i recently had a discussion of barrel choice with a friend.  i have been a stainless fan and have no problems with others...

he remarked that the stainless barrels are "harder" than stainless.  ???  i wondered later if this is a question of metallurgy so i asked my brother who is a "master" machinist, whatever that is...  he and dad are both fab guys, but know little about the firearms industry.

my brother basically told me there are so many grades of stainless it is hard to compare until he knew more specifically what i was talking about.  he also mentioned that stainless is one of his least favorite materials to work with - he cited it as "hard to work with" and that small parts especially were prone to warping and falling out of spec during the machining process.  

he also said he liked what some people call "chrome moly" as it was not necessarily hard, but tough - he said it machined well and was a "tough" metal and probably made a great barrel choice.  

when asked about chrome lined barrels he was curious if it is plating and how it is applied as well as how thick.

anybody able to shed light on this?

i'm not looking for "chrome moly only..." or "ss forever, dude"...  i'd like to know more from the technical end what the scoop is on this...

thanks,


septic tank

I looked up this (first search & later on a different gun (hunting) forum’s older posting.
(I cited the original site / forum.  There was a highly technical discussion on your very question, from a forum member who was a machinist, BUT I could not find it).  Sorry.

--  Kriegerbarrels.com
“Q: Which is better Chrome Moly or Stainless Steel?
A: For the most part neither one is better than the other. The only difference we find is that sometimes the chrome moly might take a little longer to break-in and might have a little more affinity for copper or seems to show it easier. In terms of barrel life and accuracy, we can find no difference.”

--LongRangeHunting.com
(Dated: 10/2004) Searched for:  “chrome moly or stainless steel”)
SUMMARY ONLY (Pros & Cons…)
Chrome Moly
-Tighter barrel tolerances…
…(Tens of thousandths of inch)
&
Stainless Steel Barrels
-More corrosion resistant
-Longer Throat life

BM 4150 chrome lined are the best overall

Sabre CMV chrome lined are the best overall

Thank god this has NEVER BEFORE been discussed!

Dunno if this helps or not but what the hell...4130 for instance is used in making high-end bicycle frames and is also the steel Kel-Tec uses for their Sub2000, though I'm not sure what they use in the SU16. RRA, DPMS, etc. use 4140 in their barrels and is considered a great commercial grade barrel steel. Good for approx. 10,000 rds. Then you have 4150 which is "Mil-Spec" and is used by Colt, Bushmaster, etc. I guess it's suppose to sustain FA for longer periods and offers greater wear resistance. CMV is basically 4150 with added Vandium for even GREATER wear resistance. Sabre and I think Denny's newest barrel is CMV. These barrels (if chrome-lined) have gone 30,000 rds. before being considered worn.

Chrome-lined means the chamber / bore has a hard chrome coating for added wear resistance, ease of cleaning, and increased corrosion resistance. I believe I read that HC was harder than the barrel steel itself though I don't know that for fact. Seems like it'd chip.

Stainless supposedly isn't expected to last half as long as "chrome-moly" but offers the best corrosion resistance and accuracy potential.

Stainless and chrome moly have similar life expectancies. SS will decline in accuracy slowly, CM will drop off all of a sudden. If you aren't shooting hipower you probably wont notice the difference. Chrome lined lasts a LONG TIME. CM tends to rust if you dont care for it well, unless you live in a very, very dry climate.

Quoted: Thank god this has NEVER BEFORE been discussed!

got any of the answers?  if so, i'd owe you a favor for providing them.  



thanks,

septic tank

i checked out benchrest.com and a few others and they are mostly putting in print that it is a personal preference.  looks to me like actual performance and cost may be a flip of the coin.  

i'm not too concerned about this, as i own both.  i need to check out some metallurgy sites if there is such a thing to learn a little more about that.  i was trying to find and understand the attributes of each barrel so i can decide which one i want for which application.  besides, i just wanted to actually KNOW it and understand it, rather than take someone's opinion for it.

if the benchrest guys are all saying the same thing, and it doesn't matter which way a guy goes, then why is 85% of arfland under the impression there is one or the other that should be used and nothing else?  the only thing i can figure out is the impression of non-chrome lined barrels in initial shipments of M-16's to Vietnam and the associated problems - most people must be under the impression that chrome lining is as necessary now as then...

any more info would be appreciated.

thanks,

septic tank

Bravo Co. I believe posted a link to a 4100 series metallurgy chart in one of my or somebody's barrel thread a few months back but I couldn't find it and google didn't help either. The main difference was increased amounts of carbon in the higher series metals and less copper...oh shit...found it, here it is:

www.ar15.com/forums/topic.html?b=2&f=138&t=148891

He also posted some info from a Colt barrel he sent off to have destructive metallurgy testing done to. Wish I could find it.

