<span style="font-weight: bold">Found this article has some good info i believe
Author: Wood, Dennis A. from - 07 American gunsmith
Rebarreling choices: there are numerous decisions to be made before an action is rebarreled, including the type of steel, how it's rifled, rate of twist, and contour. Only after those decisions are made is it time to pick a barrel maker.(Back to Basics)
We're fortunate to have a multitude of barrel makers putting out excellent barrels for us to choose from. The choice we make when selecting an aftermarket barrel usually depends on the intended use along with the price for fitting and chambering the barrel to the action. Once we understand of...
.the process putting the rifling into a steel tube and which method suits our needs best, the choice for the intended application becomes much easier and a customer looking to us for advice will know that he came to the right place.
Barrel Steel
There are two types of barrel steel most commonly used these days for smokeless-powder firearms--chrome molybdenum (4140) and stainless steel (416). Both materials have their advantages and supporters of each material make some very good arguments.
Rifle-barrel blanks are usually rolled into shape during the forming process, which introduces a lot of "stress" into the barrel metal by compressing the molecular structure of the steel. To relieve the stress or "normalize" the barrel steel, a heat-treating process is used to take the barrel temperature close to the critical point and then let it normalize to ambient temperature.
Chrome molybdenum is an alloy steel containing chromium and molybdenum --not surprisingly--along with carbon, silicon, and manganese, and with trace amounts of phosphorus and sulfur. There are twelve grade designations as defined by the American Society for Testing and Materials ASTM) that comprise the 4100 series of chrome-moly chemical compositions that contain both chromium and molybdenum along with gradually higher amounts of carbon and manganese. The silicon, sulfur, and phosphorus contents are listed as the maximum amount contained, but remain the same throughout the eight grades that are considered most appropriate for barrel steel. When we get into the so-called nickel, chromium and molybdenum grades of steel, the grade designations reach the 4300 series. The accompanying chart breaks down the most common chromium-molybdenum steel compositions, with the 4140 type used most often for barrels that will receive a blued surface coating. As you can see, the differences in carbon and manganese are slight and overlap as they get higher from one type to the next. If the 4140 barrel steel chemical composition doesn't fall within the chemical range as listed, then it's not officially 4140 barrel steel.
The stainless steel used most for barrel blanks is the 416 series, and here we don't see many choices available from the steel mills. What is supposed to make the 416 stainless steel durable is the high content of chromium, although more manganese is added to the chemical mixture to make machining the barrel steel easier. Several barrel makers have told me they see no difference in the life span of a stainless-steel barrel compared to one made of chrome-moly.
Rifling Styles
Back when I was in my early teens, I always made a stop at the Trading Post in uptown Racine, Wisconsin, whenever I was near the store. This was a gun shop of sorts, and I liked to ogle all the guns I was unable to own at the time. But what I found really interesting was what went on in the store's lower level. Down in the basement was the shop where Barrett "Boots" Obermeyer was busy cutting rifling in barrel blanks. Obermeyer used "cut" rifling in his famous R5 rifled barrels having five lands and grooves, somewhat like the 1917 Enfield barrels. Obermeyer is no longer in Racine, as his barrel making is now done in Bristol, Wisconsin, and he's kept very busy making pressure and test barrels.
With this style of rifling, each groove is cut one at a time with multiple passes until the lands remain at a height of .005 inch or so. This is a tedious process, but there are advantages to this method of rifling a bore. Cutting the rifling into the bore does not produce any additional stress in the barrel steel, so this process can be done on a fully contoured barrel, even one that's been fluted. Once the rifling is complete, the bore is hand lapped with a long, pure-lead lap embedded with the correct grit to achieve a smooth internal finish. Cut-rifled barrels are said to be the most accurate barrels, and barrel life exceeds that of barrels using other methods to create rifling.
Other barrel makers who use the cut-rifling process include Krieger Barrels and E.R. Shaw. The machinery used to cut rifling into a barrel blank bore isn't made any longer, so what's in use today is what was in use when this type was a more common method of rifling.
