K.L. Davis
08-09-06, 21:53
I guess if I post the first part of this, I will be more inclined to finish it. Lots still left to do... but for now, here is a start.
==================================
Barrels of fun…
I wanted to put together a single reference that addressed the most common questions about barrels – naturally, this thing could go on for pages and pages, but I want to try to keep it reasonable and hopefully take care of the 90% or so of the more frequent questions.
First off, there are essentially two types of barrels (imho): Match Grade and Batch Grade. Which one you want is of course dependant on what you want to do… all too often the standard of a rifle is immediately judged by its group size – “What sorta groups are you getting with that?” is almost always one of the first questions asked of the owner of any new rifle.
Truth is, we have had little more to go by and the gun magazines have used group size as a standard of reporting for years – without that data, most articles would be rather boring:
“I spent three hours at the range with the new Twundelated Arms UbberShuter 150. I shot it mostly at mound of dirt about 35 yards away, and then shot a few old cans lying around, accuracy was great! I could even hit the tiny Red Bull cans with no problem.”
But gun rags don’t really print the bad news…
Nearly all the AR rifles in existence are capable of “practical accuracy” for a carbine, defense, battle, plinker type rifle – that is that nearly any barrel is capable of hitting a 8” target at the functional limit of the sighting system… even using my patented side leaning, semi-fetal, three and half point, one leg akimbo, you can’t see most of me shooting position.
On the other hand, the AR platform is well known for its accuracy potential and if the goal is to build as accurate of a rifle as possible, then by all means start there and shop for exactly that, the most accurate, repeatable barrel you can get/afford. But for most of us, the list of priorities is probably more like (a) Service Life, (b) Corrosion Resistance (c) Fine Tuned Accuracy Potential.
Hopefully, by taking a look at how a barrel is constructed, how it works and what some of the possible choices are, you will be at least a little more comfortable with defining what you need.
The functions a barrel performs
In most cases, the barrel and chamber of a rifle are one piece, so we will operate with that assumption throughout this. Working from the chamber end, the barrel obviously supports the cartridge in the chamber. Directly in front of the chamber is the throat, which supports the bullet as it starts its travel and guides it into the rifling – think of it as a funnel between the chamber and the bore. This is a very key part of the potential accuracy of the rifle... if the bullet is "set" in the bore slightly crooked, it will stay that way in flight and degrade accuracy -- a barrel really wears out in this throat area, even after the throat is "shot out", the bore and rifling just a few inches in front of the bore is usually still in great shape.
The rifling simply starts the bullet twisting or spinning… the Rate of Twist (RoT) is given as a ratio of how far the bullet must travel to spin on full revolution (360 degrees), this is normally written as 1:7 or 1/7 or One in Seven – simply meaning that the rifling makes one full turn in every seven inches of barrel.
Most of the work of the rifling is done within the first few inches… maximum chamber pressure occurs within this same length of barrel as well, and from there on the barrel simply contains the pressure and allows the bullet to accelerate faster and faster. Naturally, a longer barrel produces a faster muzzle velocity; however, there is a point of diminishing return. A barrel can be too long, the pressures start to fall as the bullet moves further down the bore and there is a tipping point where the bullet will actually start to slow down before it leaves the barrel. For the AR platform, you probably would not want to pack around a rifle with that long of a barrel through.
Conversely, shorter barrels have lower muzzle velocities… again on the AR/5.56 rifles; it seems that as barrels are reduced below 16 to 18 inches, the velocities start to fall off exponentially. This is highly dependant on ammo selection however, but is a good rule of thumb.
In the gas operated rifle, there is a port somewhere along the bore that bleeds off high pressure gas to operate the rifle – the location and details of this port is topic for another conversation, but on the AR they are normally located 7.5, 9.5 or 13 inches from the chamber, for a Carbine, Mid-Length and Rifle gas system respectively.
At the very end of the barrel, where the bullet leaves the muzzle, is the crown area… this end of the barrel must be as close to perfectly square as possible and free of nicks or burrs. If it is damaged or crooked, the gas pressure from the muzzle blast will bleed off on one side of the bullet first and disrupt its stability – not at all good for accuracy.
