Would somebody explain about barrel resonance?

M

MeekAndMild

New member
Would someone explain about barrel resonance?

I went to the range with a 308 rifle I've had for several years to sight it in with new bullets. Prior to this I've only shot Remington brand 150 gr. bullets in it. It groups about an inch at 100 yards with sandbag rest.

So, figuring bigger is better I sighted in some 165 grain Remington Core-Lokt soft points. They give a nice 3/4 high by 1 wide group...3 inches high and 4.5 inches to the right. :eek:

I double checked and the 150 grain bullets I usually shoot at the range were dead center in the gun's usual pattern.

Turning the scopes aiming screws put the gun on center with the 165 grain bullets but now I"ll have to resight it for the 150's at the end of hunting season.

So how can a barrel resonate so much to throw off the aim so much with 15 grains difference? Or is it the speed of the bullets doing it? (No, the crown is not nicked, yes the barrel is free floating and is full weight, not light weight, no it is not a short barreled gun, no I don't rest it on the barrel to shoot.)
 
hmmm ... best way I can explain it a sine wave on an ocilliscope.

You've got that wave action of the barrel. Depending on what part of the frequency the bullet departs the barrel, you get the bullet "tossed out" at a slightly different point of barrel "tip up."

OK, I suck at trying to describe it :( ... anybody else?

Too, using a heavier bullet, all things being equal, will cause a bit more recoil which could tend to toss the bullets a bit higher.
 
If you were on a swing, going back and forth, and if you were trying to throw a rock at something while on that swing the best time to throw would be that time when you were at the top of the swing. There is a brief pause, just as you stop before you start in the other direction.
If your barrel is whipping or oscellating ect. the point of most consistent accuracy is if the bullet leaves the barrel when it is at that pause either the top or bottom of the sine wave, so to speak. You can accomplish this with a movable barrel weight like the BOSS, or by adjusting the tension on the bedding screws or forend tension device like an old Remingtom 40X, stepped barrels at critical node points like the old Mausers or by simply chosing another load.
Trick is to stabililize the rifle so the barrel harmonic properties do not change (pillar bedding, glass bedding, glueing the action to the stock, ect.). That way when you find the magic load it stays that way.
 
As my feeble brain understands it, one of the major factors is velocity. Working up the most accurate load in a rifle basically means finding the velocity that results in the bullet leaving the barrel at the top of that swing. When you change bullet weights, the velocity changes, as does the relationship between the bullet and the lands. This means that we no longer have the bullet leaving at the top of the swing. Check out this article from the California Varmint Callers Association.

http://www.varmintcallers.com/accuracy.htm
 
Thanks for the explanation.

I wonder if you could get the same result as a BOSS with an adjustable weight which clamped onto the end of the barrel? I recall seeing several long thin military guns of the past which had weights clamped on them.
 
Gale McMillan once said that a perfect barrel doesn't need "outside assistance". Since most of us don't spend the money for that sort of barrel, we resort to various tricks. The Browning Boss is one of them.

My trick is to free-float the barrel, and then install a shim out at the front of the forearm. I use about a five-pound pull to separate barrel and stock to allow insertion. I use kitchen wax paper, as the heat of firing makes the wax melt just a tad and it sticks in place.

A barrel behaves as a spring, and my shim--or a Boss--acts as does a shock absorber on a car. It makes the vibrations uniform from shot to shot, and generally improves group size.

Even so, it is still common that different loads hit at slightly different points of aim. The difference is more often vertical than horizontal, however.

I pick a load for hunting season and stick with it. If it's different from my other loads, I figure it's easy enough to change my sight-in back to what it was before. Heck, it's an excuse to shoot. :)

Regards,

Art
 
My trick is to free-float the barrel, and then install a shim out at the front of the forearm. I use about a five-pound pull to separate barrel and stock to allow insertion. I use kitchen wax paper, as the heat of firing makes the wax melt just a tad and it sticks in place.
Click to expand...

That's a good idea.

I found out last night that part of the problem may have been that my stock has warped with age just a tiny amount so the right side of the forend was putting pressure on the right side of the barrel instead of being entirely free floating when the humidity changes.

