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 **** 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?
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 **** 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?
