The importance of the proper use of a chronograph is not one that I would ever understate. One of the biggest influences on the performance of a given cartridge of any caliber is the speed of the projectile as it leaves the muzzle. There are devices out there, called Chronographs, that can be used to measure the speed of the projectile as they leave the muzzle of the rifle. Setting up the chronograph properly and what the different results mean are a little less known to the average shooter. So this post will explore what a chronograph is, how to set it up, and how to interpret the information the chronograph gives us when using one!
Examples of Chronographs
There are many chronographs out there. Names like Shooting Chrony, Competitive Edge Dynamics, Oehler, and Magnetospeed are among the most common. Until the unveiling of the Magnetospeed chronograph most of the chronographs on the market, regardless of the manufacturer, had the same design. A set of triangular ‘screens’ are set up with sensors at the bottom of the inverted triangle. You fire the rifle, pistol, whichever through the two screens. The distance between the screens is fixed and known. The two sensors measure and read the passage of the bullet and math compares the times with the distance to produce a velocity.
Above is a photo of a CED Millenium 2 chronograph that we reviewed a while back. It helps illustrate the basic design and configuration of the chronograph. Most chronographs use the same design. The differences you will see in price reflect the quality of the sensors and electronics. The purpose of the screens is to block sunlight and provide consistent lighting conditions for the sensors. Varying light conditions can often lead to erratic sensor readings producing inaccurate velocities. The chronograph above is equipped with a set of Infrared Screens which provide continuous and consistent infrared light to the sensors. This helps mitigate issues with lighting.
Many times you will see the user of a chronograph build a box to put the chronograph inside. Then the only light source is that of the infrared screens and sunlight and clouds are not even able to enter the equation. While that likely provides much more consistent readings, it also increases the amount of hassle in setting one up. Alignment of the screens is also important. If the screens are not level, or the bullet passes through them at an angle, this can also affect the accuracy of the readings. You should be getting the picture with regard to the level of work required to use the basic chronograph setup to get good data by now.
Chronographs Born Again
With the unveiling of the Magnetospeed chronograph, life for all the rifle shooters got a lot easier.There was a technology shift and new design implemented that allows shooters to strap the sensor, called a bayonet, to the 6 O’ Clock position under the barrel near the muzzle. The Magnetospeed reads the actual presence of the metals in the projectile as it crosses the sensors and then interprets the readings. Not only is the whole package smaller, it it much easier to set up! It also provides very accurate and reliable readings since the sensors don’t rely on lighting conditions to detect the passage of the bullet.
Bryan Litz and Applied Ballistics actually tested the accuracy of a number of chronographs and published an article with the results. Among the most accurate tested was the Magnetospeed chronograph. Some people have suggested that you may alter barrel harmonics by hanging the device on the end of the barrel. Most users report either minimal or no shift in point of impact. I’ve noticed a very slight shift in point of impact that may have actually been caused by me and not the chronograph. Regardless of the source of the shift, in my experience the rifle still groups as well with the chronograph mounted as without, which is the important part.
When you use one or read about guys using a chronograph when doing load development for a rifle you hear terms like Standard Deviation (SD), Muzzle Velocity (MV), Extreme Spread (ES), but what do those mean? Understanding what the chronograph is telling you is an important part of using one, especially during load development. The goal when developing a hand loaded cartridge for a specific rifle is to have the most consistent shooting ammunition as possible. You want consistent velocities generated so that your results down range are also consistent and predictable.
The most important and often discussed result is the SD. You want a consistent and low standard deviation. Bryan Litz talks a lot about the consistency of ammunition and the ability to make first rounds hits at distance. SD of the ammunition is a major factor. Standard Deviation is a term that describes how far off from the average of a string of fire each shot is. If I fired a string of ten rounds with an SD of 10fps that means each round is expected to be 10fps off the average velocity of that string of fire.
Extreme spread isn’t what you see late at night on Cinemax! It describes the maximum difference in speed between the fastest and slowest rounds measured in a string of shots. This is also important because a high ES will result in large velocity differences which will result in different points of impact at distance. You have to have a tight SD/ES if you want consistent hits at long range. Most people consider single digit SD the benchmark for long range performance. You would try to develop a hand loaded cartridge that when fired over a chronograph has a Standard Deviation of 9fps or less. If you manage to get a load shooting that tight the ES will be reasonably low as well. I’ve heard under 30fps is good, obviously the lower the better, I try to keep it under 20fps for the ES.
Obviously a chronograph will tell you what the average speed is, and that is equally important because that is what you will be using as your muzzle velocity. Its 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 will often distort the readings if the screens are too close. This is another benefit of the Magnetospeed! The velocity you measure is your muzzle velocity!
Wrapping Up Chronograph Use
If you are in the market for a chronograph I highly recommend the Magnetospeed. It’s one of those inventions that really had me saying, “Why didn’t I buy this sooner?” It is that much easier to set up and use reliably. I’m not knocking the conventional chronographs. I still keep my CED M2 because I can use it to measure pistol velocities that can’t be measured with the Magnetospeed. However there is a lot of care and effort that go into properly setting up a regular screen type chronograph, so if you have one make sure the screens are level and the bullet is passing perpendicular to the sensor windows. Also be mindful of your lighting conditions.
When you get into reloading watch the numbers the chronograph gives you and really strive to hit the single digit standard deviation benchmark. I’ve yet to have a rifle that’s put together properly not shoot well with a hand load that has a single digit SD. We’ll get more into how to tweak hand loads in future articles so if that’s what you are interested in, we’re going to be discussing that in the future!