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Musings on barrel life...

 
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Old 03-03-2007, 02:38 AM
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Musings on barrel life...

First, my sincerest apologies for the rediculous length of this, my first post here...

I have been lurking unregistered around here and a few other places for some time, as well as researching and educating myself on many of the topics that interest me. One of which is firearms in general, and in particular, the intersection of accuracy and practicality.

On that note, I have noticed a tendancy for two camps, the overbore velocity crowd, and the barrell life concious crowd. I will leave out the various motivations, levels of education and experience and socioeconomic differences between these camps, and get right to the meat of the topic...

There are several bits and peices out there that pertain to extended barrell life. I am just beginning to form a picture in my head about all of this, and would appreciate some perspective.

1: number of grooves. Lilja barrels claims (and I have no reason to doubt that claim, quite the contrary, I have every reason to believe it) that 3 groove barrells will erode more slowly, and hold accuracy longer.

2: method of rifling. Alternatively, Krieger claims (and again, I have no real reason to doubt) that single point cut rifling will outlast other methods.

Now. From a pseudo scientific standpoint, I can understand both claims, but my understanding is limited. Here goes. With three grooves, as opposed to the six, there is significantly less surface area for the abrasion of the bullet, and much more importantly, the hot gasses and pressure to erode. Less surface area means two things. First, it means that erosion has less area to affect, essentially eliminating all of the erosion on the eliminated surface. Second, as erosion does not happen uniformly, less surface area means less eroded area, and thus less disuniformity, meaning loss of accuracy is put off until the erosion "catches up" with that eliminated surface area.

With the cut rifling thing, one has to understand a bit about grain structure and carbide distribution in steel. Luckily enough one of my other hobbies (bladesmithing) has taught me a thing or two about this. When a barrell is button rifled, the grain structure gets displaced, putting a given amount of shear stresses into the grain structure. Now, with a well made barrell, this isn't an accuracy problem, at least initially, as those stresses can be made rediculously uniform, resulting in the very accurate button rifled barrels we all see today.

However, over time, as the steel heats and cools, some pretty neat processes are going to happen. One is that the steel temperature forms a gradient, hot at the bore surface, significantly cooler at the outside of the tube. Since steel expands when it heats, having this uneven tempreature places even more shear stresses on the barrel. It is not out of the question that such stresses could cause microscopic fractures in the grain structure, which will ofcourse, propagate until worn away by barrel erosion. However, while they will be worn away before propagating into actual cracks(which is why we don't see button rifled barrels breaking) such miscrofractures would be more vulnerable to the process of erosion. Take into account the average size of the carbide clusters formed in the relatively complex steel alloys used in modern rifle barrels, and this problem gets even worse. A carbide cluster cut out in rifling is not a problem. However, a carbide cluster forced to displace the otherwise nice, orderly ferrite / cementite grain by button rifling will cause even more shear stresses, which will exaserbate this problem.

As to why the single point method would have longer barrell life than, say, multi point cut, well, I have a hard time stretching for that one, except to say that single point creates a more even, precisely shaped surface, meaning that when erosion does take place, it is starting from a cleaner base.

Finally, there are a few other, slightly more technical points I wanted to address.

Steel selection. As with all things in life, there are sacrifices, tradeoffs, and balances to be made. While there is little doubt that stainless barrells last longer, one stainless is not equal to another. Chromium content is primarily responsable for the corrosion resistance of stailess steel. However, it is also responsable for the brittleness of said steels. I have not done a comprehensive study, but the common barrell steels I have looked up have a rediculously large variation in Chromium content!

Also, what may actually be an overlooked factor by many is the biggest culprit in barrel wera, carbon migration. The very hot gasses in the throat will raise the very surface of the throat to a temperature at which it will take on or lose carbon. Typically, since these gasses are carbon rich, this area will carburize, which results in a micro thin layer of increased hardness and increased brittleness, which will break away after repeated shooting. This process can be controlled to a degree with the addition of Nickel to the steel. Nickel is quite effective at preventing carbon migration. High nickel bearing steels cannot, for example be color case hardened, as the nickel prevents the steel from taking on enough extra carbon in a reasonable amount of time. However, in a brief search, I was only able to find one (out of four) stainless commonly used for rifle barrels that contains any nickel at all!