Makeing the barrel
We are about to enter into a strange land of voodoo, secrets, pride and ego... the debate over which of these techniques (or even how to apply the technique) has raged on for years -- and will outlive any of us. The truth is, all of the below listed techniques work just fine for barrels for the AR... so we will dash through this rather quickly, keep your arms fully inside the cart throughout the ride and do not stop to talk to strangers...
First thing is making the barrel blank, a round piece of metal with a rifled hole running through it… there are only a handful of companies in the U.S. that actually drill barrel blanks – from there, these blanks are sometimes finished in house, or sold to outside shops that cut the chamber, length and profile of the barrel. Drilling a hole deep (long) enough to make it through a 20 plus inch piece of stock is beyond the abilities of many shops – if fact, the equipment used for this task is called a “gun drill”, as outside of a firearms barrel, there is not a whole lot of need for a hole as small, long and straight as a rifle bore.
If you have ever drilled a hole several inches in any material, you know that the drill will tend to wonder off… so the shops that make the initial hole in the barrel blanks know their game well – however, the drill will walk a little, even under the best effort of the shop. This is actually one way that shops grade the blanks, by how far the hole is off center when it exits the blank… After drilling, the hole is reamed using basically the same process as the drilling.
After drilling and reaming, the blank is rifled and may be rough shaped. Whether the blank is shaped now or fully profiled by the shop that cuts and chambers it, the biggest concern is that the barrel is profiled to the new center bore – the hole that was drilled, not the physical center of the blank… this ensures that the bore is centered in the finished barrel, with the same amount of material all the way around. As obvious as that sounds, I have seen barrels that were simply chucked and turned down… you could actually look at the muzzle and see that the bore was not in the center of the barrel.
Now the blank is rifled… The rifling is most often a traditional rifling cut, one with lands and grooves. Current wisdom holds that a barrel should have an odd number of lands and groves; this prevent two lands, or high spots, from squeezing the bullet directly across (180 degrees) from each other – which would deform the bullet more than if a land is directly across from a groove.
Rifling is cut or formed in the barrel by one of a few common ways: the most common are Button Rifling, Broach Cut and Single Point – which of these is better is an ongoing debate that is well outside of the scope of what we are talking about here however.
Button Rifling - One of the most common methods, button rifling uses a hardened steel "button" that has the inverse rifling pattern in it... this button is pushed or pulled through the barrel blank and the rifling is essentially "ironed" into the insode of the bore. Button rifling is quick and can make a very nice finish -- the most noted pitfall to the method is that the button creates radial stress in the blank... the barrel must be properly stressed relieved before it is turned down, or the diameter of the bore can expand slightly.
Rifling Button
http://img382.imageshack.us/img382/5282/riflebuttonwm8.jpg
Broach Cut - A broach is a long rod with star shaped cutting teeth on it that match the rifling pattern... this is forced throug the barrel and the teeth shave away metal, cutting the rifling into the bore. Broach cutting does not stress the barrel as much as button rifling, but in some cases can leave a slightly rougher surface.
Rifling Broach
http://img255.imageshack.us/img255/1046/gangbroachxw7.jpg
Single Point - This is another cutting technique, in which each groove is cut one-at-a-time. Single point is obviously one of the more labor intensive methods, but can be very accurate.
Other methods of making the rifling include Hammer Forging and Chemical Etching... Chemical Etching is popular with pistol barrels, but is not used much on AR barrels -- the only application I have seen use it are barrels that are made from materials that are too hard to rifle using other techniques. Very simply, the rifling is etched in by a anode/cathode or acid etch process.
Hammer Forging - Hammer forged barrels are really very good barrels... a hard steel die, with the reverse rifling pattern in it, is placed in the drilled and reamed bore, then a machine that is best described as *big and noisey* literally hammers the barrel's outside surface, forcing the metal into the rifling pattern of the die.
Types of Rifling - Most AR rifles will have "conventional" rifling, there are some barrels that use what is usually called radiused or polygonal rifling as well.
The most common (conventional) rifling consist of the familar square cut lands and grooves, looking down the bore you can make out the sharp edged, flat top of the lands (high part) and the bottom of the grooves (low part).
Conventional Rifling
http://img331.imageshack.us/img331/9793/normalriflingch3.jpg
A more and more popular rifling that uses somewhat conventional lands and grooves, but with the radiused corners, is called Radius or Polygonal rifling. Looking down the bore, it looks rather like conventional riling... but under close inspection, one can see that the normally square corners are rounded and smooth.
