A good chronograph is an essential piece of equipment for any serious long-range shooter. Knowing how much a reloaded or factory cartridge deviates from the average velocity allows competitors to choose the load that will perform most consistently at extended ranges.
For example, small velocity deviations won’t affect the accuracy of a 308 WIN round at 100 or even 200 yards, but at 900+ yards a 20 or 30 foot-per-second (fps) difference can make or break a shot. A 168g .30-caliber projectile will drop about 337 inches at 900 yards traveling at 2610 fps at the muzzle. But if the velocity drops down to 2580 fps, that same projectile will drop an additional 10 inches at 900 yards.
Identifying which load or factory cartridge maintains the most consistent velocity is a key piece of information, and a reliable chronograph is the only way you’re going to get that info.
If you’ve been suspicious that the $50 Chrono you bought on Amazon isn’t throwing the right numbers, the Two-Box Chrono from Autotrickler might be just what you’ve been looking for.
Why Is It Better?
The Two-Box Chrono was developed by Canadian long-range competitive shooter Adam MacDonald. MacDonald jumped into competitive shooting about five years ago after a career in software and electronics development and won the Canadian F-class championship in 2018.
The Two-Box Chrono (along with the Autotrickler and Shot Marker – Read GunsAmerica Review of AutoTrickler AutoThrower System HERE) is a key part of MacDonald’s scientific, evidence-based load development strategy. While other chronographs have an inherent random error of +/- 6-10 fps, MacDonald guarantees the Two-Box Chrono will maintain a repeatability error of only 0.5 fps SD or 0.02%.
The science behind the device is relatively simple. The Two-Box Chrono uses acoustic rather than optical sensors, which can record when the supersonic sound wave hits each box down to the microsecond. The sound wave is extremely strong and precise, so when the boxes are placed 15 feet apart, MacDonald can guarantee a margin of error of less than 1 fps.
“There’s literally no way the sensor itself can be ‘wrong,’” MacDonald told me via email. “The shockwave hits the sensor like a brick wall, it records the time exactly, and that’s it.”
Other chronographs, by contrast, have more uncertainty in the measurement which produces a random error in the result.
“Optical sensors have to trigger from the shadow of the bullet which may not be clearly identifiable,” MacDonald continued. “Doppler radar has to average many measurements and do a lot of math, and there is a margin of error. The acoustic approach is much more black and white.”
Setting up the Two-Box Chrono is relatively simple. Users place the first box 6-12 feet in front of the muzzle and the second box 2, 6, 10, or 15 feet beyond the first. The cable comes with yellow markers to ensure the boxes are spaced at the correct distance.
Both boxes must be within five feet below the bore line. If you’re shooting prone, this isn’t a problem. I found it helpful to use a board underneath one or both boxes to help with alignment (more on that below). If you’re shooting from a table, both boxes include tapped holes for mounting to tripods or bolting to another kind of raised surface. MacDonald cautions that whatever mounting system is used must be rock solid to keep the boxes from moving during shooting. If the boxes are screwed into tripods, he recommends hanging weights underneath to be sure they won’t move.
Once the boxes are placed securely, they must be aligned so they are parallel with the bullet path. The package comes with two magnetic peephole sights which should be aimed below the target an equal distance to the rifle’s height above the boxes. In other words, if your rifle will shoot three feet above the boxes, you should aim the boxes three feet below the target.
The chronograph will still work if the boxes aren’t aligned, but all measurements will be either slightly below or above the true velocity. This isn’t a problem if you’re measuring which load produces the lowest standard deviation, but if you’re looking for actual velocity, you’ll want to align the boxes.
Finally, turn the first box “On” and choose the correct distance from the menu (2, 6, 10, or 15 feet; be sure to press the “O” twice to confirm). The lithium-ion battery lasts for 60 hours and charges in three hours. MacDonald mentioned that if you ever arrive at the range and the boxes are dead, plugging them into your car’s USB port for about 15 minutes should provide enough juice for that day at the range.
Uses, Advantages, Disadvantages
I found the Two-Box Chrono to be relatively easy to set up and use. From the prone position, I simply used a long board to steady the boxes and stapled my paper targets about two feet from the ground. This setup ensured that the boxes were parallel with the path of the bullet at 100 yards, and the results appeared to be accurate even with the boxes only two feet apart.
Setting up at a public range was somewhat more difficult. It takes a minute to align the boxes and carrying two tripods was a hassle. Range goers are among the nicest folks I’ve met, but I felt bad making everyone stop shooting so I could measure and align my chronograph.
Shooting at a public range also made tracking my shots more difficult. The boxes are extremely sensitive, so they picked up my shots along with everyone else’s. This isn’t a deal-breaker, since everyone else’s shots registered wildly different velocities than mine, and it wasn’t difficult to differentiate my shots from theirs. Still, when I reviewed the data, I had to churn through everyone else’s shots to find my own.
Users should also be aware that the Two-Box Chrono only registers shots traveling faster than 1250 fps. Since it uses the supersonic sound wave to track velocity, if your round isn’t travelling faster than the speed of sound, it won’t show up. This isn’t a problem for long-distance rifle shooters, but if you’re looking for a chronograph to measure pistol rounds, .22lr rounds, or subsonic 300 Blackout rounds, you’ll want to look elsewhere.
I don’t fault the Two-Box Chrono for any of these apparent disadvantages: MacDonald designed the device for developing loads for long-distance shooting. For that application, you won’t find a more precise Chrono for $250. Especially if you have access to a private range and can perfect your setup procedure, the Two-Box Chrono will deliver near-perfect velocity readings every time and ensure that you can find the perfect load for your next match.