensors, you will get bad readings. So, plan on buying a moderately priced tripod, and bring along a tape measure and carpenter’s level.
At the very least you’ll need to measure the height of the pistol’s barrel in its resting position and adjust the height of the chronograph’s triangular shooting area.
When you read about people using chronographs we typically hear terms like Standard Deviation (SD), Muzzle Velocity (MV), Extreme Spread (ES). So, what do these terms mean?
The most important and often discussed result is the SD. You want a consistent and low standard deviation. Standard Deviation is a term that describes how far off from the average of a string of fire each shot is. If one were to fire a string of ten rounds with an SD of 10fps that means each round is expected to be 10fps off the average velocity from that same string of fire.
Extreme spread describes the maximum difference in speed between the fastest and slowest rounds measured in the same string of shots. This is also important because a high ES will result in large velocity differences which will change points of impact at a distance. You have to have a tight SD/ES if you want consistent hits at long line. For pistol rounds a low double digit SD is very desirable for precision rounds. You should try to develop a hand loaded round that when fired over a chronograph has a standard deviation of 50fps or less. If you manage to get a load shooting that tight, the ES should be reasonably low as well. I've heard under 75fps is good—obviously the lower the better.
Chronographs are principally designed to tell you what the average speed is, and that’s important because that is what you will be using as your muzzle velocity. It’s important to realize if using a conventional chronograph with screens that you have to tell the ballistics software how far the screens are set up from the muzzle. If you measure the velocity at 15ft from the muzzle, that isn't your muzzle velocity. There is speed degradation between the muzzle and the point where the chronograph measures the speed. Why do you have to set them up away from the muzzle? The muzzle blast, or better said its flash, will often distort the readings if the screens are too close.
However there should be a lot of care and effort that goes into properly setting up a regular screen type chronograph So if you have one, make certain the screens are level and the bullet is passing perpendicular to the sensor windows. Also be mindful of your ambient lighting conditions.
During the winter months if there’s snow on the ground, I’ll place my chronograph inside a cardboard box with a hole on either side to replicate the triangular screen shooting area. Overpowering light from snow reflected from the ground, and I’ve seen this happen as well in the summer over pea gravel, can easily overwhelm sensors.
Since we’re competitive shooters we have little choice but to reload. Watch the numbers the chronograph gives you and strive to obtain a very low standard deviation as a benchmark.
As well, a chronograph can tell you about potential failures in your reloading process. A chronograph won’t tell you where to fix problems but it might force you to reevaluate separate elements of how you reload. Such as, does my powder drop work consistently, is my crimp pressure varying due to different case lengths, are my cast bullets consistently sized and has the energy of my current powder lot changed from the previous one? And let's not forget the big one: I've got The Load for my gun and I don't want to lose it.
Although intended for long range rifle shooting, the following link provides an in depth look at the process of using chronographs. It’s published by Applied Ballistics LLC.