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Got a PCP? Then you NEED a chronograph!

Alrighty now.....you've picked up your new PCP, either new from the store or new to you from another owner. Out the door you go, targets in hand, a full reservoir of air in the gun, and a pocket full of pellets. At 20 yards you set up your target and run through 20-30 shots, with the pellet producing remarkable patterns that tells you the gun shoots the same place every time! What a gun! You've found the Holy Grail of airguns.....it never misses!

The next day, or even later the same day, you pop a pest bird at a given distance. Then on the next pest you get a clean miss! Huh? All your testing up 'til now tells you the gun is shooting the same place no matter where in the fill you are. What gives?! (I'll bet you the pests were at different distances from you, the shooter).

Did you test your gun at different ranges? Remember you are shooting a PCP (pre-charged pneumatic) that has, from shot to shot, a different amount of air pressure driving the pellet. Unless your gun comes with a regulator, it will have a bell curve if you plot out the velocity. And that's why your shots will register at different impact points once you stop shooting at a short-range, given distance all the time.

There was a recent post on the yellow forum that got me thinking about how this was a great topic for the newer air gun shooter. My thanks to Robert for planting this blog topic in my brain, and this is why you NEED a chronograph to better understand your air gun. You see, at 20 yards, there will hardly be a perceptible difference in the points of impact because you are so close to the target. The differences from shot to shot will really show up when you start shooting at longer ranges.

Here's a link to the entry discussing Robert's results, and I challenge you to follow some of his testing procedures to determine how your gun shoots at different ranges and different pressures.


Why You Need a Chronograph


As you can see from the author's testing, there is a sweet spot in the fill pressure where there is little more than 20 fps difference for a series of shots. That's what you want for repeatable performance. At short ranges, you'll hardly notice any difference, but if you stretch out your range, you'll begin to notice quite a bit more variance in the point of impact, though you may be rock solid in your hold and aiming technique. It isn't you, it's the nature of a PCP air rifle. Pay particular attention to the graph that is contained in the post referenced above for a good visual aid on how declining pressure affects your point of impact.

Some things you need to know when you use a chronograph are:

1. The first sensor starts the clock, and the second sensor stops it. There is an oscillator inside the gun that is quite steady, and the onboard computer know how many oscillations between the start and stop points triggered by the shadow of your projectile passing over the sensors.

2. In order for #1 above to work, you need a constant light source. Fluorescent bulbs flicker and mess up the readings. You need diffused sunlight or an incandescent bulb that gives a steady light supply for the chrony to work correctly.

3, Muzzle blast can set off the chrony and produce an erratic measurement that isn't true. Back up just a little in order to make the chrony work better. Point blank range isn't all it's supposed to be when measuring velocities. Back up a foot or two from the sensors.

4.  Shooting from a repeatable rested position is best in order to get accurate reading that don't vary because the shooter is moving too much, or not holding the gun in the same position.


Here is a link to Airgun Depot's selection of chronographs. You will find great prices on these useful tools for the shooter. Enjoy!



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