Note: This is why threads like this should be a little more tolerated. Not only does it give everyone something to read / bitch about, including newbies who are afraid to ask or haven't thought of yet, but it sometimes resurfaces old information that's helpful to everyone.

Quoted: Bravo Co. I believe posted a link to a 4100 series metallurgy chart in one of my or somebody's barrel thread a few months back but I couldn't find it and google didn't help either. The main difference was increased amounts of carbon in the higher series metals and less copper...oh shit...found it, here it is: www.ar15.com/forums/topic.html?b=2&f=138&t=148891 He also posted some info from a Colt barrel he sent off to have destructive metallurgy testing done to. Wish I could find it. Note: This is why threads like this should be a little more tolerated. Not only does it give everyone something to read / bitch about, including newbies who are afraid to ask or haven't thought of yet, but it sometimes resurfaces old information that's helpful to everyone.

thanks!

A question along these lines I asked a long time ago that went virtually unanswered was-----If a barrel is chrome lined, what does the material matter (4150 vs. 4140)--i've heard for a long time people here say chrome lined AND 4150 is the only way to go, but with CL being so much harder than the barrel steel anyways, what would it matter?? If you did ever wear thru the CL, the barrel is toast anyways.

i hadn't thought of that.  i suppose that is true.  

anybody know how thick the chrome lining is?  .0005"  or .005"  or .001"?  

???

I learned something today - good thread

Quoted: And while chrome-lining a CM steel barrel will increase its resistance to both wear and corrosion, the above explains why the barrel steel is still important. -Troy

do you know the thickness of the chrome?  is it a plating process?  at what stage is it applied and why?

is there a place where i can research this further?

thanks,

septic tank

Fuck it I'll play "parrot"...at least I tried to help the guy out and provided the link.

If you look at metal composition tables you'll find that 4140 and 4150 overlap a lot in their chemical compositions -- except at the extremities. You'll find that they are within a nominal range of one another in things like tensile strength, modulus of elasticity, etc.

What do those steel testing requirements mean in terms of barrel performance?  I have no fucking clue.

I do know that 4150 barrels tested to failure take more rounds and slightly higher heat to actually fail than 4140, which is why I believe it was chosen in the milspec.

That being said, in realistic field conditions, the gas tube and maybe the gas block, would end up failing before the barrels, thus never allowing a barrel to fail in the manner shown in bench trials.

Or at least that's what I have gathered in my reading of the testing and testing methods of barrels, and a former pursuit of an engineering degree.

All I can add is in personal experience the Chome moly/C lined barrels heat faster and cool faster.

Stainless heats slower but cools slower so it seems to suggest you might be able to overheat a stainless barrel more easilly due to temperature stacking.  

Stainless seems to have a much longer reasonably accurate life- so I suggest the carbon wrapped Stainless barrels as they cool faster than chrome moly and should last longer than straight stainless-- in light of that they should be the best product availible.- Not at a price the government would find attractive but then they have always been more busy lining their own pockets and padding the Air Force budget to buy true top of the line equipment for soldiers.

You cant hard chrome stainless also... though there are better barrel liners than hard chrome which will adhear to stainless.

I hope it's okay for me to piggy back this question onto this thread.

Does anyone know what goes into creating sub-MOA accurate chrome-lined barrels, like the barrels on the FN bolt guns are rumored to be?

--sub-MOA accurate chrome-lined barrels--     I LIKE IT !  

i'm not actually obsessed, but close...  i've been working at learning more about this for a few days now and found some interesting posts in other forums and other places on the net.  i'll post them here and would like any feedback you can provide.

septic tank

posted by solarqualityguy on benchrest.com:

This is a most-interesting topic that will always be hot. As a trade Metallurgist and Quality Manager for a major US heat treating company I can shed some light on this topic.