Because of the time-consuming nature of the cut-rifling process and because the barrel maker pretty much has to make his own cutting tools, a process known as "button rifling" came on the scene. A carbide button, shaped sort of like a tiny football with grooves and lands ground into it, is pulled or pushed through the barrel from rear to front until the rifling is cold formed into the barrel.
As long as the carbide button stays on track, the rifling is consistent and a good rifle bore is produced. Button rifling displaces or swages the barrel steel outward and "work hardens" the internal surface being formed in the barrel. This takes much less time than cutting multiple passes into each of the grooves, one at a time. As the rifling is cold-formed into the bore, stress is once again induced into the barrel metal.
With button rifling, barrels are also lapped to even out any tight or hard spots in the barrel interior. The rifled bore is then measured with an air-gauge to determine uniformity. Barrels are then sorted, so you will see varying grades of button-rifled barrels. The higher-grade button-rifled barrels are those that are most consistent in bore size throughout their length, therefore the price of these match-grade barrels will be higher. Cut-rifled barrels do not have different grades, as the process produces consistent groove diameters and land widths each and every time with virtually no residual stress left in the barrel steel. Button-rifled barrels are usually stress relieved after the rifling process.
The hammer-forging process was developed by the Germans just prior to World War II to quickly "hammer out" barrels for the war effort. With this process, a mandrel with a mirror image of the rifling is inserted into a slightly oversized hole in the barrel blank and a series of hardened rollers hydraulically compress the outside diameter until the inside diameter forms around the mandrel. With this process, even the chamber can be formed with a specific mandrel for a particular caliber. These days, virtually all of the major firearms manufacturers use the hammer-forging process to install rifling in their production barrels.
This cold-forming process will impart a lot of stress into the barrel metal, which is why some hammer-forged factory barrels will move around a bit when they heat up from firing if the stress in the barrel isn't previously relieved somehow. With even the best procedures used to remove stress in barrels created by the hammer-forging process, some residual stress remains in the steel.
With any barrel blank, a hole must first be cut through the blank with a deep hole drill, leaving around .005 inch or so for a hole reamer to finish up the bore to provide a smooth internal surface. With the hammer-forging process, a very smooth internal finish is produced during the forming of the rifling because a honing step is added after the reaming process. It takes only three or four minutes to hammer forge rifling into a barrel, which is why firearms manufacturers use this process to make their barrels.
Barrel Break-In: Myth or Magic?
Much has been written about how a new barrel should be broken in, and opinions run the gamut. Some of the recommended procedures call for bore scrubbing after each shot fired for "X" number of rounds, and then bore cleaning after every 10 rounds or so. Obviously, some bore cleaning will be in order after some number of bullets have been sent downrange. Since bores have been lead-lapped on all barrels that are cut- or button-rifled, what would be the purpose of scrubbing the bore after each round during a so-called break-in period? Lead lapping has already removed or smoothed out the annular rings in the barrel caused by the deep-hole drilling and reaming process, and hammer-forged barrels have been honed to smoothness before the rifling was cold formed inside the barrel. So why do we need to break-in this new barrel?
The one area that may cause some concern is just ahead of the chamber. The throat or leade which was cut during the chambering process (which has nothing to do with the barrel maker) may have caused some rough spots in the form of roll-over burrs on the now tapered rifling in the throat. The chambering or throating reamer cuts when rotated in a clockwise direction, so it will leave roll-over burrs on the trailing edge of the cutting action which would be the left side of the lands. If there are any rough areas caused by the chambering process, tiny bits of metal will be shaved off the bullet as it passes over these areas and deposit themselves somewhere along the rifling. Following bullets passing over these small bits of material will pound them tighter onto the bore's internal surface. As a result, some bore cleaning at a more frequent interval may be necessary with a new barrel until the flame temperature and sandblast effect of the gun powder from 50 or so fired cartridges burn off any burrs that may be present in the throat area.