Steel talk gets me onto a pet topic of mine, that being cryogenics. There are all sorts of wild claims about this out there. I can think of two things, metallurgically speaking, that cryo actually does that MAY have an effect on barrel life and / or accuracy. First, when steel is hardened, it is taken from the parent microstructure, called austenite, and transformed into the desired (in this case) "room temperature" microstructure, called (again, in this application) martensite. However, since we are dealing with fairly complex alloys, some of which are precipitation hardening, and I have doubts that any barrel maker does all of their own heat treating in house (rather, such operations are farmed out to industrial heat treaters) there is the distict possibility of retained austenite distributed in the grain structure. This is bad for barrel life and accuracy (although it may not be bad to a noticeable degree). The reason it is bad, is that retained austenite will both interrupt the orderly martensitic grain structure, and it has little to no wear resistance compared to the martensite we are after. The reason austenite is retained is due to the depression of the martenside finish temperature below ambient, and by bringing the steel down to that temperature or below, that austenite can be transformed into martensite. However, if this process is not followed up by a high temperature treatment, that martensite will be full of stresses, and could have a very bad overall effect on accuracy, as those stresses naturally seek equilibrium. I fully believe that not understanding that last factor is the culprit for reports of decreased accuracy after cryo. Second, The work of Dr. Fredrick Diekman has shown that cryogenically processed steel will precipitate very tiny eta carbides upon reheating to 300 F or above. These carbides are rediculously small and even when compared to the heavy carbides commonly found in steel (like chromium carbide, for example), but they can have a significant impact on the wear resistance of the steel. Of course, to have proper, maximum benefit, the cryo should be incorporated into the original heat treat as the barrel is first being manufactured. After rifling, lapping, chambering, and installing is a less than optimal time for this process by a long shot.

Finally, surface treatments, of one sort or another have a great deal of potential. I have always wondered if it may be worth nitriding a bore? would the rediculous hardness be just doo damn brittle and wreck an otherwise good barrell? What about Chromium nitride PVD coating? massively increased wear resistance (as opposed to stainless steel) a very low coefficient of sliding friction, and a high service temperature!

What about other coatings, Microlon, Sprinco Plate +, Molyfusion, Tungsten Disulfide? Typically billed as a "lubricant" these coatings have the unintended benefit of providing a very thin, but nonetheless significant insulating barrier between the pressure, heat, and abrasion active in the barrell and the substrate steel, and they can be much more easily renewed!

Then there is very fine firelapping, a la tubb. While I wouldn't dream of using such a system in a new handlapped barrell, once the throat erodes, the tubb TMS seems like a decent option to at least extend some of the accuracy life of a top quality tube!

I may be trying to reinvent a wheel that was designed by those far more competent than I am, but that's a problem I'm quite prone to anyhow. It is by reinventing the wheel over and over again that I learn, and I like to think that by doing so, I can help others learn right along with me.

Ultimately, where I am going with this, is, on a practical, real world level, just how well could barrell life be extended. Erosion is a fact of life, and cannot be completely eliminated. However, with careful consideration of all of the above, would it not be possible to finally bridge the gap between the overbore speed junkies and the barrel life crowds? Could we maybe see on the horizon "super tubes" that wear a 6mm/06 AI as slowly as a normal barrel does a .243 Win?
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Old 03-03-2007, 07:33 AM
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Re: Musings on barrel life...

Welcome to the forum. Nice post, can you elaborate on the 4 stainless steels you researched and one had nickel?

Thanks,
Don
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Old 03-03-2007, 07:59 AM
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Re: Musings on barrel life...

welcome aboard, great stuff, very interesting.i'd also like to know who and which with regards to the stainless barrels. i thought i understood what you said,but i might have to read it again a time or two,you know, just in case my wife has any questions about it.

as a side note. when the barrel wears out, most barrel makers will sell you another.
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Old 03-03-2007, 08:28 AM
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Re: Musings on barrel life...

Great subject and information.
All I can say is please come up with a plating process to extend barrel life without hurting accuracy. Please do this for us.

I have followed this subject as well. I'm now pretty sure that accuracy loss, the point of declaring a barrel whipped, is reached with bore constriction due to carbon impingement/displacement/buildup in the stress cracking(don'y even know what to call it). I think carbon is 'digging in' tenaciously, and eventually raising the surface. This constricts the bullet more at the breach than at the muzzle. And if the bore isn't tightest at the muzzle, forget accuracy. It's gone.
I think this falls in line with barrel life being extended for a bit through lapping(either firelapping or JB past). It also explains why barrels still 'die' even though the lands have not moved yet(plenty of cooling time between shots).

I have used tungsten coating for many years. But I don't keep rifles long, and haven't reached a barrel's end. With coatings like this, and now Boron Nitride, I worry about their affects regarding carbon in the metal surface. For all I know, they might contribute to the problem.
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Old 03-03-2007, 10:04 AM
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Re: Musings on barrel life...

Hart and Krieger both use crucible 416R. An excellent steel, made by an excellent mill, but alas, no nickel.

Krieger used to offer marrels in 410, as does several other manufacturers. 410 has greater carbide content, and thus is harder on tooling, dulling reamers and cutters very quickly, and this, not inferior barrel performance is why krieger stopped using it. However, again, no nickel.

Lilja used a 416 mod that is functionally equivilant, but not identical to crucible's 416R. again, no nickel.

Many factory rifles, even high end ones use 420 or a modification thereof. A decent (although not completely ideal) barrel steel that has the added advantage of being less expensive. Do not hold it against them though, a small step up in quality from 420 causes a cost increase that is quite disproportionate!

Finally, Crucible offers the unique 174 SXR. This is a high purity grade of 17Cr-4Ni Mod that has 4.8% nickel in it. This is the only barrel steel I have ever seen with Nickel. 4.8% is a REALLY LOT of freaking nickel for a steel to carry too!