So I reamed out the channel better then varnished it with urathane varnish. The next step will be to shoot the gun as is and see what the patterns are like then add back a square of leather exactly in the middle underneath it if they are not acceptably small.

I can deal with the vertical changes but the side to side variation was really upsetting.
 
How about this. 1 Get some target knobs for the windage and elevation ajustments from a place like the blue press. 2. keep a log book of how each type of round strikes the target. Do this for different ranges. One thing to remember is to count how many clicks both up and to the right or left you have to move your sight. This is a simple way of keeping track of how each type fires thrue your rifle. The Marine corp snipers have been doing this for quite a while. They almost always get a one shot, one kill solution out of this system. Because they know how many clicks to go to get on target at any range.
 
MeekAndMild, wax paper won't absorb moisture, as wll leather. No rust. It's also unaffected by gun oils and solvents. SFAIK, some thin plastic would work just as well.

Marine 1, I sure won't argue with your system, but I tend to think in terms of a hunter's needs. That is, for my .243 and '06, I sight in for two inches high at 100 yards. That's dead on at 200 and around six inches low at 300. ("Battle sights" or "point blank" range.) I have found that for my '06 the book is pretty close, and I use two feet low at 400; four feet low at 500. All I then need to know is the fairly-exact distance, out beyond 300; and the wind. Guessing the holdover is the easy part.

:), Art
 
One solution to the problem of fore end warpage in a wooden stock, is to ream out a channel in the fore end and put a layer of bedding material in the bottom of the channel. When the bedding material is set up put a stiff aluminum rod or billet into the channel and completely fill the channel and cover the rod with bedding material. The rod should extend from near the forward action bolt to near the tip of the forend. After the "stiff arm" rod and its bedding material is completely cured you can procede to hog out the barrel channel and reciever area to allow clearance for the barrel and room to bed the receiver into the stock. Procede to glass/pillar bed the action and the first inch or so of the barrel to the stock. Once a barreled action is properly set into a wooden stock with a properly installed "stiff arm" the fore end will not warp or place uneven pressure on your barrel. We have rifles installed decades ago in walnut, rosewood or maple stocks that have been hunted in all types of weather, rain and sun and have never once shifted zero.
 
To add further injury to your shooting accuracy, the barrel not only vibrates, but, because of the twisting rifling, the bullet imparts a torque on the barrel which can make it wobble as well as sine-wave oscillate.

Anyhoo, the vibrating string thing is an apt teaching tool. But the idea of reducing the vibration is only part of the deal. If you look at a sine wave or a vibrating guitar string, you will notice that there are nodes, places where the string does not vibrate. The rifle barrel does not necessarily vibrate only at, say, the recoil lug. It will have nodes all the way down the barrel at regular intervals (does anyone remember that old film clip of a suspension bridge oscillating in many places during an earthquake (or was it a wind storm?)?

Now, let's say we could adjust those isolations (with a BOSS adjustment or bullet change or powder change or primer change) so that we could move that non-vibrating node closer to or right on the muzzle of the barrel. The barrel would be vibrating all over the place behind the muzzle but the muzzle would be relatively calm.

Unfortunately, most of us, without a BOSS cannot simply dial this in as if we had a rheostat and a CRT to monitor the results. We have to play hit or miss with reloading techniques, five rounds at a time.

Rick
 
While it may seem amazing that such a difference can occur with 15 grains of difference in the bullet, another way to look at the problem is to think of the rifle as a highly precise machine tool in regard to systems theory. With such a high level of precision, a change in any one facet will affect the end result. A change in powder or powder burn rate will also create changes.

Look at it this way, if you didn't have a precision instrument, you might not notice the effect on your shooting.
 
wax paper won't absorb moisture, as wll leather.
Click to expand...
You're right. If I need to use the leather as a damper I will soak it in varnish first...as an expedient to last until the end of this hunting season.

In the end I want to rebed the stock. I once made a banjo with exactly the sort of tension rod riddleofsteel talks about in the neck. After 25 years the banjo neck is still straight and it plays as in tune as a banjo can. :)

But. :eek: I found a little crack in the stock as well starting from the bolt notch area. So it will either be more repair or get a new semi inletted one and start over. Now just to get through the next 2 months.
 