Radius Rifling
http://img93.imageshack.us/img93/7343/radiusyc0.jpg
There is a variation of conventional rifling that is sometimes used... it involves canting or tapering one of the edges between the lands and grooves. Normally called ratchet, canted or C-Cut rifling, it is designed to reduce spin drift and is something that very few AR shooters will need to bother with -- but you may see it on high end barrels designed for superior accuracy.
Ratchet Rifling
http://img224.imageshack.us/img224/7087/ratchetriflemo4.jpg
Choices of Material
About 99% of barrels are made up of one a few different types of steel alloy -- of these, the choice is pretty much between Chrome-Moly steel and Stainless Steel.
Chrome-Moly - This name is used to describe a broad family of steels, the most common in use for the AR are 4140 and 4150... the 41xx indicates the AISI-SAE numbering for Chromium-Molybdenum alloy (ergo the name Chrome-Moly), these steels are alloyed with about 0.50% to 0.95% Chromium and 0.12% to 0.30% Molybdenum. The second part of the number indicates that percentage of Carbon (in tenths of a percent). Therefor 4140 is about 0.40% Carbon and 4150 about 0.50% -- the extra carbon in 4150 makes the steel a little tougher and it resists heat better... but it is harder to work and machine.
4150 is one of the "MilSpec" steels... another that is used in a lot of military barrels is CMV (Chrome-Moly-Vanadium) -- as the name implies, this is a chrome-moly steel (very close to 4150) with the addition of about 0.20% to 0.30% of Vanadium. CMV is reported to perform better under high heat.
Carbon steel barrels (chrome-moly) are the bread and butter of barrel materials for the AR, chrome-moly has made great barrels for many years and will continue to do so for many more. The only real downside to carbon steels is that they are prone to rust and corrosion problems.
Stainless Steels - We all know stainless steel, very high levels of chromium and nickel make these steels very resisitant to rust and corrosion, they are used in nearly every type of manufacturing around... the most common choices of stainless for a barrel are 410 and 416. There are a few "families" of stainless steel, but all of that is really academic to the discussion -- but 410 and 416 are Martensitic steels... simply they have lower percentages of chromium than other types of stainless steel.
The most common stainless in use for AR barrels is 416, this steel is plentiful, not expensive and machines nearly as well as the carbon steels -- 416 is rather strong, and has decent corrosion resistance; however, it is one of the least corrosion resistant of the stainless steels. Another choice barrel makers offer is 410, it is a little tougher and more corrosion resistant, but is harder to machine.
Grade 630 stainless, often called 17-4PH, is a great steel that offers excellent corrosion resistance and hardness. 630 is tough, but machinable, and tempers well. It is a precipitation hardened (PH) or age hardened steel and does not distort significantly during the hardening process; however, one consideration for the use of 630 is that the steel manufacturers do not recommend using it for applications that will expose it to cycling temperatures above 700 (f) -- In some cases this is not an issue, however in an AR that may see "heavy fire", 630 (17-4PH) is not a great choice.
So which is better, CM or SS? Again, this depends on what you want/need... a few things that seem to hold true about the two are that: CM barrels may take a little longer to break in and can be more prone to copper fouling; Due to the way the metal erodes in the throat area, the acuracy of CM barrels tends to degrade at a more steady rate, where SS tends to fall off quickly towards the end of its life; The throat area of CM barrels can work harden, this makes recutting the chamber and "bumping" the barrel back difficult -- but AR barrels can not be bumped back anyway (due to the gas port); SS barrels tend to be more prone to scratching but naturally is better at resisting corrosion (over an unplated CM bore); Many barrel manufacturers do not recommend using SS in sub-zero cold; Either SS or CM is capable of the same accuracy and will have about the same service life.
Bore Platings
Without question, the most common bore plating is Hard Chrome (HC)... it has been a requirement on military rifle barrles for decades. HC plated barrels are often refered to simply as "chrome" barrels and easy to confuse with the name Chrome-Moly; keep in mind that a Chrome-Moly steel barrel may or may not be chrome lined, the two are not mutually inclusive.
