Wandering Shots as Barrel Heats Up

B

Bart B.

New member
Got to thinking this out a day or so ago. I dawned on me that while I've often stated that an out of square receiver face relative to the chamber axis makes a high point on it the hard spot on the barrel's tenon shoulder. Pressure builds there as the barrel heats up and expands.

But what if the bore's not perfectly centered on a barrel that's perfectly fitted to a squared up receiver face? The heated up metal on its thick side's gonna expand more than the thin side. It'll bow around its thin side. And that'll make the muzzle axis move away from where it was when the barrel was at ambient temperature.

There are ways to find out if a barrel's thicker on one side of the bore than the other; spin it on perfect centers in a lathe and put a dial indicator on its mid point. Any wobble on the dial might be indicating out of balance.

Thoughts, comments and things I might not think about are welcomed.
 
One of the reasons the Mauser barrels were stepped was to reduce the wandering POI effect of a barrel that had a wandering bore as it heated up.

Some Mauser barrels have quite a bit of wander. This is very noticable when a long barrel is shortened. The bore can be far from the center of the new muzzle.

In the old days barrels were straightened using a hand turned arbor, and Savage still uses the old method. This requires highly skilled craftsmen.
At some point it was decided that so long as the last six inches of the bore of a milspec barrel was straight it would be accurate enough for government work.

If you want the best barrel be willing to pay for it, and buy from a highly respected barrel maker.
 
Straightening is quite an interesting subject and most heat treaters that quench and temper bar steel (such as used in rifle barrels) used to do it using machines that required operator skill. Some operators never get that skill and "overbend" the bar (barrel) requiring overbending to get it back. I've watched one of our heat treater's operators just mutilate one of our shafts trying to get it straight.

Fortunately there is new automatic straighteners that do not require operators. They measure the runout and get the high point up to the press. The press comes down the exact amount needed to straighten the barrel when the pressure is released. I'm sure some of the rifle makers use these and some do not. We would only allow one "reverse bend" if the initial press was too much. Otherwise it weakens the shaft (barrel).

Maybe more then anyone wants to know about straightening but I was charged with solving this problem once (now retired).
 
From your previous posts I assume you are pondering on a bedded action, free floated barrel. To this post I will add that a bad bedding job can cause stringing groups. Some actions need relieving at critical points. Some barrels need to be partially bedded. It takes knowledge and skill to properly bed an action.
 
Big Al, no, I'm not pondering on anything other than a barrel whose bore ain't centered in it. Doesn't matter where, nor how straight, it is; I think it'll still bend as it heats up. Just like a bimetallic thermometer whose needle is attached to a coiled flat strip of dissimilar metals; each metal expands a different amount for a temperature change so that spring moves the needle.

As far as bending barrels and bedding barreled actions is concerned, I don't think they have any effect on how a barrel with an off-center bore bends as it heats up. How much the stock touches it will effect how much it bends and gives it something to bounce off of as it whips and wiggles from firing.
 
If I understand the process of Hammer Forging of barrels correctly, would that mean that all hammer forged barrels will have equal barrel wall thicknesses? I wonder what barrel making process would be most likely to produce perfectly centered bores?
 
Ok, after re-reading your post I see what you are wanting to discuss. I don't know how much of a difference there would have to be to cause noticeable group change. If the bore was off center more than a few thousandth of an inch I could see where the difference in expansion might cause stringing. But I would think it would be less than an out of square receiver would cause. I would be curious to see testing that included barrel wall thickness and centering of the bore as a factor.....what do you suppose the factory tolerances are for bore centering.
 
If I understand the process of Hammer Forging of barrels correctly, would that mean that all hammer forged barrels will have equal barrel wall thicknesses? I wonder what barrel making process would be most likely to produce perfectly centered bores?
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All barrels have the initial "hole" gun drilled through the bore, even hammer forged barrels BUT the hammer forging process WOULD probably make the best barrels in regard to consistently centered bores IMHO
 
I'm a believer that barrels drilled to make the bored blank, reamed to uniform the bore dimension then rifled to put the grooves in are best centered in the barrel when it's all done on the same machine that spins the barrel. The gun drill, reamer and rifling tools all align on the center of rotation. Then the outside could be turned to profile desired with the bore spinning on centers at each end.

This is how the most accurate barrels are made these days; either button or cut rifled ones. No hammer forged barrel to date has equalled what the best of these do. . . .at least as far as I know.

But this is microscopically off thread topic......
 
Got to thinking this out a day or so ago. I dawned on me that while I've often stated that an out of square receiver face relative to the chamber axis makes a high point on it the hard spot on the barrel's tenon shoulder. Pressure builds there as the barrel heats up and expands.

But what if the bore's not perfectly centered on a barrel that's perfectly fitted to a squared up receiver face? The heated up metal on its thick side's gonna expand more than the thin side. It'll bow around its thin side. And that'll make the muzzle axis move away from where it was when the barrel was at ambient temperature.

There are ways to find out if a barrel's thicker on one side of the bore than the other; spin it on perfect centers in a lathe and put a dial indicator on its mid point. Any wobble on the dial might be indicating out of balance.

Thoughts, comments and things I might not think about are welcomed.
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I found from gunsmiths and barrel makers that the gun drilled hole wanders inside the tube. Also, the exit hole and entrance hole are always offset. I was told by a barrel maker how he sorts barrels in categories of “hunters” and “target” by spinning drilled blanks between centers and observing the wobble.

A gunsmith told me of a mutual acquaintance who had bought a Krieger barrel. The barrel was installed correctly but shot so far left that it ate up all the windage on the gentleman’s sight. Another barrel was installed. The gunsmith marked the take off barrel, rethreaded and rechambered it so the left offset was pointed 90 degrees up. That barrel was installed on a palma rifle and shot great at 1000 yards. I think it was shooting high but at that distance, it just took less elevation.

