Barrel harmonics and airgun accuracy
by B.B. Pelletier
Dok Watson suggested this topic. He says he had never heard of barrel harmonics til he got into airguns. Since then, he's learned a lot. It was the same for me, Dok.
What are barrel harmonics?
A guitar string vibrates in a certain way when plucked. Those waveforms (they look like the waves in the ocean) determine the sound the guitar string makes. It turns out that many things vibrate in the same way. In a former profession, I worked with vibration dampeners to kill the vibrations made by steel beams and windows, so people could not listen in on private conversations from outside buildings!
Airgun barrels have vibration patterns, too, and they can dramatically affect the accuracy of a gun. Here's how. When a barrel is free to vibrate any way it wants, it tends to vibrate exactly the same with every shot. As long as nothing is done to disturb the vibration patterns, such as a tuneup or using a different pellet, the barrel will try to vibrate the same every time.
The shooter has an effect!
By holding the airgun differently for every shot, you are changing the vibration patterns. It's exactly the same as placing your finger on a guitar string at a different place. You get a different note. If you place your finger at the same place repeatedly, you will get the same note over and over, which is how guitars make music. If you hold your airgun in the same way every time, you get the same shot - meaning the pellet leaves the muzzle at the same place in the vibration pattern every time, so it tends to go to the same place downrange - every time.
But it is impossible for anyone to grip an air rifle tightly with both hands and repeat that hold shot after shot. It's the same as trying to hold ten fingers and your shoulder and your cheek against the guitar strings in exactly the same pattern, time after time. Don't bother trying - it's impossible!
So, the airgunner does just the opposite - he holds the gun as loosely as he can and allows it to vibrate as much as it wants. And, if he does a good job of not influencing the airgun, his groups get better. I'm talking about all airguns - not just spring-piston guns, though those are the ones most affected.
We're only beginning
A good hold lets the gun do its best, but that's just where it starts. You can also tune the vibration patterns of airguns to optimize their accuracy. I have tested adjustable harmonic tuning devices on several makes of air rifles and found that they not only work - they work dramatically well! Unfortunately, there isn't much information relating to airgun harmonic tuning, but the Browning company makes their BOSS (Ballistic Optimizing Shooting System) for firearms, and you can find lots of information about it.
The Browning BOSS is a proven way to adjust barrel harmonics.
Barrel tuning relates to length
Like guitar strings, the length of a barrel defines its vibration pattern. A tuning device like the BOSS lets the shooter make small changes in the length of the barrel, or more specifically, where the vibration nodes occur. Some airguns do this on their own through their design. A rifle with an air reservoir connected to the barrel changes vibration patterns as the reservoir pressure drops, which causes the reservoir to move. That can have a big effect on where the pellet goes, and it's a good reason why a free-floated barrel (no contact with the gun other than at the breech) is good.
Now you understand Dok's concern with the barrel-mounted bipod of the Crosman Nightstalker. Other barrel-mounted accessories, such as muzzlebrakes, will also change the vibration pattern. In the case of the Nightstalker, however, it seemed to make the gun MORE accurate, not less. You see, they don't necessarily make things worse; they simply change things. The shooter has to be aware of what he's doing to his gun when installing such accessories.
Vibration tuning is not an exact science. Even the top scientists in the free world could only guess what the effects of their work might be. We had to test each installation rigorously to determine whether we had been successful or what else had to be done. And, so it is with airguns!