Shooting Chronograph Assassin
May 02, 2016
"Hello, my name is Patrick, and I am a serial chronograph murderer." No, not the very spendy chronograph timepiece you wear on your wrist, the shooting chronograph that registers bullet velocity. Name a brand, and I've whacked it with one payload or another. Killed 'em all, dead.
A few weeks ago, I was at an LE class where we were demonstrating to the assembled students just how ballistic gelatin works. I had a particular load I wanted to test, so I stepped to the line. A friend of mine wanted to get a photo of the event, but his camera (his phone, actually) couldn't do high frame rate, so he wanted a countdown. "Three, two, one '¦ ." On zero, I mashed the trigger. You can fill in the rest.
Shooting chronographs work by housing circuitry that generates a very high-frequency vibration. This is used as its clock. The sensors, called skyscreens, note the shadow of a bullet passing overhead. You tell the program in the mystery box how far apart the screens are, it measures the time elapsed between the two shadow-passing events, and it spits out a speed. So far, no problem. The difficulty arises in sensitivity. You see, depending on time of day, time of year, weather and — I swear — phase of the moon, the screens will be more or less sensitive to a bullet's shadow. In the shade, they pout. Indoors, they get psychotic, as fluorescent lights make them crazy. The allowable area above the screens, where a bullet can pass and be "seen," changes not just from day to day but hourly and even minute to minute.
You can be hunting again and again for the sweet spot where readings come regularly. This is the "chrono dance," and you do it unless you build a special box wired for 120 outlets to hold your screens with incandescent lights over them. That's a box that is hard to schlep to and from the range each time you want to check bullet velocities. I deeply envy range labs that have built-in shooting chronographs such as this.
Add to the dance festivities the varying bore-to-sight distances of different firearms, and you can have bullets passing perilously close to the equipment, even skipping off the edges of them.
The imperative of manufacturing does not always mean the same thing. Some companies use separate sensors, remotely wired to the calculating box with its readout. Others put all of it in one unit: sensors, hardware, batteries, readout. That's when the real fun begins.
The shooting chronograph I killed in that LE class was a Competition Electronics ProChrono Digital. It is one of the "all in one" designs, with sensors, hardware and readout all in one plastic tube. It was drilled end-to-end with a very nice 9mm entrance hole and a larger exit. When we picked it up, parts rattled inside. Sigh.
The shooting chronographs with separate sensors fare a little better but not always. You still have the overhead shades and the legs that hold them. Plus, you have the screens themselves, small plastic boxes about the size of a pack of cigarettes. Some years ago, I was chronographing shotgun loads using a PACT chronograph. It has the electronics on a bench-top box, and the screens, affixed to a rail on the tripod downrange, have attached cables to signal the clock.
I had gone to the trouble of fabricating a plywood screen, a short wall with a hole through it, to keep the muzzle blast from toppling my chrono on its tripod. This worked well, but then I started checking the speeds of the various slug loads.
After a few shots, I got the dreaded "No stop signal detected" showing on the readout. I peered over the plywood barrier to discover a lack of screens, legs and shades. The plastic carnage generated by the sabots was impressive. Yes, the sabots were "only" pieces of lightweight plastic, but they were hard enough and traveling at some 1,400 fps, and they scraped all the chrono hardware off the top of the base rail, leaving it bare on the tripod. The expensive hardware was fine, but the day's work was done, and I still had to buy new screens and shades. The next time I was chronographing shotgun loads, I had an improved plywood shield and spare hardware.
One very compact shooting chronograph design is the Shooting Chrony. It is a steel box that folds up for storage. It has the hardware and screens inside, and you simply unfold it, install the shades and stand it on a tripod, fence post or other stand.
The demise of the Chrony was spectacular. I was checking the speeds of tyrannosaur-level .44 Magnum loads, 315-grain bullets in the close vicinity of the speed of sound. Anyone who tells you he has never crushed the trigger, flinched or yanked a shot out of the group is a big, fat '¦ well, you know. On the fatal shot, the chrono fairly leapt away from me, taking the lightweight tripod with it, snatching the readout box by its cable off the shooting bench. Parts flew, as did bad language. The steel shell was crumpled, and as for the parts that had moments before been inside the shell, they were now scattered dust, drifting in the wind. There was nothing to salvage, not even the steel rods for the shades, as they had been bent by the impact. I couldn't even find the battery.
I've been handloading for more than 40 years, and for most of that time I've had one or another shooting chronograph to use to help fine-tune loads. The earliest one actually required that I shoot it. It used transparent plastic screens, each with a conducting, waffle-like pattern in it, and when you shot the screens, you stopped the two timing circuits. If you didn't shoot the screens, you didn't get a velocity. Since then, however, shooting a chronograph has been a bad thing.
If you're checking speeds, use a rest to reduce the chances of a trigger crush pulling a shot low and into your screens or chrono. Pick a spot on the hill to aim at, or use a target, and take the time to get the chrono positioned at a comfortable level. Remember, when you pick up an AR, the bore is 2.6 inches lower than your line of sight. (You know how I know to tell you this, right?) Also, if someone wants to know what his ammunition is doing, shoot it over the chrono yourself. As a friend of mine remarked, "It's Murphy's law, not his suggestion." Let someone who hasn't done it before take a few shots over your chrono, and in a short time there will be one through your chrono.
So, you (or your shooting buddy) have just drilled your shooting chronograph. Now what? In a lot of instances, it's not a problem, and even if it is, the chrono can be repaired. Phone or email the maker, and ask for a repair estimate. Those who make chronos with separate sensors and boxes consider the sensors disposable, and the manufacturer will rightly charge you for them, but the all-in-one makers often offer discounted repairs. Competition Electronics only charged me $28 to fix my friend's shooting chronograph, but I think the company nicely overlooked charging me for the labor involved.