“No mere gadget, the chronograph is one of the most powerful tools imaginable in load development and problem diagnosis. A bullet’s velocity is one of the major contributors to its behavior, and if you don’t know what its velocity is, you may never understand that behavior. But knowing why the bullet behaves as it does, you can take intelligent corrective action that wasn’t available to you before.”
– Dan Hackett, Precision Shooting Magazine
Most of these tips are intended for chronographs that use a natural or incandescent light source (Ex: Shooting Chrony, Competition Electronics ProChrono, CED Millennium 2, Oehler 35P, etc) and may not all be applicable to chronographs using infrared light sources or other types of projectile sensors.
- You’ll get the best readings with even lighting. Best case scenario is overcast skies, and worse is direct sunlight on the skyscreens.
- Remove diffusers in overcast or shaded conditions. The diffusers are only necessary in sunny conditions, and by removing them in other conditions you’ll allow more light into the chronograph.
- On sunny days place the chronograph in the shadow of a building or opaque wall. Ensure the sensors have a clear view of the sky, but the chronograph itself is in the shade. You could alternatively add sun shields to create a shadow (see Advanced Techniques section for more details). The goal is to eliminate direct sunlight on the chronograph, while still ensuring the sensors have a direct view of the sky. However, avoid placing the chronograph in the shade of a tree. The uneven, dappled sunlight that filters through the tree will cause more problems than it will solve.
- Avoid low light conditions, such sunrise and sunset. During those times the extreme low angle of the sun can cause additional issues due to reflections.
- Clean skyscreens. If you’ve used your chronograph more than a dozen times, there is likely a build-up of dust on the sensors. Use a Q-tip or canned air to clean the lenses, being especially careful not to scratch them.
- Ensure chronograph is level
The formula the chronograph uses to calculate the velocity always assumes the bullet path is perpendicular to the skyscreens (i.e. like Path 1 shown in green). That means the chronograph assumes the bullet travels exactly 12.0” from the time the bullet was seen passing over point A until the chronograph saw it pass over point B. If the bullet was shot at an angle, it would have to travel farther (in our example ½” farther) and would therefore take longer to travel between point A & B. So for Path 2 the chronograph would record a slower time, but since it thinks the bullet just traveled 12” … it would say the bullet was going slower than it really was. Here is a concrete example:
Times chronograph recorded for the bullet to travel between points A & B:
- Time 1 = 0.000333 seconds (Path 1, green line)
- Time 2 = 0.000347 seconds (Path 2, red line – longer because it had to travel slightly farther)
The unlevel bullet path in this example would result in a chronograph reading that was over 4% slower than it was actually travelling. Although I used an extreme example to illustrate the concept, even slight angles can have dramatic effects in the velocity readings the chronograph produces.
- Ensure sensors aren’t tilted. If using a Shooting Chrony, ensure it is completely unfolded. This would result in a similar margin of error as the example above, because the real distance the bullet traveled doesn’t match what the chronograph is assuming it did. But in this example, the reading would actually be around 4% faster than the bullet was really travelling.
- Position the chronograph 15-20’ from the muzzle to avoid interference from muzzle blast. To get true velocity at the muzzle (what most ballistics programs need), you can typically add .75 mph per foot. For example, if the chronograph was 20’ from the muzzle and displayed a reading of 3,200 fps you would just add on 20*.75, which is 15. So your true muzzle velocity would be 3,215 fps.
- Paint all surfaces flat black to avoid erroneous readings caused by reflections, including all chronograph and tripod surfaces that would be in direct line of sight with the bullet as it passed over the chronograph.
- Use a quality tripod for a stable platform.
- Avoid shooting over reflective surfaces like snow or some types of sand. May be able to reduce reflection by laying out a large, dark colored tarp under the chronograph setup.
- Replace batteries regularly to ensure stable power.
- Understand the flight of your bullet will be lower than your sight picture. Most scopes are mounted 1.5” higher than the centerline of the barrel. Always aim much higher than the middle of the shooting area.
- Avoid using near florescent lamps. Although we can’t detect it with our naked eye, they actually vary in intensity many times a second and chronographs will detect these changes rendering them virtually useless.
- To work indoors, chronographs need a lot of even lighting … which is hard to pull off with incandescent lamps. A white background with even amounts of light over the sensing area works best. Some chronographs offer optional accessories that help make them more reliable indoors.
Attach flat black sun sheilds made of foam board or cardboard to the guide rods to create a shadow. I bought 2 pieces of black foam board for $4 at a craft store for mine. This will help prevent direct sunlight from shining on the chronograph, but still ensure the sensors have a direct view of the sky. Warning: In windy conditions, these shields could transform your chronograph into a very expensive kite.
Knowing that chronographs prefer even lighting, you can also help by adding a translucent light diffusing cover just a few inchces above the included diffusers. This would only be necessary on sunny days. The plastic diffuser sheets commonly used flourescent lighting works well for this, and can be found in 2′x4′ sheets at local hardware stores. Note: You shouldn’t mount or even attach the diffuser sheet directly to the chronograph. Instead create a frame that holds the diffuser sheet, so you can simply set up the chronograph underneath it.
If you are using a CED Millennium Chronograph, you can purchase upgrade parts to replace the standard visible light sensors with infrared skyscreens. Infrared sensors eliminate the need for sunlight, and can even record velocities in total darkness … which is exactly what some people opt to do. If your chronograph has infrared sensors you can actually place it in what is referred to as a “coffin” with shoot-through windows, and virtually eliminate all problems caused by lighting and reflections. Coffin setups only work with infrared skyscreens, so this setup wouldn’t work for the Shooting Chrony or ProChrono. Here is a good example of a coffin I found in a forum post:
To increase the accuracy of the velocity readings (or at least your confidence in the measured velocity), shoot through 2 chronographs in a “close tandem arrangement” to ensure they register the same velocity. You should add 2-4 fps to the velocity reading of the chronograph that is further downrange to account for the slight velocity loss that occurs in just a few feet. You probably shouldn’t see more than 5 fps difference after the adjustment.
For more info on chronographs, I suggest reading the Oehler 35P Instruction Manual … it contains a ton of great info written first-hand from the leading expert in this field, Ken Oehler. Most of the info is applicable to any chronograph, not just the 35P … although the 35P is the best personal chronograph in the business.