The purpose for HC plating is simply corrosion prevention, a chrome lined barrel is very easy to maintain, clean and is very resistant to rust and corrosion; however, this is not to say that they will not rust if neglected. The HC plating can be applied to just the bore, or the bore and the chamber both -- the complete bore and chamber plating is the most common, and the most desirable. The byproduct of HC plating is that it gives the bore a very hard surface and enhances barrel life.
HC plating has a bit of a bad reputation, many say that it degrades accuracy... the issue is that the plating is only as good as the shop that does it and it can do more harm than good, if not properly applied -- With today's hightech plating systems, it is quite possible to get a HC lined barrel that is every bit as accurate as non-plated barrel, but this may still be the exception, rather than the rule.
There are other "exotic" platings that have been used in AR barrels, some of these include: Nickel Boron (NiB), Melanite, Nye-Carb and others. Results have been mixed in real world use however and so far none of these has complete enamoured the firearms industry.
Chambering and Finishing - In Progress
Summation
So... after all of that, what barrel do you need? At the start, I talked about Batch v. Match and what the intended use of the rifle is. To this end, we talked about defining the priorities which are pretty much one of three groups... but first let me say this, there is no hard and fast rule about any of this -- if there was, there would not be the choices of barrels that are out there. Records are made and held with barrels that are made from chrome-moly and stainless, as well as rifled with any of the above mentioned techniques -- in far too many cases, the rifle is mechanically capable of accuracy far above what the shooter is; to steal a perfect line "it is the singer, not the song."
Anyway, first define what you need... keeping in mind the three things I mentioned at the start: (a) Service Life, (b) Corrosion Resistance (c) Fine Tuned Accuracy Potential, we will show these listed in priority.
Accuracy, I want the most accuracy potential possible (C,A,B) - Pick a premium barrel, but be ready to pay a premium price. You are most likely to end up with a stainless or non-plated chrome-moly barrel; expect radiused or ratchet rifling; a match grade chamber; and great attention to detail in the selection of the steel, machine work, lapping and finishing. This type of barrel is the most suseptable to damage from neglect or mistreatment and will have the shortest "service" life -- but keep in mind the original expectations. ======= THERE IS MORE TO FOLLOW =============
but I am lazy
==================================
Barrels of fun…
I wanted to put together a single reference that addressed the most common questions about barrels – naturally, this thing could go on for pages and pages, but I want to try to keep it reasonable and hopefully take care of the 90% or so of the more frequent questions.
First off, there are essentially two types of barrels (imho): Match Grade and Batch Grade. Which one you want is of course dependant on what you want to do… all too often the standard of a rifle is immediately judged by its group size – “What sorta groups are you getting with that?” is almost always one of the first questions asked of the owner of any new rifle.
Truth is, we have had little more to go by and the gun magazines have used group size as a standard of reporting for years – without that data, most articles would be rather boring:
“I spent three hours at the range with the new Twundelated Arms UbberShuter 150. I shot it mostly at mound of dirt about 35 yards away, and then shot a few old cans lying around, accuracy was great! I could even hit the tiny Red Bull cans with no problem.”
But gun rags don’t really print the bad news…
Nearly all the AR rifles in existence are capable of “practical accuracy” for a carbine, defense, battle, plinker type rifle – that is that nearly any barrel is capable of hitting a 8” target at the functional limit of the sighting system… even using my patented side leaning, semi-fetal, three and half point, one leg akimbo, you can’t see most of me shooting position.
On the other hand, the AR platform is well known for its accuracy potential and if the goal is to build as accurate of a rifle as possible, then by all means start there and shop for exactly that, the most accurate, repeatable barrel you can get/afford. But for most of us, the list of priorities is probably more like (a) Service Life, (b) Corrosion Resistance (c) Fine Tuned Accuracy Potential.
Hopefully, by taking a look at how a barrel is constructed, how it works and what some of the possible choices are, you will be at least a little more comfortable with defining what you need.
The functions a barrel performs
In most cases, the barrel and chamber of a rifle are one piece, so we will operate with that assumption throughout this. Working from the chamber end, the barrel obviously supports the cartridge in the chamber. Directly in front of the chamber is the throat, which supports the bullet as it starts its travel and guides it into the rifling – think of it as a funnel between the chamber and the bore. This is a very key part of the potential accuracy of the rifle... if the bullet is "set" in the bore slightly crooked, it will stay that way in flight and degrade accuracy -- a barrel really wears out in this throat area, even after the throat is "shot out", the bore and rifling just a few inches in front of the bore is usually still in great shape.