Another process that has received some press over the last few years involves "fire lapping" a barrel to smooth out rough spots. With this procedure, grit-impregnated bullets are fired down the barrel, causing a lapping effect on any rough or tight spots in the barrel. Rifling lands are thin with crisp corners made to imprint the bullet and create a spinning effect that guides the bullet accurately toward the target. Lapping off these crisp corners and reducing the height and/or width of the lands could do more harm than good if the fire-lapping process is overly done.
Fire-lapping kits are available for both centerfire and .22-rimfire calibers, with grit-impregnated bullets for centerfire rounds and coarse- to fine-grit ammunition for the .22 rimfire. Keep in mind that the lands in most .22-caliber rimfire barrels are only around .002 to .003 inch high, so how many of these grit coated bullets would it take to affect the leade and rifling in these guns? In my opinion, this would be a drastic measure for a very rough barrel that should probably be replaced anyway.
Fire-lapping has been reported to move the throat or leade forward up to 1/4 inch after the process was performed. This additional gap will cause the bullet to "jump" until it engages the rifling, which is not conducive to the best of accuracy, since the farther the bullet must jump to engage the rifling, the more of a chance there is that the bullet may tip slightly.
Short-Chambered Barrels
There are at least three barrel makers that I'm aware of who offer semi-finished barrels for popular rifle actions. These barrels are fully contoured, crowned, threaded, and chambered to within .050 inch or so of final fit. All that's needed is to remove the old barrel, install the replacement barrel, and then run a chambering reamer through the action and finish cutting the chamber until headspace is correct. Sounds relatively simple.
When customers approach me when they are considering having a rifle action rebarreled, the first question I ask is, "What are you hoping to gain over what you have now?" There are three common answers: "I want a different caliber"; "to get better accuracy"; or "my barrel is shot out and I need a new one." If the customer says he thinks the barrel has been shot out, we discuss how much shooting he's done over the time he's owned the rifle. Rarely does a deer hunter who sights his rifle in once a year and then maybe fires a couple of rounds during hunting season put enough rounds through his barrel to shoot it out. More often than not, a thorough cleaning of the barrel will get it back to the accuracy he was once accustomed to. On the other hand, a varmint hunter, especially one whose target is prairie dogs, can burn a lot of ammo during his trips to a prairie dog town, and may shoot the throat out of a barrel rather quickly.
Getting better accuracy or changing the caliber of his rifle usually involves more questions and discussion. Top-notch accuracy expectations will require that the front face of the action be trued up perpendicular to the bore centerline, and the engaging faces of the locking lugs be lapped to full contact with the mating surfaces in the action. The bolt face might need to be squared up so it too is flat and will direct a cartridge straight into the chamber. Just changing calibers may not require the above expected degree of accuracy, so in that case the short-chambered barrel may deliver all the accuracy the owner is expecting to see and willing to pay for.
The main benefit of actually fitting a new barrel is having the barrel-shank threads turned to fit the receiver threads as precisely as possible. Firearms manufacturers allow themselves some degree of manufacturing tolerances that fall within safe limits for the firearm's function. When re-barreling older firearms, especially those manufactured before the advent of computer-controlled machinery, you must consider how these barrels and receivers were threaded and adjust the fitting of a new barrel to that specific action. This means that the threads will need to be cut into the new barrel shank to accommodate a specific fit. This can't always be done with a short-chambered barrel, because "you get what you get," and the end result may not be any better than the accuracy achieved by the original barrel with the original chambering. When an action is "custom" chambered, the throat can be cut to a specific length to accommodate a certain length/weight bullet, whereas most factory chambers are cut to accept the longest bullet for that particular chambering.
