Chronograph instructions and tips
by B.B. Pelletier
I need to be humbled periodically to maintain my perspective on things. Fortunately, for me, I was created with many imperfections that make frequent humbling a certainty.
I was taking a .22 semiautomatic rifle apart several days ago to clean the action, and I got to the part where you remove the last drift pin and all the major and minor parts fly apart like a satellite that’s been hit by a particle beam. No chance to see where everything went because they all got disassociated at the same time.
When this happens, I have several mantras to address the situation. No. 1 is I imagine the item was assembled by a 19 year-old girl named Tiffany, while she is also talking to her coworkers, drinking a Slurpee and texting her best friend. Tiffany can put this thing together in 27 seconds and can spot (without thinking about it) when part 51b has been reversed in its slot, which is good because Tiffany isn’t really into thinking.
If that one fails and I still have parts lying all over the table, I think of Ishmael, who uses no special tools to assemble this item. He has a hole in one of the upright girders supporting the roof where he assembles these items all day long. It was blown through the steel girder 37 years ago with an acetylene torch, and it isn’t quite round; but time and use have polished the edges of the hole, and it’s the perfect assembly tool that was used by Ishmael’s father for the same purpose. With it, he can assemble a pallet-load of these things, whatever they are, before tea time.
When that one fails me, there’s only one thing left — the Machtar chant of assembly (see the movie Galaxy Quest). As it happens all too often, even this potent bit of magic refused to work, leaving me with a pile of parts that purportedly had once been a semiautomatic rifle. Had I not seen it in that condition, I would have doubted it.
I got the gun back together by scrutinizing each part and imagining the relationship it had with the other parts (see, mom, I can use that lump on my shoulders for something other than a hatrack!), but I hate it when that happens. Complex parts should self-assemble, like a wine glass filmed in reverse after shattering.
But this isn’t about me fixing a gun. It’s about me being humbled, so I’ll remember what it’s like to approach something new for the first time. Trepidation, you are my middle name!
So, when a Pyramyd Air customer asked for some pointers on the use of a chronograph the other day, I felt I had to spring into action. Here is his exact request:
I’ve read a good percentage of your BLOGs & articles (plus videos), but no-where do I see the distance specified to set-up a chrono for muzzle velocity for springer airguns, pistols & rifles. I use a ProTach Classic Chrono, with 36″ between “start” & “stop” sensors (originally for hand-loading). I’ve searched the net for an airgun industry std. (like for fire-arms), with no success. One article, on the net, said set the “stop” sensor @ 3 ft. from the muzzle ~ that’s impracticle!! How-about-it, B.B., Tom or Robert B.!! Rich
Where to place the start screen
Rich, if this was five years ago, I wouldn’t have a clue what to tell you. That’s because you’re coming from the world of firearms. I began using chronographs with airguns. Only very recently have I started using a chronograph for my firearms, and only recently have I learned the difficulties of figuring out where to place the start screen.
I typically place the start screen about one foot from the muzzle of the airgun. That’s almost ANY airgun, mind you, except for a big bore and one other exception I’ll mention in a moment. A couple months ago, while I was chronoing some centerfire handloads, I rediscovered why my Oehler 35P came with 15-foot cables. Even when the skyscreens are placed 10 feet from the muzzle, the muzzle blast from a .43 Spanish round will move them like a slinky in Shakeytown!
When it came time to test the Benjamin Rogue, I was prepared to move the skyscreens way downrange from the muzzle to keep from blowing them apart from the air blast. Even though the start screen was situated about 10 feet from the muzzle, the entire skyscreen assembly shook violently every time the rifle fired. So, I understand Rich’s question at the most visceral level.
Rich, a spring gun discharges only the tiniest fraction of pressurized air that a pneumatic puts out, so you can place the start screen a foot from muzzle of the most powerful spring rifle or pistol you can find, which would be a Whiscombe JW80 generating 32 foot-pounds in .25 caliber. Ain’t nothin’ badder than that out there (in spring guns, that is), and your chrono will never miss a beat!
CO2 guns — the other exception
CO2 guns, however, often have a visible exhaust that can fool the skyscreen. Whenever I chrono one of them, I back up about 18 inches from the start screen. This holds true for the weakest pistols as well as the more powerful rifles. You don’t get an incorrect number from them, at least not from my Shooting Chrony Alpha model. What you get is an error message that screen one, the start screen, was unable to detect the passage of the pellet accurately. Back up a few inches, and the problem is solved!
The rest of the smallbores
As far as the other smallbore air rifles and pistols are concerned, 12 inches is all the distance you need between the muzzle and the start screen. This holds true for a catapult gun throwing a 3-grain lead shot at 86 f.p.s. as well as an AirForce Condor belting out a .25-caliber 43.2-grain Eun Jin pellet at nearly 1,000 f.p.s.
Watch where you’re shooting!
A funny story that is directly related. Many years ago, I was running an M203 grenade launcher range for my company at Hohenfels training center, West Germany. The M203 is an underslung weapon that attaches to the bottom of the M16 rifle. It lobs a 40mm grenade out several hundred meters and has been called the hip-pocket artillery of the infantry.