The rifling simply starts the bullet twisting or spinning… the Rate of Twist (RoT) is given as a ratio of how far the bullet must travel to spin on full revolution (360 degrees), this is normally written as 1:7 or 1/7 or One in Seven – simply meaning that the rifling makes one full turn in every seven inches of barrel.
Most of the work of the rifling is done within the first few inches… maximum chamber pressure occurs within this same length of barrel as well, and from there on the barrel simply contains the pressure and allows the bullet to accelerate faster and faster. Naturally, a longer barrel produces a faster muzzle velocity; however, there is a point of diminishing return. A barrel can be too long, the pressures start to fall as the bullet moves further down the bore and there is a tipping point where the bullet will actually start to slow down before it leaves the barrel. For the AR platform, you probably would not want to pack around a rifle with that long of a barrel through.
Conversely, shorter barrels have lower muzzle velocities… again on the AR/5.56 rifles; it seems that as barrels are reduced below 16 to 18 inches, the velocities start to fall off exponentially. This is highly dependant on ammo selection however, but is a good rule of thumb.
In the gas operated rifle, there is a port somewhere along the bore that bleeds off high pressure gas to operate the rifle – the location and details of this port is topic for another conversation, but on the AR they are normally located 7.5, 9.5 or 13 inches from the chamber, for a Carbine, Mid-Length and Rifle gas system respectively.
At the very end of the barrel, where the bullet leaves the muzzle, is the crown area… this end of the barrel must be as close to perfectly square as possible and free of nicks or burrs. If it is damaged or crooked, the gas pressure from the muzzle blast will bleed off on one side of the bullet first and disrupt its stability – not at all good for accuracy.
Makeing the barrel
We are about to enter into a strange land of voodoo, secrets, pride and ego... the debate over which of these techniques (or even how to apply the technique) has raged on for years -- and will outlive any of us. The truth is, all of the below listed techniques work just fine for barrels for the AR... so we will dash through this rather quickly, keep your arms fully inside the cart throughout the ride and do not stop to talk to strangers...
First thing is making the barrel blank, a round piece of metal with a rifled hole running through it… there are only a handful of companies in the U.S. that actually drill barrel blanks – from there, these blanks are sometimes finished in house, or sold to outside shops that cut the chamber, length and profile of the barrel. Drilling a hole deep (long) enough to make it through a 20 plus inch piece of stock is beyond the abilities of many shops – if fact, the equipment used for this task is called a “gun drill”, as outside of a firearms barrel, there is not a whole lot of need for a hole as small, long and straight as a rifle bore.
If you have ever drilled a hole several inches in any material, you know that the drill will tend to wonder off… so the shops that make the initial hole in the barrel blanks know their game well – however, the drill will walk a little, even under the best effort of the shop. This is actually one way that shops grade the blanks, by how far the hole is off center when it exits the blank… After drilling, the hole is reamed using basically the same process as the drilling.
After drilling and reaming, the blank is rifled and may be rough shaped. Whether the blank is shaped now or fully profiled by the shop that cuts and chambers it, the biggest concern is that the barrel is profiled to the new center bore – the hole that was drilled, not the physical center of the blank… this ensures that the bore is centered in the finished barrel, with the same amount of material all the way around. As obvious as that sounds, I have seen barrels that were simply chucked and turned down… you could actually look at the muzzle and see that the bore was not in the center of the barrel.
Now the blank is rifled… The rifling is most often a traditional rifling cut, one with lands and grooves. Current wisdom holds that a barrel should have an odd number of lands and groves; this prevent two lands, or high spots, from squeezing the bullet directly across (180 degrees) from each other – which would deform the bullet more than if a land is directly across from a groove.
Rifling is cut or formed in the barrel by one of a few common ways: the most common are Button Rifling, Broach Cut and Single Point – which of these is better is an ongoing debate that is well outside of the scope of what we are talking about here however.