Conclusions
Choosing an aftermarket barrel requires a methodical approach to the selection of barrel steel and type of rifling, as well as close collaboration with the gunsmith chosen to assemble this shooting package. Only after these decisions have been made can you help the customer to choose a good barrel that will suit his needs as well as his budget.</span>
Author: Wood, Dennis A. from - 07 American gunsmith
Rebarreling choices: there are numerous decisions to be made before an action is rebarreled, including the type of steel, how it's rifled, rate of twist, and contour. Only after those decisions are made is it time to pick a barrel maker.(Back to Basics)
We're fortunate to have a multitude of barrel makers putting out excellent barrels for us to choose from. The choice we make when selecting an aftermarket barrel usually depends on the intended use along with the price for fitting and chambering the barrel to the action. Once we understand of...
.the process putting the rifling into a steel tube and which method suits our needs best, the choice for the intended application becomes much easier and a customer looking to us for advice will know that he came to the right place.
Barrel Steel
There are two types of barrel steel most commonly used these days for smokeless-powder firearms--chrome molybdenum (4140) and stainless steel (416). Both materials have their advantages and supporters of each material make some very good arguments.
Rifle-barrel blanks are usually rolled into shape during the forming process, which introduces a lot of "stress" into the barrel metal by compressing the molecular structure of the steel. To relieve the stress or "normalize" the barrel steel, a heat-treating process is used to take the barrel temperature close to the critical point and then let it normalize to ambient temperature.
Chrome molybdenum is an alloy steel containing chromium and molybdenum --not surprisingly--along with carbon, silicon, and manganese, and with trace amounts of phosphorus and sulfur. There are twelve grade designations as defined by the American Society for Testing and Materials ASTM) that comprise the 4100 series of chrome-moly chemical compositions that contain both chromium and molybdenum along with gradually higher amounts of carbon and manganese. The silicon, sulfur, and phosphorus contents are listed as the maximum amount contained, but remain the same throughout the eight grades that are considered most appropriate for barrel steel. When we get into the so-called nickel, chromium and molybdenum grades of steel, the grade designations reach the 4300 series. The accompanying chart breaks down the most common chromium-molybdenum steel compositions, with the 4140 type used most often for barrels that will receive a blued surface coating. As you can see, the differences in carbon and manganese are slight and overlap as they get higher from one type to the next. If the 4140 barrel steel chemical composition doesn't fall within the chemical range as listed, then it's not officially 4140 barrel steel.
The stainless steel used most for barrel blanks is the 416 series, and here we don't see many choices available from the steel mills. What is supposed to make the 416 stainless steel durable is the high content of chromium, although more manganese is added to the chemical mixture to make machining the barrel steel easier. Several barrel makers have told me they see no difference in the life span of a stainless-steel barrel compared to one made of chrome-moly.
Rifling Styles
Back when I was in my early teens, I always made a stop at the Trading Post in uptown Racine, Wisconsin, whenever I was near the store. This was a gun shop of sorts, and I liked to ogle all the guns I was unable to own at the time. But what I found really interesting was what went on in the store's lower level. Down in the basement was the shop where Barrett "Boots" Obermeyer was busy cutting rifling in barrel blanks. Obermeyer used "cut" rifling in his famous R5 rifled barrels having five lands and grooves, somewhat like the 1917 Enfield barrels. Obermeyer is no longer in Racine, as his barrel making is now done in Bristol, Wisconsin, and he's kept very busy making pressure and test barrels.
With this style of rifling, each groove is cut one at a time with multiple passes until the lands remain at a height of .005 inch or so. This is a tedious process, but there are advantages to this method of rifling a bore. Cutting the rifling into the bore does not produce any additional stress in the barrel steel, so this process can be done on a fully contoured barrel, even one that's been fluted. Once the rifling is complete, the bore is hand lapped with a long, pure-lead lap embedded with the correct grit to achieve a smooth internal finish. Cut-rifled barrels are said to be the most accurate barrels, and barrel life exceeds that of barrels using other methods to create rifling.
Other barrel makers who use the cut-rifling process include Krieger Barrels and E.R. Shaw. The machinery used to cut rifling into a barrel blank bore isn't made any longer, so what's in use today is what was in use when this type was a more common method of rifling.