Yes, AISI 416 stainless alloy (all grades) is a prefered material for manufacturing gun barrels and for good reason. It is readily available, relatively inexpensive, and corrosion and wear resistant due mostly to its high chromium content. It has sex appeal because it's "stainless", and come on...it looks cool. Unfortunately it has sulphur in it to make it free machining. Without it it would not be a production alloy for the gun-makers. Sulphur is generally prohibited from design and use in high value products. It does not diffuse well into metal alloys and generally takes the form of "sulphide stringers" in the metal matrix. This is primarily why it is prohibited from pressure vessel use by ASME code. It tends to be inconsistent in quality and sometimes the stringers combine and cracks propogate from the sites which causes failures.

In gun manufacturing however, it is a relatively stable product I guess...most of the time. I do not have warm fuzzies over this material however, and here is why: All martensitic stainless alloys (416 is one) show a marked reduction in impact strength when tempered between 700°F and 1100°F. Ironically, and after austenitizing and quenching, that is exactly the required tempering temperature to bring 416 SS down into the hardness range of 28-32 HRC where it can be most easily machined. This is called "temper imbrittlement", and one will also realize a marked reduction in corrosion resistance from tempering in that range. Aerospace specifications prohibit tempering in this range.

Anyway, so a gun manufacturer buys 416 SS with sulfide stringers in it (bad), austenitizes, quenches and tempers it in the worst possible range (because you have to achieve the right hardness) machines it, then in use it's exposed it to shock and impact which is now its weakest attribute. But it sells and works well...most of the time. As a metallurgist who performs heat treatment on aircraft parts I shudder at the 416 gun barrel issue. But that's me.

On the other hand, AISI 4140, 4142, 4150, and 4340 ("chrome moly") have no major drawbacks to the entire manufacturing and end use application. There are small drawbacks, but none as scary as what I just described. Better yet would be 300M which is a slightly modified 4340 alloy. This alloy was developed for landing gear of aircraft using carrier runways. When I see a bar of 416 SS alloy next to a bar of 4340 alloy I see a couple of good alloys, but for different end uses. Only one that is good for a gun barrel...4340.

One last thought: I have literally dozens of nonconforming reports filled out by my shop people every year for 400 series SS alloy steel that does not respond to heat treatment. The problem is poor quality coupled with wide open AISI tolerances. China is also causing a huge problem in the global marketplace when it comes to metals. This past year I had to reject 2000 parts for a custom gun component manufacturer (whom I will not name) for a total non-response to heat treatment, i.e it did not harden at all. When he told me I was crazy and that it was "certed material" I told him to send me a copy of the cert and I would examine it. It was half written in chinese and the ASTM specification it was certified to didn't even match the chemical composition listed on the same cert. In other words, it was total garbage; worthless material and the guy had no idea. I have very few quality problems with the chrome moly grades.

posted by bglenn on benchrest.com:

If stainless is easier to machine and rifle, why does it cost more when you buy a barrel?  There is a reason why something cost more. Stainless material cost more but not that much more. Is the extra price because the manufacturer has to put more work and time into making a barrel with stainless or is it price gouging.  I think the first. When I machine stainless compared to 4140 PHT, I have to run the tooling 20 to 30 % slower to drill, tap and turn. Button rifling in a CM barrel is smooth as can be before any lapping. Now the lapping in CM may be more work, I don't know. Just my $.02 worth.

posted by solarqualityguy on benchrest.com:

Shelley, that is a very good question, however it is nearly impossible to effectively heat treat the barrel after machining. This is due to the high austenitizing (hardening) temperature and required rapid cooling process (quenching). It causes distortion on the average of .001" per inch of part length. So a typical barrel is 24 inches and it would be warped about .024" from one end to the other. This is totally unacceptable. Also, and during the phase change from austenite to martensite, there is a size change in the material. This makes that idea impractical on its own. The size change is unpredictable and is influenced by the amount of cold work in the material (from manufacturing) and chemistry. The last problem is that annealed material (heat-treated to softest state) is too gummy to machine effectively as it does not produce good chips and it galls.

