Barrel harmonics

V

velocette

New member
Sunday, at the range where I am a part time range officer, I was helping a new shooter with his "sniper rifle" It was a Remmy 700 chambered in 300 Win Mag. medium heavy barrel in an expensive aluminum frame / stock with a suppressor.
Using a bipod, he was getting erratic groups of about 4" at 100 yds.
A fellow range officer, an experienced F class competitor fired the rifle with similar results, very poor grouping. His ammo was of good quality, not match grade but good hunting grade, probably good for 1" at least @100 yds.
I was watching him and his rifle very carefully to learn what the problem was when I noticed that the barrel / suppressor was violently vibrating up & down about an inch & a half with each shot, diminishing quickly in perhaps 1/4 second. YES, clearly visibly shaking up & down with each shot. Action screws were tight, barrel was free floated. We removed the can & then he proceeded to put three rounds through the same hole @ 100 yds with no visible vibration.
It appears that the weight of the can (about 2 1/2 lbs, it was a steel can) gave the barrel the wrong weight & resulted in the harmonic shaking we observed. The can was attached to the barrel with a quick release screw on type mount & felt to be tight.
Learn something new every day, and this was really something new. I have seen high speed photography of rifle barrels vibrating, but never anything close to this.
 
As long as no bullets bump the supressor can's internals on their way out and the can's correctly fit to a decently fitted barreled action in the stock, the barrel will whip at the same frequency for every shot fired. It's about the most repeatable thing in the system. A heavy weight on the muzzle can easily make it more visible, especially with skinny barrels.

That said, with the groups' it's producing in another rifle as well as that one without the can, I suspect a mechanical problem in the fit of one or more parts to each other. The difference between two tests is one had the can and shot bad but without the can, it shot good. What's the difference? Problems follow the part that's bad in design or fit.
 
velocette said:
The can was attached to the barrel with a quick release screw on type mount & felt to be tight.
Click to expand...

Any chance the bullet's hitting the suppressor on it's way out? Any evidence of damage?

I had a friend who has a suppressor with this same set-up. He screwed it on and I watched him meticulously make sure it was screwed in all the way. Apparently, it wasn't, and the first round bounced off the can's crown. Had I been watching the end of the barrel, I'm sure it would've made for visible "harmonic changes".
 
Any chance the bullet's hitting the suppressor on it's way out?
Click to expand...

If this is true, I don't see how it could NOT hit:

the barrel / suppressor was violently vibrating up & down about an inch & a half with each shot
Click to expand...

I'd try either different loads, or shims to change how the can fits the barrel to reduce that movement
 
The Browning BOSS system IIRC has a weight that can move forward or back to reduce that type of movement and get better accuracy. Would cutting back the barrel length a bit help ypou ?
 
The Browning BOSS changes the frequency a tiny amount the muzzle axis wiggles at in the vertical axis so bullets leave at a different angle. The amount of wiggling doesn't change any significant amount.
 
We suggested to the shooter that he find a can that was lighter and one that screwed on and fitted tightly.
His barrel vibrated visibly with all the loads he tried when the can was on the barrel. No visible vibration without the can.
Occam's Razor: The weight of the can harmonized with the barrel's natural vibration & created the problem. Solution, change the can, remove the can.
$200.00 gone to the govt. to screw up a good rifle.

Roger
 
My question is- who made the can?
Reputable, well known manufacturer (which could still have a manufacturing defect) ? Well designed and precision made suppressors can increase accuracy- not the other way around. The fact that they're widely used on military sniper and DM rifles is proof enough...

Just learning about these recently on Silencertalk as I've been playing with the idea of applying for the stamp and making my own. But, they've got to be done "right". Here is one write up on suppressors and accuracy- or lack thereof:

The most critical moment in a bullet's flight path is just after it exits the barrel, where the highly elastic and more rapidly moving gasses overtake it and press upon its base. The first baffle in a suppressor is called, appropriately, the blast baffle. This is the most critical component in the entire baffle stack. The blast baffle is subject to a great deal of heat, stress and impact. Many baffles have asymmetrical surfaces, and these can bounce the blast of high-pressure gas around in a way that disturbs the stability of an exiting bullet. When we first started experimenting with asymmetrical S, Z and K style baffles this phenomenon became painfully obvious. Keyholing, tumbling and baffle contact were common because the bottle-shaped blast of muzzle gas overtook the exiting bullets, deflected off the asymmetrical surfaces, and then deflected the bullets. Accuracy was not good. Some manufacturers haven't learned this lesson yet, and their suppressors are plagued with inherent instability and resultant accuracy problems.

The blast baffle must have a perfectly symmetrical, coaxially aligned surface and bore. It must be made of fairly tough steel, stainless steel or inconel. If it is made of a soft material like copper, brass, titanium or aluminum, the high-velocity impact from unburned grains of powder will peen the surfaces - eventually reducing the size of the bore orifice to the point where destabilizing bullet contact results.

A properly designed blast baffle will strip and deflect much of the bottle-shaped blast of high-pressure gas that envelops and pursues the departing bullet. For this reason, one can logically expect an increase in practical accuracy when a properly designed suppressor has been installed. Also, the weight of a heavy steel unit tied to both the center and end of a rifle barrel does beneficial things for harmonic barrel vibration - dampening out much of it. These two factors greatly increase the practical accuracy potential of a suppressed rifle. The properly suppressed rifle becomes very stable and reliable. Larger internal clearances reduce the likelihood of baffle contact in the event that the suppressor or barrel get slightly damaged or bent.
Click to expand...

I would suggest he contact the manufacturer, to return the unit for testing and evaluation.
 
1.5" of muzzle movement would translate to far more than 4" in group size(just guessing more like 10'). A "medium heavy" barrel simply won't flex that much.
The most flex I've seen was on an FAL, under slo-mo the barrel flex was quite noticeable although nowhere near 1.5". Oddly, the groups were around 4".

I'd look at the front rest the rifle was sitting on. I watched a guy struggling to sight in a borrowed 338 Win mag and finally determined the front sling swivel was hitting the edge of the front rest causing the erratic POI.
Bullet strikes on the can would be very noticeable(if it didn't blow the end cap off) and would likely show up as keyholing on the target.
Another cause could be shooter input. The 300 Win mag has significant recoil and failure to consistently support the rifle can cause POI shift and/or increased group size. I quit using my 300WM match rifle simply because I could no longer manage the recoil effectively(resulting in vertical stringing or change in POI).
 
I think it kind a depends, Mobuck.

While the barrel may move that much, where is it when the bullet actually
leaves the bore?
 
Sometimes the simplest answer is the best one. Rifle shoots one-holer without the can and shoots 4 inch groups with the can. Hmmmm. The can needs repair/replacement.
 
I'd try the suppressor out on a different rifle. But suspect fit first. The weight of the suppressor will affect the harmonics but that affect should be relatively consistent.
 
"...Erratic groups of about 4" at 100 yards...barrel/suppressor was violently vibrating up & down about an inch & a half with each shot..." Sounds like a new shooter with a .300 Mag. Too much gun. However, 2 1/2 lbs. of useless weight on the end of a relatively slender barrel will do that.
"...Any chance the bullet's hitting the suppressor on it's way out?..." Anything will be instantly damaged beyond repair.
 
Its obviously related to the can. It could be a flaw in the can itself or in the mount or in the barrel threading. If it were me, my first step would be to send the can back to the manufacturer for testing. If they find nothing wrong, then maybe a gunsmith should look at the threading.... probably not the guy that cut it. He could try a few other loads to see if any improve... but i doubt that much variation is all harmonics.
 
You must log in or register to reply here.
Back
Top