A chronograph is indispensable for reloaders and especially during load development, but if used incorrectly, it will be of no use.
A Chronograph measures bullet speed by recording the time a bullet takes to travel the distance between two sensors. Most reloaders use chronographs with optical sensors that measure light intensity. As soon as a bullet passes the sensor, the device detects the change in light intensity. Chronographs usually have two sensors, one for the beginning and the other for the end of the measurement. Start and stop. Since the sensors are at a fixed and known distance from each other, the device can measure the bullet speed. From there the reading in feet or meters per second is provided.
The reliability of measurements is determined by a number of factors. We can distinguish between two aspects of measurement, namely the accuracy or correctness of the measurements against the repeatability or the precision of the measurements. Accuracy is whether we measure the actual bullet speed. Accurately measured velocity is very important for trajectory calculations, especially with longer distance shots, as well as for using QuickLOAD or Gordons Reloading Tools for load calculations. By doing calculations with inaccurate measured bullet speeds, inaccurate and unreliable results will be produced.
The components and electronics in almost all modern-day chronographs are capable of accurate measurements. Therefore, the physical placement or mounting of the sensors in the apparatus during manufacture has the greatest effect on the accuracy of measurements. The distance between the sensors must be controlled very precisely. Most chronograph’s sensor spacing is one foot or 304.8mm.
A a 1mm error in sensor spacing can cause a 0.3% reading error. At a speed of 2 800 fps, the error is then 9.2 fps, luckily with most modern chronographs like the Chrony F-1 we use below, the sensor distance remains fixed if it is folded fully flat and correctly level.
If the sensors are inclined or not at right angles to each other, the velocity measurement will vary depending on the height of the bullet above the sensors.
If the sensors are not level, there may be an error with the position or time at which the bullet is detected and the sensors activated, which also produce an incorrect velocity.
Sometimes the sensors activate slightly too early or too late due to reflection of light from the bullet or glint, the physical properties of the bullet or due to insufficient light. The glint can be eliminated by coloring the bullet in black with a permanent marker, this is rare however.
The further apart the sensors are, the more repeatable and precise the measurements and the smaller the errors. Older design chronographs were sold with the sensors separate so you have to align and mount them yourself, creating a gap for greater user margin of error. Most modern chronographs available today, come in a housing where the sensors are spaced and fixed in the whole unit from the factory, which lessons the margin of error.
The repeatability or precision of bullet velocity measurements is important for reloaders to measure the statistical uniformity of their cartridge’s performance.
A very sturdy photography tripod with level indicator is a great base to mount a chronograph on.
Always try to shoot through the center of the chronograph window or measurement area. It is important for both the measurements and to avoid shooting the chronograph. There are apparently only two types of chronograph users, those who have already hit a chronograph and those who are still going to.
Confirm the cross hair position before each shot. Measure the distance from the muzzle to the center of the sensors. If the sensors are too close, the shot can affect the operation of the sensors, which can lead to incorrect readings. Keep the muzzle at least 3m away from the chronograph. The distance is limited by the available length of the connecting wires. Keep the distance between different range sessions constant to keep the results comparable. Programs like Gordons Reloading Tool and Quick Load calculate your muzzle velocity based on the distance of your chronograph measurement from the muzzle, this value is for you to input, so remember to write it down.
The Chrony F-1 is one of the chronographs we use and highly recommend, it is basic in function, but offers most of what we need, folds up to a small size that can easily be stored in your range bag, and has a cord so you have the screen displaying the speed right next to you during your range session.
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Pay close attention to the following aspects for reliable measurements:
- Always use a fully charged battery as the chronograph’s operation and measurements are totally dependent on electrical power. If the battery’s voltage drops, it can affect the measurements. We always keep a extra spare 9V battery in the range bag together with the Chronograph.
- Always use the sunscreens as a background above the sensors so that the bullet can be “easily seen” by the sensors.
- Shoot only with plenty of bright light. The sensors are light sensitive and require bright light to see the bullet properly.
- Avoid shadows that move across the sensors. This can trigger the sensors in error. In particular, do not place the device in the shade of trees. (Keep the light conditions on the sensors constant and even).
As always, get out there and shoot shoot shoot!