To answer your harder is better question, yes harder is better...for WEAR, but not for ductility (toughness). Putting distortion issues aside, if you made a barrel from AISI 4140 then heat treated it to 50-54 HRC (which is the max hardness for that grade), it would wear great but probably fracture during the first fire. Thus the need for impact resistance and the high tempering temperature.

The ideal barrel would be something like what the Germans produced during the late world war II period in their artillary weapons. By the way this is not widely known. They made barrels from CM, then vacuum plasma ion nitrided (PIN) the ID to produce a shallow "case" hardness to prevent wear. An ion nitrided case is about 60-70 HRC (wow). The base material still has impact resistance due to the tough core that was previously hardened and tempered to approximately 28-32 HRC. I have never seen a hand-held barrel like this, but that does not mean it does not exist. PIN processing is performed at a measly 900°F and does not require rapid cooling so distortion is not a problem. However, is is tough to get a plasma discharge glow in the ID of a small tube such as a barrel. What the germans did was to blank off the ends of the large barrel and use it as the vacuum chamber. They then hook a power supply to the part to heat it to the required temperature then inject the nitrogen gas inside the barrel which creates the plasma glow, nitriding the surface.

I would be intersted to know f anyone has tried this with a hand-held firearm.

posted by mho:

Solarqualityguy, interesting posts. I know your opinions somewhat fly in the face of what has been proven to work (for BR) in practice, but interesting nevertheless.

From your point of view as a metallurgist, handled the same way (same amount of rounds fired, same caliber, same frequency of fire), is there anything that indicates that a barrel made of one material (SS vs CM - whatever the standard steels are) should last longer than a barrel made of the other?? I once heard, that when a CM barrel corrodes in the throat, it is likely to do so at least as fast as a SS barrel, but that the effect would be much more gradual. For non-BR applications, such a barrel would still be useable even though corrosion was quite advanced. A SS barrel, on the other hand might resist corrosion somewhat longer, but then suddently "loose" chunks, and from then on be ready for replacement. True or not, I have not got a clue??

Metallurgically speaking, should a cut barrel last longer than a buttoned, and does the width of the lands (as in 3-groove) have any bearing on how long the barrel lasts??

Finally, it has been my experience, that CM barrels have a tad more affinity for copper than SS barrels do. Again from a metallurgic point of view, does this make sense in any way??

posted by solarqualityguy:

Please understand I have no question that stainless barrels work; they work very well. I have no question that there are a good many 416 stainless barrels out there that will never fail, and last a good long time. The issue I have is with the consistency and quality of the barrel. I do not think I am flying in the face of what has been proven to work, I am just somewhat reserved about the ultimate failure rate of stainless barrels vs CM grades. Please also bear in mind I am an aerospace metallurgist where one failure is possibly going to kill hundreds of people so I tend to be a bit dramatic on even the potential for one failure.

Now I will try my best to answer your excellent questions:

#1 It is my opinion that a SS barrel will wear better than CM as long as the hardness is the same. I am sure someone out there will say I am wrong, and that some condition may exist where I am wrong, and I have been wrong before (once) but the higher Cr content is of great benefit for wear properties.

#2 I have never heard of a CM barrel corroding more in the throat, but then I am not the expert per say on barrel corrosion, more on the alloys they are made from. Corrosion is a complicated thing and is influenced by many factors: material chemistry, material condition (annealed, as forged, etc.), environmental conditions, cleaning procedures and chemicals. I would say a SS barrel will corrode slower than CM in a barrel, period. Your statement about chunks coming out is scary, and normal corrosion does not usually do this. That sounds more like intergranular corrosion where the metal alloy is affected by factors such as poor heat treating atmosphere control. Another problem 400 series SS alloys are susceptible to is hydrogen embrittlement from process like passivation where hydrogen is absorbed in high concentrations by the metal making it brittle. I could see how chunks might fly if that were the case. CM grades are not prone to hydrogen imbrittlement.

