Barrel stress cause by contouring ?

B

Boyd Allen

Active member
Barrel stress caused by contouring?

The one barrel characteristic that is almost impossible for a gunsmith or shooter to measure or observe in any way (except possibly by looking at targets), is the amount of stress that a barrel has in it. I have read accounts of problem barrels being re stress relieved with the result that they shot better. My question is whether the rate of material removal that is common when barrels are contoured by their manufacturers induces stress in the metal that might affect accuracy to some degree. A related question is if it does, does this stress reside in the surface of the barrel, and if it does, could it be removed by taking light cuts to some depth, or would it have to be purchased uncontoured, and turned down? Would abrasive contouring be better in this respect?
 
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I always thought that stress relieving was to keep the barrel from moving impact as it heated up. Stresses could be anywhere, depending upon what machining and forging the barrel was subjected to. Like button rifling would leave stresses near the bore, but also work harden the bore, making it theoretically more wear resistant. We need to do a study on this. Send me 10 barrels, half cut rifled, half button, and 10 gallons of liquid nitrogen.........never mind.
 
Boyd,

The barrel makers could answer this question but you won't get one answer. Some seem to stress relieve twice and some once. Does it make a difference between cut and buttoned...maybe, I don't know. I think most say on their websites their process.

Hovis
 
Not a gunsmith, but......

I have never contoured a barrel, so take this with a grain of salt. However, it is my understanding that contouring before final internal dimensioning is best. A hammer forged barrel supposedly will have slightly less internal diameter in the portion most heavily contoured (toward the muzzle) while a button rifled barrel will grow internally in the same area. The latter condition is certainly not what you want for accuracy. I don't think there is much effect with a cut rifled barrel, but I believe anyone doing cut rifled barrels is doing outside contouring before final internal finishing. I also know that you want to cut and crown it at the point of greatest constriction as shown by slugging. I know that the question was more oriented toward whether the barrel becomes less straight or if it is more prone to "heat walking" if contoured as a last step. I have not read about such things. It is my understanding that the barrels that generally shoot best come from the top suppliers of COMPETITION barrels and that they all contour first.
 
Let me give you an example that will possibly illustrate what I am asking about. First of all, let me say that I am not talking about bore growth from contouring, or walking per sae. I am talking about the fine difference between a good custom barrel and a really fine one, both shot in benchrest rifles.

A long time back, a new barrel maker, who had built his stress relieving furnace, followed the time and temperature advice of a friend, who had been in the business for a number of years. and one of his barrels became part of a rifle that was tested by an accomplished shooter, over a period of days, shooting it some every day. On those occasions, the barrel would perform very well for its first five shots, and then degrade into mediocrity. It was not a fouling issue. After this pattern repeated itself several times. After some discussion, it was decided that there might be an issue with the stress relieving that had been done as a part of making the barrel (button rifled), so the barrel was returned to the manufacturer for additional stress relieving, in a nitrogen atmosphere, so as not to cause scaling of the bore. After that was done, a ball hone was run through the bore, lightly, as a precaution, in case there was some minor scaling, and the barrel was reinstalled on the rifle. The change was dramatic, the fellow who was doing the testing told me that if not for the color change in the finish of the barrel, that showed that it had been heated, that he would not have believed it was the same barrel. It was that good. Some time after that, someone else told me of another barrel (button) that had been returned to a well known manufacturer because of accuracy issues, that the same thing was done to, with similar result. Recently, a friend, who builds rifles, told me that he has used barrels from one of the manufacturers that makes lapped button barrels, that this particular makers barrels show heat color, that the other makes that he has used do not, and that those barrels, parts of well made magnum hunting rifles, do not require any warming shots to come to their most consistent points of impact. The bores of these barrels look like other lapped barrels, and do not have any heat color, which leads me to believe that they were stress relieved, a second time, after contouring, and before final lapping. It seems to me that there might be a small advantage in this procedure that would be useful in competition. That was the reason for my original post, the pursuit of information about small but possibly significant differences in how barrels are made.
 
I do know that button barrels must be rifled full diameter, and then stress relieved afterwards. Then they are contoured and lapped. If you're asking would it be beneficial to stress relieve after the contouring, quite possibly, but I don't think many if any at all barrel makers do so. That would be a case Cryo by the user before chambering if they wanted that benefit IMO.
 
Some years back, I believe it was Doug Shilen that said that contrary to all the hype, that while cryo could convert residual austinite in 416r stainless, that he challenged anyone to prove that it would stress relieve it. I believe that he made an offer of money to go with his claim. I never heard of anyone offering that proof or collecting the money. I have no doubt that cryo can make a beneficial change with regard to the homogeneity of the steel, and I have seen claims of increased accuracy, but I know, for sure, of one case where it was made worse. The barrel was a custom stainless barrel that had proven extremely accurate on a rail gun, and he was pursuaded to have it treated to possibly extend its life. It came back less accurate than it had been, enough so that it was basically useless for competition. On the other hand, treating steel before it is drilled has worked out really well for Krieger.