Because of the time-consuming nature of the cut-rifling process and because the barrel maker pretty much has to make his own cutting tools, a process known as "button rifling" came on the scene. A carbide button, shaped sort of like a tiny football with grooves and lands ground into it, is pulled or pushed through the barrel from rear to front until the rifling is cold formed into the barrel.
As long as the carbide button stays on track, the rifling is consistent and a good rifle bore is produced. Button rifling displaces or swages the barrel steel outward and "work hardens" the internal surface being formed in the barrel. This takes much less time than cutting multiple passes into each of the grooves, one at a time. As the rifling is cold-formed into the bore, stress is once again induced into the barrel metal.
With button rifling, barrels are also lapped to even out any tight or hard spots in the barrel interior. The rifled bore is then measured with an air-gauge to determine uniformity. Barrels are then sorted, so you will see varying grades of button-rifled barrels. The higher-grade button-rifled barrels are those that are most consistent in bore size throughout their length, therefore the price of these match-grade barrels will be higher. Cut-rifled barrels do not have different grades, as the process produces consistent groove diameters and land widths each and every time with virtually no residual stress left in the barrel steel. Button-rifled barrels are usually stress relieved after the rifling process.
The hammer-forging process was developed by the Germans just prior to World War II to quickly "hammer out" barrels for the war effort. With this process, a mandrel with a mirror image of the rifling is inserted into a slightly oversized hole in the barrel blank and a series of hardened rollers hydraulically compress the outside diameter until the inside diameter forms around the mandrel. With this process, even the chamber can be formed with a specific mandrel for a particular caliber. These days, virtually all of the major firearms manufacturers use the hammer-forging process to install rifling in their production barrels.
This cold-forming process will impart a lot of stress into the barrel metal, which is why some hammer-forged factory barrels will move around a bit when they heat up from firing if the stress in the barrel isn't previously relieved somehow. With even the best procedures used to remove stress in barrels created by the hammer-forging process, some residual stress remains in the steel.
With any barrel blank, a hole must first be cut through the blank with a deep hole drill, leaving around .005 inch or so for a hole reamer to finish up the bore to provide a smooth internal surface. With the hammer-forging process, a very smooth internal finish is produced during the forming of the rifling because a honing step is added after the reaming process. It takes only three or four minutes to hammer forge rifling into a barrel, which is why firearms manufacturers use this process to make their barrels.
Barrel Break-In: Myth or Magic?
Much has been written about how a new barrel should be broken in, and opinions run the gamut. Some of the recommended procedures call for bore scrubbing after each shot fired for "X" number of rounds, and then bore cleaning after every 10 rounds or so. Obviously, some bore cleaning will be in order after some number of bullets have been sent downrange. Since bores have been lead-lapped on all barrels that are cut- or button-rifled, what would be the purpose of scrubbing the bore after each round during a so-called break-in period? Lead lapping has already removed or smoothed out the annular rings in the barrel caused by the deep-hole drilling and reaming process, and hammer-forged barrels have been honed to smoothness before the rifling was cold formed inside the barrel. So why do we need to break-in this new barrel?
The one area that may cause some concern is just ahead of the chamber. The throat or leade which was cut during the chambering process (which has nothing to do with the barrel maker) may have caused some rough spots in the form of roll-over burrs on the now tapered rifling in the throat. The chambering or throating reamer cuts when rotated in a clockwise direction, so it will leave roll-over burrs on the trailing edge of the cutting action which would be the left side of the lands. If there are any rough areas caused by the chambering process, tiny bits of metal will be shaved off the bullet as it passes over these areas and deposit themselves somewhere along the rifling. Following bullets passing over these small bits of material will pound them tighter onto the bore's internal surface. As a result, some bore cleaning at a more frequent interval may be necessary with a new barrel until the flame temperature and sandblast effect of the gun powder from 50 or so fired cartridges burn off any burrs that may be present in the throat area.
