Curious case of barrel harmonics : can someone explain?

[Scarecrow3]

So this is a 6 shot grouping fired at 10 yards from my precihole sx-100. The shots in green are the first three while those marked by red are the next three in the same order as the first three.

From what I understand, the barrel harmonics mentioned in this (http://www.stocks-rifle.com/harmonics.htm) link pertain to gunpowder firearms and not to airguns, at least not in the same intensity. Could someone explain this?
Thanks

[pradeep2374]

It looks like random variation associated with your rifle hold. What position did you shoot these? Try and see if this phenomenon persists when using a rifle rest.

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[brihacharan]

Here's an extract from an article written by BB Pelletier a well known authority on Air Guns....
Barrel harmonics and AIRGUN ACCURACY
B.B. Pelletier

What are barrel harmonics?
Air-gun 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 tune-up or using a different pellet, the barrel will try to vibrate the same every time.

The shooter has an effect!
By holding the air-gun 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 air-gun 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. So, the air-gunner 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 air-gun, his groups get better. I’m talking about all air-guns – not just spring-piston guns, though those are the ones most affected.

We’re only beginning
You can also tune the vibration patterns of air-guns 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!

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 air-guns do this on their own through their design.

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 air-guns!

I hope this explains to some extent about "Barrel Harmonics"....
As for me I put on a "Muzzle Brake" on all my 3 ARs & have experienced good accuracy....however consistency in hold is important while shooting to get good groupings....
Briha

[ganeshn]

Its fluke may be, you may not be able to get skewed patterns like this each time.harmonics play insignificant role in comparison to varying breach lock, loose stock screws,varying double recoil to alone account for the skew or the 30mm spread.harmonics generally would create a random POI.

[sohampaul11]

IMHO in the 1st 3-shot group, you were anticipating recoil(or not going through follow through,perhaps) and the lower one may be due to your tightening the grip while you were squeezing the trigger and for the next 3-shot group you repeated the same mistake. At this stage it is most likely your holding pattern that's most likely to go wrong. Barrel harmonics plays an insignificant part at the stage and range at which you are shooting. Tighten the 2 allen screws near the front of the stock and the bigger screw at the rear,behind the trigger guard,it generally solves the inconsistency of this rifle.

This is a 10-shot grouping before tightening the screws on my Preihole NX100-


image upload no compression

This is a 10-shot grouping after tightening the screws(Pardon the 2 flyers)-


screenshot

Range- 10 metres
Weapon- Precihole NX100 Club Elite(5000+ shots,so,it has been properly broken in)
Pellets- Preipell .177 Match
Stance- Standing offhand

[brihacharan]

Hi sohampaul11,
Great going!!!!
Don't worry about 'flyers'...it happens till you settle down to a proper hold...which you've achieved!!!!
Briha

[sohampaul11]

Thanks briha ji! Appreciate your comment!