Attached under the rifle, the M203 grenade launcher lobs 40mm grenades out to 350 meters. It uses special high-angle sights, which the firer must not forget to use!
Here’s the thing about the M203. It shoots only a few hundred yards, while the M16’s 5.56mm cartridge can shoot several miles. Naturally, the rifle shoots much flatter than the grenade launcher, so the grenade launcher comes with its own set of sights designed to elevate the weapon to a very high angle to get the needed range. If you were to use the rifle’s sights, the grenade would hit the ground just a few yards downrange — and that would be a bad thing.
This young man demonstrates the correct angle for the M203 grenade launcher.
The firing positions on the M203 range were simulated foxholes with bermed bunkers in front and on both sides of each shooter. These berms were made of railroad ties that held back mounds of compacted dirt. Each firing position was protected from the others so that if anything bad happened, only the one position would suffer the consequences. On this day, I found out why — to my chagrin.
Even though I briefed each relay of shooters before they went to the firing points about using the M203 quadrant sights and not the rifle sights, and even though each firing position had an NCO to watch the shooter, we had an incident where a shooter forgot and used his rifle’s sights to engage a target. The grenade came out of the launcher and hit the railroad ties that were about 12 inches in front of him.
No, he didn’t blow himself up. The designers of the M203 grenade anticipated this event (it’s fairly common) and made the grenade to be armed by spin. It has to travel a certain distance downrange before the centrifugal force of it spinning from the rifling arms it, and 12 inches isn’t far enough. After the range was called cold and evacuated, I went to inspect the firing position, where I found a crumpled grenade lying in the dirt, next to the abandoned weapon. Just from the sheer velocity of the projectile, the grenade had dented the railroad tie about two inches!
Bad things can happen
I won’t tell you how I fixed the situation, but my point is this — when the sights and the bore are not aligned at close range, bad things can happen. The same is true with chronographs! If you’re shooting into a pellet trap that’s three feet away and you sight through the scope, you’re going to shoot your chronograph because the bore is three inches below the scope. Don’t think you’re smarter than that, because everyone who uses chronographs shoots them sooner or later. By sighting through the scope, you’re almost guaranteed to put a pellet through the guts of the electronics package.
Instead, sight by instinct, looking at the orientation of the barrel relative to the target, and of course to the skyscreens. Do this both for the elevation above the skyscreens as well as for the line the pellet takes across both screens.
People sometimes place a chronograph downrange to calculate the terminal ballistics at a certain distance; or, if another chronograph is used near the muzzle, the ballistic coefficient of the projectile. But they forget that downrange the projectile can go wherever it wants. More chronographs have been ruined this way than any other. Figure that it is only a matter of time before the downrange chronograph is hit.
Lighting for a chronograph
The best light for skyscreens is an even light. A totally overcast day is perfect, as is a day with clear blue sky (as long as the sun does not shine directly on the skyscreens). But a day with puffy white clouds that move around is bad, and you’ll have to use the diffuser filters above the skyscreens.
For artificial light, incandescent bulbs that shine evenly are the best. Bulbs that shine by exciting either a gas or a phosphor, such as fluorescents, cannot be used. They will set off the skyscreens.
I personally have found that by reflecting a 500-watt incandescent light off a white ceiling, I get the optimum in indoor chronograph lighting.
Here’s a small lighting tip. Don’t use strobe flashes near the chronograph, because they will set off the skyscreens. So will the arc from an electric welding torch.
On the level
When you shoot through the skyscreens, it’s important to be as close as possible to perpendicular to the path of light to the screen. If you shoot on an angle — up or down doesn’t matter — the path through the screens will be longer than if perpendicular and the recorded velocity will be lower.
You’re in charge!
Most chronohraphs run on batteries. I should not need to say it, but always carry a spare for when the battery dies. It’s discouraging to be out on a range, only to have the battery die and not have a replacement — especially when the whole reason for going to the range was to use the chronograph.
A few chronographs use infrared sensors in their skyscreens and need infrared light sources in order to work. If you lose one of these special-purpose bulbs, all the bright lights in the world will not make up for it. Keep spares close by.
What about that neat little Combro chronograph that attaches to the muzzle of the gun? How good is it? Well, I once owned one and can comment. It does work and you do get a number from it. And whenever there’s a number, people stand around and believe it.
But here is the deal. Oehler, which is admittedly the leader in commercial chronograph technology, separates his skyscreens (the third one in the middle is the stop screen for a second channel that checks the other one) by at least 24 inches. The machine’s clock speed (the frequency at which the crystal oscillates) is four megahertz. While the pellet flies between the start screen and the stop screen, the oscillator is counting at the rate of four million cycles per second. At that rate, it can parse time into small packets. The Combro has screens that are only a couple inches apart and a clock speed they don’t publish, but which must be slower than the Oehler. The number you get from this device is at best a close approximation — a best guess.
Aside from that, the Combro uses IR sensors, will not operate well in strong daylight and is difficult to fit to the muzzle of the gun. If it’s misaligned when mounted, it can be hit by the projectile. It’s not suited to use with firearms.
I answered Rich’s question in one paragraph in this report, then I went on to discuss other common problems encountered when using a chronograph. If you have any other questions or would like to know more, please make a comment to that effect.