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Chrome lined barrel FAQ
http://www.sksboards.com/smf/index.php?topic=36794.0
Since its seems a common topic I am going to make this a general rundown and put in the FAQ section.
Chrome lined barrels are generally thought of as trading accuracy for longevity, and that would be correct generally, but not specifically. In other words just because its chrome lined doesn't mean it wont shoot well, or that if its not chromed it won't last.
_
1st off, a primer on how barrels are made. More indepth explanation can be found here - http://www.riflebarrels.com/articles/barrel_making/making_rifle_barrel.htm
A barrel starts out as a chunk of specifically heat treated round bar stock of chrome moly steel. To get the required hardness of 4140 or 4150 they add harder (and denser) elements of Chromium and Molybdenum within the steel as its smelted. 410 or more commonly 416 stainless is also used on a lot of barrels, being relatively the same hardness as 4140 chrome moly steel. From the forging company, usually about 1" in diameter for small arms, but varies depending on what the purpose is. These are purchased in bulk lengths by barrel makers, cut to length and bored under size to whatever spec the caliber is. This is done on a deep bore drill, basically a specialized mill. Then the blank is rifled. To get the bore to the correct diameter, they use reaming, to improve consistency of the bore and produce a smoother finish than the higher speed drill leaves. Its the reaming process that has impact on accuracy on most barrels, some barrels are not reamed at all, just inspected for minimum tolerance and sent out. Some are reamed once, and others are reamed several times depending on the specs they are building to.
Next comes rifling. The oldest method was to actually cut the rifling one groove at a time into a barrel. The most common rifling now is called button rifling. Its made by pushing a football shaped plug with the grooves on it through the bore which pushes the metal of the barrel to form the rifling. Another way is called hammer forging, where they insert a full length plug with the grooves and hammer the outside of the barrel so it forms around the plug. (Read more on it here - Hammer forging) All of these produce a traditional flat groove style rifling. There is another style that is a non edged type instead of the square edged type called polygonal rifling, it produces a smoother bore with flat "lands" that induce the bullet to spin. Glock is one maker who uses this type.
After rifling, some barrel makers will lap the barrel at this point. Armalite claims to lap their barrels before going to chroming to improve consistency of the rifling and bore.
After rifling the chrome layer process is completed. It basically puts a layer of chrome down on the interior of the barrel and welds it to the steel, chrome is roughly twice as hard as 4150 steel. If done correctly it should never separate like chrome does on bumper, its not a separate layer but bonded. They do have to accommodate the extra thickness of the chrome layer, so they slightly oversize bores for barrels intended for chroming.
_
This is why chrome lined barrels tend to not be as accurate. Any time you put a layer of something over an contour, it magnifies it, so any imperfection in the barrel surface is made more so with the extra layer that adds to the surface. There was in the early days of chrome lining process problems keeping the lining thickness uniform as well, though the process has become much more automated and a lot of that has ceased to be an issue.
For normal shooters, chrome lining is not an accuracy killer. The barrel quality control underneath the lining is far more critical than the lining itself. In many cases the lining being an accuracy issue is only one to those like match shooters trying to shave 1/4 MOA off groups.
The main advantage of Chrome is being much harder and impervious than 4150 steel, will hold up to wear a lot longer giving a much longer effective lifespan of a barrel. Its also much easier to clean as powder residue and lead will not work into the pores, like it does in bare steel.
However a chrome lined barrel can still wear and reduce accuracy quickly depending on the use its subjected to. Most wear occurs in the throat, where the most heat and contact friction is, if the wear is sufficient it can cause the bullet be less stable as it travels down the barrel, even chrome lined rifles will have reduced practical accuracy life in full auto use.
Chrome lining also does not protect the crown, which can have a major impact of accuracy of a barrel.
The bottom line is, for any utility or defense rifle there is no reason to not opt for chrome lining where offered. But its not necessary either. A lot of rifles have lived generations without chrome lining, and if you are into accuracy then without question the more accurate barrels are not chrome lined.