I guess it would take a metallurgist to answer my original question. If rapid removal of steel during factory contouring were to stress the surface of the finished barrel, how deep into the steel does the damage extend, and can it simply be removed by more gradual turning? I believe that one barrel maker is heat stress relieving after contouring and before finish lapping, and from that (if I am correct) , I gather that he must believe that there is some benefit in doing so. If he is right, than contouring must introduce some stress, at least when done at the rate that the economics of barrel manufacturing dictate.
 
This doesn't address the stress relieving question but my first bbls were 1.25" straight and turning them down to something more reasonable is the worst swine of a job I have ever contended with in gun tinkering.
 
Finish turning with a positive rake angle tool is pretty low stress to the part. All most all of the plastic deformation is located in the chip, there is superficial residual stress in the surface of the part though. There are some photos from Cincinnati in the Machine Shop Practice set of books which shows gridded coupons that visually illustrates the rake angle's effect on the amount of plastic deformation of the surface of the part. Actually it shows a surprising lack of plastic deformation in the part with the most positive tool.

Grinding is actually a pretty high stress operation unless you limit the depth of cut very low and flood cool. Because there is no chip formation there is a lot of surface pressure between the part and the grinding wheel which creates shear and compressive stresses on the parts surface. Then the heat creates all forms of stresses cause different depths of the part cool at different rates.
 
Thanks. I was thinking of the process that is usually used to put a final finish on barrels using what is commonly called a barrel spinner and a flexible belt grinder, but set up for faster stock removal with at coarse belt.
 
When it comes to stress relief on button rifled barrels I think I would be safe in saying that G.R Douglas just about wrote the book on that one,especially on chrome moly. When you use one of their rough turned blanks they are blue from heat. When they rough turn them the chips are also blue. In other words they don't take light slow cuts. Thru the years I have profiled more than a few of their tubes and have never had a problem. Admittedly these were hunting and varmint barrels although I did turn a heavy varmint barrel down to light specs. once and my daughter agged a .186 with it. I guess Tim Gardner or Stan Taylor from Douglas would be the ones to ask, at least when it comes to buttoned barrels.

P.S.- The CM barrels are plugged on both ends for the process,the SS barrels are not.
 
I talked with Stan Taylor this morning an he informed me that ALL of their barrels are plugged. Don't know where I got that idea!
 
June 27, 2013
4 years ago I got a Shilen select match #7 6mm 10" twist from Brownells...
Could I turn down the #7 barrel for a hunting weight [from 112 ounces
down to 55 ounces], or will it warp and need straightening that I do no
know how to do?
TIA
Clark..
Click to expand...

July 1, 2013

comments@shilen.com via comcast.net

to Clark
You can re-contour a barrel as long as it has not been cut.
Click to expand...

I have still not done it.
I was hoping to learn something from this thread.
 
You might wish to consider that contouring does not induce stress, it relieves stress induced by previous means, primarily button rifling, in a largely uncontrolled manner, proper post Lapp stress relief attained in a furnace, still does not totally eliminate it.
 
tim said:
You might wish to consider that contouring does not induce stress, it relieves stress induced by previous means, primarily button rifling, in a largely uncontrolled manner, proper post Lapp stress relief attained in a furnace, still does not totally eliminate it.
Click to expand...
Mr. Lilja would agree with this (he is an engineer, by the way). I once asked him about the stress induced in fluting, and his response was that fluting usually more evenly redistributes residual stress, which I took to mean that fluting would be a plus. As a careful man, he wouldn't go that far, in general terms.

Now of course, Lilja barrels are button-rifled, so when I kinda pushed a little, as in "But Mr. Lilja, sir..." he allowed fluting, as with any mechanical operation, might induce a little stress, but he was still of the opinion it's primary effect was to more evenly redistribute the residual stress.
 
I should have added that in regards to button rifled barrels, some of the best evidence of this is the fact that any contouring or metal removal done post production almost always results in the bore dimension getting bigger as well as less uniform.
 
One of few "hummers" I have had

Was re-contoured before it was chambered. It was a Hart. Thinking back, it may have been the best barrel I have owned to date. I realize this may not answer the question but apparently the re-contouring didn't hurt it any, if you know what I mean.

Pete
 
Pete,
My main concern is contouring done at the manufacturing, and that, because of the rate of stock removal, and possibly the geometry, and sharpness of the cutter. I would assume that if a barrel is recontoured by a smith that the rate of stock removal and possibly cutter might be different. Perhaps taking a little off with a sharp HS tool, using light cuts, fast feed, and finishing with a spinner, might actually improve the overall stress of a barrel? Yes, this is only conjecture.
Boyd
 
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