This last grade is a complex precipitation hardening, exotic steel. Crucible's literature indicates that at the same strength levels, it is very much tougher and more corrosion resistant than even their own 416R (used by Hart and krieger, and an almost identical product by Lilja). Why this isn't the premium barrelmakers steel of choice, I'm not sure, but I have a couple of theories:

1: being quite a new offering, nobody wants to risk their reputation on using it when the proven high performace 416R works quite nicely.

2: being a rich, exotic alloy, it's probably quite expensive by comparison. If this is the case, even the top makers may blanch at the price tag for what they may percieve is a very small if any improvement.

3: being an exotic precipitation hardening steel, it takes a very patient heat treater, with very tight temperature controlls to get the most of it. Without these, it will not perform upt to par.

4: This last one I'm not so sure aout, but it may contribute. Crucible's literature on 416R tout's the alloy's fine polishability as a selling feature for high quality rfle barrels. No such mention is made in their literature on 174 SXR. While I personally believe that this is because they devoted that space to touting the alloy's other great features and properties, it may in fact take a less fine finish. On the other hand, the chemistry appears to be great for this (which is why I believe this was overlooked so they could tout the other great properties). I just don't know about this one for sure.

Also, crucible's own CSM 21 (billed off as a mold steel) has practically identical chemistry (it's another 17Cr-4Ni Mod) is touted by their literature as having a high degree of polishability. That really makes me wonder about the 174 SXR.

Finally, I must wonder at the precision barrel maker's dedication to very extended barrel life. This is not to question commitment to quality, nor to question integrity, but rather a simple, cold look at economics. Why would a barrelmaker, say krieger, offer a single point cut rifles, three gorrve barrel in a steel that significantly outlasts most others? Certainly they could charge more for it, but the laws of supply and demand dictate that they couldn't charge very much more, or people would order some other high quality tube. Probably, the profit margin would be shrunk, even with the price increase. Less profit margin, and because it lasts longer, they won't have as much business in the future! This is NOT a successful business model! Before I get flamed for this, I must say that I don't think these guys are getting rich at our expense. Far from it! But hey, barrel makers gotta eat too y'know! If they go out of buisiness by offering a product that lasts too long at too low a profit margin, they won't be there to help us out when that product does eventually wear out!


MikeCR - you have it almost right! What happens is, as the carbon cracks and carburizes the throat area, that area erodes away, causing it to be looser (not tighter). Eventually, this area gets so loose, so rough, and so long, that the bullet 'tilts' in the bore before encountering good, clean, uneroded rifling. Try loading a .277 bullet into a .284 cal cartidge sometime and see what king of accuracy you get! a bullet not held snugly cebtered, it won't fly predictably, let alone straight!
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Old 03-03-2007, 10:53 AM
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Re: Musings on barrel life...

Dzaw,
Very interesting information. Besides the manufacturing procedues and material discussion there is also an interesting occurence of "hummer" barrels as mentioned in the Shilen site. Some barrels are simply more accurate than others - from the same manufacturer and batch. Also some clean-up much easier than others - no doubt this is related to smoothness and ?.

I have been a member of both types of barrel users - used to hotrod my Weatherbys to get true Weatherby performance, now I shoot my little .308 Wins and clean them every forty shots and try to "baby" them for maximum life.

Throwing away a new piece of Weatherby brass after only one firing was pretty dumb, about as dumb as worrying about barrel life when there are such incredible barrels being manufactured by Broughten, Lilja, Rock, Schneider, Krieger, Bartlein, Hart, Obermyer and a few others. I believe a sign that one has been pushing for maximum velocities will inevitably be a .210 ring burned into the bolt face around the firing pin hole. Several of my rifles have this badge of stupidity.

I have some very nice digital photos of the inside of shot-out barrels if anyone would like to post a couple. A borescope can be a very scary tool - shows what is really going on inside your barrel.

Thanks for sharing, enjoyed reading your info and welcome to our site.
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Old 03-03-2007, 11:33 AM
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Hummers

I have encountered this phenomenon...

I have a theory on this, but I'm not entirely sure how valid it may be. See, industrial heat treaters, like most barrel makers (likely) use, typically aren't tooled up for small run precision treating to spec. More often than not, they take the steel grade info and intended application, and run it through an ANSI standard process in automated or semi automated machinery. While this typically results in relatively good consistancy of product, it may not be optimal, and the occaisional real hummer as well as the occaisional real lemon could be the result.

By studying metallurgy a bit more deeply, and constructing their own HT equipment, I think a real improvement could be had by those who don't do this already.

I could be dead wron in this. For all I know, these barrel makers do their own HT in house. However, if that were the case, I would expect that they would advertise and tout that fact as broad and as far as they do their machining excellence. As we say in the bladesmithing community, the best steel, shaped by the best smith, and given a mediocre heat treat makes a mediocre blade. Decent steel, shaped by an absoloute novice, and given an optimal heat treat makes an heirloom quality blade. The heat treating really is that critical to steel performance.
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