Button Rifling - One of the most common methods, button rifling uses a hardened steel "button" that has the inverse rifling pattern in it... this button is pushed or pulled through the barrel blank and the rifling is essentially "ironed" into the insode of the bore. Button rifling is quick and can make a very nice finish -- the most noted pitfall to the method is that the button creates radial stress in the blank... the barrel must be properly stressed relieved before it is turned down, or the diameter of the bore can expand slightly.
Rifling Button
http://img382.imageshack.us/img382/5282/riflebuttonwm8.jpg
Broach Cut - A broach is a long rod with star shaped cutting teeth on it that match the rifling pattern... this is forced throug the barrel and the teeth shave away metal, cutting the rifling into the bore. Broach cutting does not stress the barrel as much as button rifling, but in some cases can leave a slightly rougher surface.
Rifling Broach
http://img255.imageshack.us/img255/1046/gangbroachxw7.jpg
Single Point - This is another cutting technique, in which each groove is cut one-at-a-time. Single point is obviously one of the more labor intensive methods, but can be very accurate.
Other methods of making the rifling include Hammer Forging and Chemical Etching... Chemical Etching is popular with pistol barrels, but is not used much on AR barrels -- the only application I have seen use it are barrels that are made from materials that are too hard to rifle using other techniques. Very simply, the rifling is etched in by a anode/cathode or acid etch process.
Hammer Forging - Hammer forged barrels are really very good barrels... a hard steel die, with the reverse rifling pattern in it, is placed in the drilled and reamed bore, then a machine that is best described as *big and noisey* literally hammers the barrel's outside surface, forcing the metal into the rifling pattern of the die.
Types of Rifling - Most AR rifles will have "conventional" rifling, there are some barrels that use what is usually called radiused or polygonal rifling as well.
The most common (conventional) rifling consist of the familar square cut lands and grooves, looking down the bore you can make out the sharp edged, flat top of the lands (high part) and the bottom of the grooves (low part).
Conventional Rifling
http://img331.imageshack.us/img331/9793/normalriflingch3.jpg
A more and more popular rifling that uses somewhat conventional lands and grooves, but with the radiused corners, is called Radius or Polygonal rifling. Looking down the bore, it looks rather like conventional riling... but under close inspection, one can see that the normally square corners are rounded and smooth.
Radius Rifling
http://img93.imageshack.us/img93/7343/radiusyc0.jpg
There is a variation of conventional rifling that is sometimes used... it involves canting or tapering one of the edges between the lands and grooves. Normally called ratchet, canted or C-Cut rifling, it is designed to reduce spin drift and is something that very few AR shooters will need to bother with -- but you may see it on high end barrels designed for superior accuracy.
Ratchet Rifling
http://img224.imageshack.us/img224/7087/ratchetriflemo4.jpg
Choices of Material
About 99% of barrels are made up of one a few different types of steel alloy -- of these, the choice is pretty much between Chrome-Moly steel and Stainless Steel.
Chrome-Moly - This name is used to describe a broad family of steels, the most common in use for the AR are 4140 and 4150... the 41xx indicates the AISI-SAE numbering for Chromium-Molybdenum alloy (ergo the name Chrome-Moly), these steels are alloyed with about 0.50% to 0.95% Chromium and 0.12% to 0.30% Molybdenum. The second part of the number indicates that percentage of Carbon (in tenths of a percent). Therefor 4140 is about 0.40% Carbon and 4150 about 0.50% -- the extra carbon in 4150 makes the steel a little tougher and it resists heat better... but it is harder to work and machine.
4150 is one of the "MilSpec" steels... another that is used in a lot of military barrels is CMV (Chrome-Moly-Vanadium) -- as the name implies, this is a chrome-moly steel (very close to 4150) with the addition of about 0.20% to 0.30% of Vanadium. CMV is reported to perform better under high heat.
Carbon steel barrels (chrome-moly) are the bread and butter of barrel materials for the AR, chrome-moly has made great barrels for many years and will continue to do so for many more. The only real downside to carbon steels is that they are prone to rust and corrosion problems.
Stainless Steels - We all know stainless steel, very high levels of chromium and nickel make these steels very resisitant to rust and corrosion, they are used in nearly every type of manufacturing around... the most common choices of stainless for a barrel are 410 and 416. There are a few "families" of stainless steel, but all of that is really academic to the discussion -- but 410 and 416 are Martensitic steels... simply they have lower percentages of chromium than other types of stainless steel.