#3, As far as product life, that's a big one. There are numerous failure modes. There is wear, fatigue, corrosion, and creep. They can exist in combination, and many times do. It is fascinating how they can work together. Creep means over time the material under load fails. Guns should not experience this failure mode. Corrosion is even not a big deal here in the gun world, however, if one does not clean a gun it will be. Fatigue and wear are the big deals here, and fatigue is where I have major doubts about 416 SS. Wear is something we all easily understand just by looking at old sneakers. Wear happens through use, and the harder the material, the better for wear. CM at the same hardness as SS should not wear as well as the SS. As for the effect of the manufacturing procedure, now we get even more complicated. It is known that cut rifling produces less stress than buttoning. But stress is not always bad. Compressive stress from case hardening or shot peening is a good thing as it keeps the material underneath the surface under a healthy load. I do not know for sure if button rifling does this same thing or not, however, it works the metal, which is good. Working the metal makes it harder and can create grain flow in the metal and is a good thing for wear, fatigue, and even directional strength. Buttoning may work in the same way, my gut feeling on it is that it is a very good process for barrel longevity. Cut rifling removes metal and does not work it per say. Metal removal is somewhat of a quality concern because of the tools are not sharp and there are minor defects in them they will create stress risers in the metal which can be a barrel killer. Especially if there is sulphur in the steel like in 416 SS. But it is my understanding that buttoning is not as precise as cutting, so if that is the case, and over time, there could be uneven stress created from inconsistent rifling twist with buttoning. I think I have raised more questions than answers here! All in all I would say that in theory, for longevity, a quality buttoning job is better.

#4 You are right about the copper thing. CM is much harder to polish to a smooth surface, so the copper has a tendancy to build up unless the surface is identical to the SS barrel surface. I believe it is strictly a mechanical thing. I am unaware of any metallurgical affinity to diffuse copper more in one alloy or the other because of chemistry. But I will look that up.

ok, ok...  

i was going to copy and paste in several passages from several other threads and forums, but the ones i already posted came from here:

benchrest.com - stainless/cm

this particular thread sheds a great deal of light on what i was after and is surprisingly well rounded considering they are benchresters...

i'll opt not to add all the others posts and threads.  this particular one pretty much covers a great deal of it - the guy sounds like he's right, but who knows...

enjoy!

oh, by the way, the action doesn't start until page 3 of the thread!

septic tank

"On the other hand, AISI 4140, 4142, 4150, and 4340 ("chrome moly") have no major drawbacks to the entire manufacturing and end use application. There are small drawbacks, but none as scary as what I just described. Better yet would be 300M which is a slightly modified 4340 alloy. This alloy was developed for landing gear of aircraft using carrier runways. When I see a bar of 416 SS alloy next to a bar of 4340 alloy I see a couple of good alloys, but for different end uses. Only one that is good for a gun barrel...4340."


All I think of is results you will see in use-- that's all that matters; not what some metalurgist says without examining the actual results shooters are getting.  

Chome moly chrome lined has a moderate accurate life of around 4700-6700rds after which the chrome seems to be gone and the barrels accuracy rapidly detirorates.  Were talking M4 barrels at 5MOA at 10K

Stainless barrels have been giving users 10K under 1MOA and 20K under 2MOA< that says to me it could be a long time before these users see 3MOA which is what I believe the threshold for a combat gun should be.  5MOA is only useful below 300meters and I like to be able to use the gun to around 500meters if need be.  [that's kindof based on a human's 18inch shoulder width and the group needing to be smaller than 18inches to reliably produce a hit.

The point there is that a metalurgist can't tell you what you need to know obviously- the stainless barrel is a better barrel, except in terms of heating over time which won't happen unless you are going into a second combat load which is an extreme rarity that I estimate less than one in one thousand infantry soldiers today have ever had to do- it was commonplace for some soldiers in Vietnam though- so obviously it is still important as more conventional wars against a determined opponent with small arms might require those fire schedules.  For that there is carbon wrapped stainless which will allow you to have all the benefits of stainless with better cooling than chrome moly- eliminating this fringe contingency planning desire for chrome moly.  



As far as stainless costing more, I think that is simply the metal costing more.  I want to say stainless can cost $30 a lb or more. a barrel weighing around 2lbs means around $60.  

So, does anyone know how to make accurate chrome-lined barrels?  What effect, if any, would hammer forging the barrels have?