Maybe someone with a CAD model could see how vibrations and frequency modes change based on bore alignment, but given the variation in barrels, barrel stresses, manufacturing, I don’t believe that we can give a definitive answer in the real world.
 
About 1970 started the use of cold hammer forged barrels and almost all are made that way now.
In theory anyway the barrels should get stress relieve heat treat at various stages in manufacture .Stressed barrels like to return to earlier bends when they heat up from firing !
 
Mete, Winchester used hammer forged barrels in their Model 70's starting in 1964. 'Twas the Germans who came up with the process in the late 1930's.

Slamfire, this is amazing:
A gunsmith told me of a mutual acquaintance who had bought a Krieger barrel. The barrel was installed correctly but shot so far left that it ate up all the windage on the gentleman’s sight. Another barrel was installed. The gunsmith marked the take off barrel, rethreaded and rechambered it so the left offset was pointed 90 degrees up. That barrel was installed on a palma rifle and shot great at 1000 yards. I think it was shooting high but at that distance, it just took less elevation.
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Palma rifle rear sights need at least 40 MOA of windage on both sides of zero. With a 30 inch barrel whose breech is 7 inches in front of the rear sight, sight radius gets up in the 36 inch range. Which means the guy's rear sight that ran out of windage to get a zero was 4/10ths inch or more off center from the chamber axis at the back end of the action.

Do you know how much bend that barrel had to have if this is what needed to be done? Putting that barrel on a granite gauging table would have shown a 1/5 inch or more gap between its ends at the middle as it was rolled on that surface. I can't imagine a 'smith chambering or crowning such a barrel; it would wobble his lathe quite noticable. Much less the barrel maker who would have to have seen its condition as he made and finished it.

My question is, why did the barrel maker let that out of his shop?
 
Bart, you got me wondering enough that I did some research. Here is what I learned and how it affects the barrel.

Steel expands with heat at a constant that is calculated with a coefficient and the difference between starting temperature and final temperature. The same formula is used for all thickness and diameter pipes. A barrel is essentially a pipe. That shows that the barrel will expand in length the same amount regardless of thickness.

Therefore if the barrel is squared to the axis of the bore then the barrel will not move off of it axis due to thermal expansion.
 
Big Al, you are right. I'm just wondering if the pipe wall is thinner on one side than the other, would that make the thick side's length change more than the thin one. From what I've checked, it may not matter.

So, there's a good chance that a rifle barrel whose bore is off center in its profile will only change in length as its temperature changes. And there'll be no bending at all.

I may not be able to domesticate this wild idea I had.
 
Do you know how much bend that barrel had to have if this is what needed to be done? Putting that barrel on a granite gauging table would have shown a 1/5 inch or more gap between its ends at the middle as it was rolled on that surface. I can't imagine a 'smith chambering or crowning such a barrel; it would wobble his lathe quite noticable. Much less the barrel maker who would have to have seen its condition as he made and finished it.

My question is, why did the barrel maker let that out of his shop?
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All good questions I cannot answer. I don't know if the hole was horribly offset, I doubt it or the gunsmith would have said something. Maybe it was due to stresses in the barrel.

I have a 6.5 Swede Military rifle that has the barrel heats up the point of impact moved up something like 18" at 100 yards. I rebedded the thing, opened the space between the forend and barrel, may have shot it in a different stock, the barrel still walks.

I have not taken the barrel off the thing but I am thinking of sending it to a cryo treatment shop in the hope that might fix it.
 
Being a retired metallurgist and very familiar with the manufacture of long transmission shafts I'll chime in.

Any steel harder then HRC 30 will have stresses put in from machining, straightening or forming. What is typically done is extra stock is left for finishing operations and the shaft, barrel or whatever is "stress relieved" after these operations which causes them to warp. Then they are finished using processes that do not put any stresses in them. If they are not stress relieved that stress relieving will take place when any higher heat is applied.

The shafts are heated to 1650º F (typically) and quenched.

They are tempered to the desired hardness (1100º F for HRC 30-32).

They are machined, straightened, drilled, hammered or whatever and then receive the stress relieve at a lower temperature (1000º F) then the tempering temperature so as not to lower the hardness.

Then the finishing operations are done.

It takes alot of heat to cause these non-stress relieved barrels to warp (300º F or more) which is much higher then firing the rifle will yield SO the most likely cause of barrel warping is a bore which is not concentric to the O.D of the shaft or barrel. The different thicknesses will cause a change but since this temperature is low I would expect the barrel to go back to it's original shape. If you only fire shots out of a cold barrel you would not notice it walking.

My 2 cents.
 
It takes alot of heat to cause these non-stress relieved barrels to warp (300º F or more) which is much higher then firing the rifle will yield SO the most likely cause of barrel warping is a bore which is not concentric to the O.D of the shaft or barrel. The different thicknesses will cause a change but since this temperature is low I would expect the barrel to go back to it's original shape. If you only fire shots out of a cold barrel you would not notice it walking.
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Well I was wondering, they used to "straigthen" barrels. I have seen pictures and a machine at Springfield Armory, I think I have seen pictures of sporting barrels being straightened. How does straigthening effect a barrel?, are there internal stresses that show when the barrel is fired and as it gets hot?
 
I remember years ago selling point for Shilen was they don't straighten barrels.

I was talking to gunsmith that does my barrels and he said last 8 to 10yrs barrels have been better in part customer are little smarter. You can do a search find all kinds of information how to straighten barrels etc.

One thing I notice ordering barrels they seem to more interested send you a barrel finished length so all you have to do is crown it.
 
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