More info on general qualities of barrels and manufacture methods.
link to more on barrel steels from Obermeyer barrels
http://www.riflebarrels.com/articles/barrel_making/details_of_accuracy.htm
Chrome lined barrels are generally thought of as trading accuracy for longevity, and that would be correct generally, but not specifically. In other words just because its chrome lined doesn't mean it wont shoot well, or that if its not chromed it won't last.
_
1st off, a primer on how barrels are made. More indepth explanation can be found here - http://www.riflebarrels.com/articles/barrel_making/making_rifle_barrel.htm
A barrel starts out as a chunk of specifically heat treated round bar stock of chrome moly steel. To get the required hardness of 4140 or 4150 they add harder (and denser) elements of Chromium and Molybdenum within the steel as its smelted. 410 or more commonly 416 stainless is also used on a lot of barrels, being relatively the same hardness as 4140 chrome moly steel. From the forging company, usually about 1" in diameter for small arms, but varies depending on what the purpose is. These are purchased in bulk lengths by barrel makers, cut to length and bored under size to whatever spec the caliber is. This is done on a deep bore drill, basically a specialized mill. Then the blank is rifled. To get the bore to the correct diameter, they use reaming, to improve consistency of the bore and produce a smoother finish than the higher speed drill leaves. Its the reaming process that has impact on accuracy on most barrels, some barrels are not reamed at all, just inspected for minimum tolerance and sent out. Some are reamed once, and others are reamed several times depending on the specs they are building to.
Next comes rifling. The oldest method was to actually cut the rifling one groove at a time into a barrel. The most common rifling now is called button rifling. Its made by pushing a football shaped plug with the grooves on it through the bore which pushes the metal of the barrel to form the rifling. Another way is called hammer forging, where they insert a full length plug with the grooves and hammer the outside of the barrel so it forms around the plug. (Read more on it here - Hammer forging) All of these produce a traditional flat groove style rifling. There is another style that is a non edged type instead of the square edged type called polygonal rifling, it produces a smoother bore with flat "lands" that induce the bullet to spin. Glock is one maker who uses this type.
After rifling, some barrel makers will lap the barrel at this point. Armalite claims to lap their barrels before going to chroming to improve consistency of the rifling and bore.
After rifling the chrome layer process is completed. It basically puts a layer of chrome down on the interior of the barrel and welds it to the steel, chrome is roughly twice as hard as 4150 steel. If done correctly it should never separate like chrome does on bumper, its not a separate layer but bonded. They do have to accommodate the extra thickness of the chrome layer, so they slightly oversize bores for barrels intended for chroming.
_
This is why chrome lined barrels tend to not be as accurate. Any time you put a layer of something over an contour, it magnifies it, so any imperfection in the barrel surface is made more so with the extra layer that adds to the surface. There was in the early days of chrome lining process problems keeping the lining thickness uniform as well, though the process has become much more automated and a lot of that has ceased to be an issue.
For normal shooters, chrome lining is not an accuracy killer. The barrel quality control underneath the lining is far more critical than the lining itself. In many cases the lining being an accuracy issue is only one to those like match shooters trying to shave 1/4 MOA off groups.
The main advantage of Chrome is being much harder and impervious than 4150 steel, will hold up to wear a lot longer giving a much longer effective lifespan of a barrel. Its also much easier to clean as powder residue and lead will not work into the pores, like it does in bare steel.
However a chrome lined barrel can still wear and reduce accuracy quickly depending on the use its subjected to. Most wear occurs in the throat, where the most heat and contact friction is, if the wear is sufficient it can cause the bullet be less stable as it travels down the barrel, even chrome lined rifles will have reduced practical accuracy life in full auto use.
Chrome lining also does not protect the crown, which can have a major impact of accuracy of a barrel.
The bottom line is, for any utility or defense rifle there is no reason to not opt for chrome lining where offered. But its not necessary either. A lot of rifles have lived generations without chrome lining, and if you are into accuracy then without question the more accurate barrels are not chrome lined.
More info on general qualities of barrels and manufacture methods.
link to more on barrel steels from Obermeyer barrels
http://www.riflebarrels.com/articles/barrel_making/details_of_accuracy.htm
« Last Edit: December 17, 2008, 07:31:06 AM by Onepoint »
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