Another process that has received some press over the last few years involves "fire lapping" a barrel to smooth out rough spots. With this procedure, grit-impregnated bullets are fired down the barrel, causing a lapping effect on any rough or tight spots in the barrel. Rifling lands are thin with crisp corners made to imprint the bullet and create a spinning effect that guides the bullet accurately toward the target. Lapping off these crisp corners and reducing the height and/or width of the lands could do more harm than good if the fire-lapping process is overly done.
Fire-lapping kits are available for both centerfire and .22-rimfire calibers, with grit-impregnated bullets for centerfire rounds and coarse- to fine-grit ammunition for the .22 rimfire. Keep in mind that the lands in most .22-caliber rimfire barrels are only around .002 to .003 inch high, so how many of these grit coated bullets would it take to affect the leade and rifling in these guns? In my opinion, this would be a drastic measure for a very rough barrel that should probably be replaced anyway.
Fire-lapping has been reported to move the throat or leade forward up to 1/4 inch after the process was performed. This additional gap will cause the bullet to "jump" until it engages the rifling, which is not conducive to the best of accuracy, since the farther the bullet must jump to engage the rifling, the more of a chance there is that the bullet may tip slightly.
Short-Chambered Barrels
There are at least three barrel makers that I'm aware of who offer semi-finished barrels for popular rifle actions. These barrels are fully contoured, crowned, threaded, and chambered to within .050 inch or so of final fit. All that's needed is to remove the old barrel, install the replacement barrel, and then run a chambering reamer through the action and finish cutting the chamber until headspace is correct. Sounds relatively simple.
When customers approach me when they are considering having a rifle action rebarreled, the first question I ask is, "What are you hoping to gain over what you have now?" There are three common answers: "I want a different caliber"; "to get better accuracy"; or "my barrel is shot out and I need a new one." If the customer says he thinks the barrel has been shot out, we discuss how much shooting he's done over the time he's owned the rifle. Rarely does a deer hunter who sights his rifle in once a year and then maybe fires a couple of rounds during hunting season put enough rounds through his barrel to shoot it out. More often than not, a thorough cleaning of the barrel will get it back to the accuracy he was once accustomed to. On the other hand, a varmint hunter, especially one whose target is prairie dogs, can burn a lot of ammo during his trips to a prairie dog town, and may shoot the throat out of a barrel rather quickly.
Getting better accuracy or changing the caliber of his rifle usually involves more questions and discussion. Top-notch accuracy expectations will require that the front face of the action be trued up perpendicular to the bore centerline, and the engaging faces of the locking lugs be lapped to full contact with the mating surfaces in the action. The bolt face might need to be squared up so it too is flat and will direct a cartridge straight into the chamber. Just changing calibers may not require the above expected degree of accuracy, so in that case the short-chambered barrel may deliver all the accuracy the owner is expecting to see and willing to pay for.
The main benefit of actually fitting a new barrel is having the barrel-shank threads turned to fit the receiver threads as precisely as possible. Firearms manufacturers allow themselves some degree of manufacturing tolerances that fall within safe limits for the firearm's function. When re-barreling older firearms, especially those manufactured before the advent of computer-controlled machinery, you must consider how these barrels and receivers were threaded and adjust the fitting of a new barrel to that specific action. This means that the threads will need to be cut into the new barrel shank to accommodate a specific fit. This can't always be done with a short-chambered barrel, because "you get what you get," and the end result may not be any better than the accuracy achieved by the original barrel with the original chambering. When an action is "custom" chambered, the throat can be cut to a specific length to accommodate a certain length/weight bullet, whereas most factory chambers are cut to accept the longest bullet for that particular chambering.
Conclusions
Choosing an aftermarket barrel requires a methodical approach to the selection of barrel steel and type of rifling, as well as close collaboration with the gunsmith chosen to assemble this shooting package. Only after these decisions have been made can you help the customer to choose a good barrel that will suit his needs as well as his budget.</span>