The most common stainless in use for AR barrels is 416, this steel is plentiful, not expensive and machines nearly as well as the carbon steels -- 416 is rather strong, and has decent corrosion resistance; however, it is one of the least corrosion resistant of the stainless steels. Another choice barrel makers offer is 410, it is a little tougher and more corrosion resistant, but is harder to machine.
Grade 630 stainless, often called 17-4PH, is a great steel that offers excellent corrosion resistance and hardness. 630 is tough, but machinable, and tempers well. It is a precipitation hardened (PH) or age hardened steel and does not distort significantly during the hardening process; however, one consideration for the use of 630 is that the steel manufacturers do not recommend using it for applications that will expose it to cycling temperatures above 700 (f) -- In some cases this is not an issue, however in an AR that may see "heavy fire", 630 (17-4PH) is not a great choice.
So which is better, CM or SS? Again, this depends on what you want/need... a few things that seem to hold true about the two are that: CM barrels may take a little longer to break in and can be more prone to copper fouling; Due to the way the metal erodes in the throat area, the acuracy of CM barrels tends to degrade at a more steady rate, where SS tends to fall off quickly towards the end of its life; The throat area of CM barrels can work harden, this makes recutting the chamber and "bumping" the barrel back difficult -- but AR barrels can not be bumped back anyway (due to the gas port); SS barrels tend to be more prone to scratching but naturally is better at resisting corrosion (over an unplated CM bore); Many barrel manufacturers do not recommend using SS in sub-zero cold; Either SS or CM is capable of the same accuracy and will have about the same service life.
Bore Platings
Without question, the most common bore plating is Hard Chrome (HC)... it has been a requirement on military rifle barrles for decades. HC plated barrels are often refered to simply as "chrome" barrels and easy to confuse with the name Chrome-Moly; keep in mind that a Chrome-Moly steel barrel may or may not be chrome lined, the two are not mutually inclusive.
The purpose for HC plating is simply corrosion prevention, a chrome lined barrel is very easy to maintain, clean and is very resistant to rust and corrosion; however, this is not to say that they will not rust if neglected. The HC plating can be applied to just the bore, or the bore and the chamber both -- the complete bore and chamber plating is the most common, and the most desirable. The byproduct of HC plating is that it gives the bore a very hard surface and enhances barrel life.
HC plating has a bit of a bad reputation, many say that it degrades accuracy... the issue is that the plating is only as good as the shop that does it and it can do more harm than good, if not properly applied -- With today's hightech plating systems, it is quite possible to get a HC lined barrel that is every bit as accurate as non-plated barrel, but this may still be the exception, rather than the rule.
There are other "exotic" platings that have been used in AR barrels, some of these include: Nickel Boron (NiB), Melanite, Nye-Carb and others. Results have been mixed in real world use however and so far none of these has complete enamoured the firearms industry.
Chambering and Finishing - In Progress
Summation
So... after all of that, what barrel do you need? At the start, I talked about Batch v. Match and what the intended use of the rifle is. To this end, we talked about defining the priorities which are pretty much one of three groups... but first let me say this, there is no hard and fast rule about any of this -- if there was, there would not be the choices of barrels that are out there. Records are made and held with barrels that are made from chrome-moly and stainless, as well as rifled with any of the above mentioned techniques -- in far too many cases, the rifle is mechanically capable of accuracy far above what the shooter is; to steal a perfect line "it is the singer, not the song."
Anyway, first define what you need... keeping in mind the three things I mentioned at the start: (a) Service Life, (b) Corrosion Resistance (c) Fine Tuned Accuracy Potential, we will show these listed in priority.
Accuracy, I want the most accuracy potential possible (C,A,B) - Pick a premium barrel, but be ready to pay a premium price. You are most likely to end up with a stainless or non-plated chrome-moly barrel; expect radiused or ratchet rifling; a match grade chamber; and great attention to detail in the selection of the steel, machine work, lapping and finishing. This type of barrel is the most suseptable to damage from neglect or mistreatment and will have the shortest "service" life -- but keep in mind the original expectations. ======= THERE IS MORE TO FOLLOW =============
but I am lazy