Ok, I need some more help with the barrel resonanance. It has been said that a rifle barrel is like a wind chime. You have wind chimes, hollow tubes of metal cut to different lengths. Each hollow tube has a different resonant frequency or tone that is exhibited when it is stuck. Same with an antenna or organ pipes; shorter antennas or pipes are resonant at higher frequencies and the longer it gets the lower it's resonant frequency. As I understand this theory, putting a shim against the barrel simply changes the resonant frequency by allowing less of the barrel to vibrate similar to the way a guitar string changes frequency when your finger presses it against the guitar neck. The way I view the BOSS system is that you screw a tube in and out thus changing the barrel length or resonanant frequency of the barrel. Normally you would "work up a load" by changing powder charges or OAL of your loaded round in an attempt to have the bullet leave the barrel at a null in the vibration cycle. With the BOSS you are leaving the load alone and adjusting the vibration cycle instead.
That is the way I have always looked at it and I could be wrong (this happens more often than I like).
 
Your guitar string analogy id true for a true free-floated barrel. Putting a fore end shim in is somewhat akin to fretting the string at some point on the neck but not exactly. In a true free-floated barrel, no matter how thick, there is a signwave type vibration in a three dimensional pattern and a slight wobble. Fore end shims and adjustable fore end tensioners (ala old remington 40X rifles) tend to dampen those vibrations but they still exist. I once shimmed the free-floated factory barrel on an old Mauser at the shooting range with a makeshift aluminum shim made from part of a soft drink can. When you fired it the rifle it rang like a little gong as the barrel vibrated against the shim despite four pounds of upward pressure. Later I replaced the aluminum shim with one made from a block of plexiglass.
Best procedure seems to be to stabilize the action in the stock by bedding and free float the barrel. Shoot it and see how it acts. If you are not satisfied start shimming the barrel with heavy paper or thin paste board shims until you find an upward pressure that shoots well. Remove the shims and use a micrometer to measure the thickness. Cut a shim(s) out of plastic or some other stable and non porous material and slip it into the same location between the barrel and fore end. Shoot for groups and adjust the tension as needed.

As far as a new stock a Fajaen Rhinehart wood or an H&S Precision or McMillian synthetic is hard to beat.
 
444, RickD seems to have pretty good phrasing in his post...

My shim seems merely to act as a damper, reducing the amplitude of the vibrations. I guess. Dunno. All I know is that regardless of barrel length or bullet weight, groups shrink.

As far as stock warpage, me being such a cheapskate and all, if the clearance to the sides of the barrel is at least 1/16" and the clearance below is a bit more, sealing the wood with stock wax or expoxy varnish holds down any change in moisture.

If slight moisture changes from local humidity make a stock warp more than 1/16", there is some problem inherent in the piece of wood of which the stock is made. Decent wood just won't do this much changing.

Art
 
Seems that an easy enough adjustable unit could be sort of pillar bedded into the front end of the stock & maybe that's what was refred to above.

A threaded insert at the fore end pressure point which contacts a contoured (to match barrel contour) shim. Start with free float & adjust presure by just turning in the screw to make further & increased presure at the barrel.

Using a threadlocker (222?) would keep it from backing out but still allow for adjustment. Have the screw head counterbored/recessed & all set.

"Poor man's" BOSS.

Comments?
 
The old Remington 40X target rifles had a threaded insert in the forend with a screw and lock nut. The internal end of the screw had a widened nylon insert that rested against the barrel. You could adjust barrel harmonics by tightening the screw to affect grouping. Once you found the proper tension for your load you tightened the lock nut. I always thought it was a good system, especially if you shoot a lot of different loads from the rifle.
It would a simple project to bore a hole in the forend and install a threaded bushing. Most automotive supply or fastener suppliers stock screws/bolts with a nylon insert in the end. Or you can just bore out the end of a bolt that fits your insert and install a nylon or plastic insert.

Of course if you mainly shoot one or two loads as is the case for most hunting rifles, the shim method may be simpler easier and less hassle.
 
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