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AR15 Barrel FAQ

SAA Barrel Information

Disclaimer:  The following information was done by independent research and may or may not be perfectly accurate and is subject to change and correction without notice. There is no warranty expressed or implied that the following is perfect. Your mileage my vary. If you find factual errors we would be happy to correct them. Opinions expressed within are those of the author and are not necessarily that of the owners and management of SAA.

Contents
              
Barrel Steel
              
Barrel Finishes
              
Barrel Rifling
              
Barrel Profiling
              
Barrel Lapping
              
Barrel Twist
              
Barrel Length
              
Barrel Crown
              
Barrel Choke
              
Barrel Accuracy
              
AR Barrels


Barrel Steels:

4140      Chromoly            chrome/molybdenum      chromium-molybdenum Normal hardness of 13. These barrels are normally heat treated after machining to increase hardness and reduce internal stresses present after machining.

4142      4142 is a chromium-molybdenum alloy steel that has been hardened to 27 to 32 Rockwell C and is ready to use while still being machinable and of exceedingly high strength.

4150      Chromium Molybdenum with some Vanadium added to make the steel tougher but not necessarily harder. Normal hardness of 13 is the same as 4140.

416R      Stainless Steel    416R is the standard SS used for barrels. The finish can vary. It can be polished and shiny or shot peened and have a matte or dull finish and even chemically colored to a dark finish.    28-36 Rockwell C hardness is normal for 416R.

Barrel Finishes

Mag Phospate or manganese (not “magnesium”) phosphate is coating on the outside of the barrel using a sprayed or immersed salt bath to achieve a somewhat porous finish that then more readily accepts and holds some sort of lubrication to help prevent corrosion. Mag Phosphate is also called Parkerizing. A mag-phosphate finish without lubrication can rust. A Mag Phosphate coating does nothing other than protect the metal (if properly lubricated).

CL           Chrome lining is usually put in 4140 barrels. The chrome coating is electro-mechanically bonded in the barrel and chamber. The outside of the barrel is the standard mag-phosphate finish. Chrome lining will make the barrel easier to clean, less susceptible to corrosion and increases the life span. Military and other full-auto shooters benefit from chrome lining as it protects the barrel from the high temperatures generated during rapid fire.  It is generally agreed that a chrome lined barrel is less accurate than an equivalent plain chromoly barrel though most shooters will not be able to tell the difference. We are talking about ½” MOA groups  in 800 yards usually. This is because of the inconsistent chrome finish that results in the barrel. By inconsistent we meant minute variations resulting in small changes in the bullet flight path. A perfect barrel that is chrome lined will no longer be perfect.

MH         Nitride (Melonite) is a chemical process performed at high temperature (975-1100 degrees) in a cyanate salt bath. The finish is on the inside and outside of the barrel. Glock uses this process and they call it Tenifer. This type of “case” hardening renders a molecular change in the outer thickness of .0004 to .001. Most barrels will see .0004 to .0008 in case hardness. Most people consider this coating superior to chrome lining as it does not change the dimensions and is not a coating in the barrel. Chro-moly steels and stainless steels can be nitrided. A nitride finish also increases lubricity. This coating is considered the best to date for barrels as it protects and hardens the metal without “adding” to the barrel like chrome lining does.

SS           Stainless steel barrels are normally not finished but can be finished in a variety of ways. They can a shiny polished or shot peened matte finish. Other chemical finishes can be applied and your stainless barrel could be black on the exterior.

Barrel Rifling

Barrels start their life as a billet of round stock. First they are drilled using a special drill bit called a gundrill. This drill bit has a one sided cutter and a hollow center which cutting oil is pumped down to lubricate and clear the cuttings. The drill works slowly at about one inch per minute and can take a half an hour to drill a barrel. The hole is then reamed to the finished dimension before the rifling is cut.

Barrels can be rifled by cut rifling, broaching, button rifling or hammer forged. Standard lands and grooves (Enfield) are typically squared off at the corners while polygonal rifling has smooth curves between the lands and grooves. Polygonal rifling will have a hexagon (six sided) or a octagon (eight sided) pattern. Octagonal polygonal rifling can seal larger calibers (.45) better as the bore more closely resembles a circle.

Cut rifling is a specialty and requires multiple passes of the cutter to complete the rifling. The method takes the most time, uses equipment that is expensive to buy and run but this method generally does a better job and does not induce stresses on the barrel.

Broaching is also done by passing a cutter through the barrel. The broach has multiple cutters that are progressively bigger. The broach cuts the rifling in one pass. This type of cutting rifling is still widely used.

Button rifling is the most common. The button is a tapered tungsten carbide former that is pulled through the barrel with a large hydraulic ram. The button is turned as it is pulled creating the required rifling twist rate. Some barrel makers use the button push method as well. This process introduces stresses in the barrel and the barrel should be heat treated to minimize those stresses before the barrel is profiled.

Hammer forged rifling is accomplished by extruding the barrel (cold or at temperature) over a mandrel and progressively hammering the barrel down to a certain diameter. The mandrel has the reverse rifling impression on it. As the barrel is hammered the inside is pressed against the mandrel creating the rifling. These machines are incredibly expensive and only the larger manufacturers use them. Some barrel makers buy the barrels with the rifling already done. Then they just have to chamber and profile. Some of the more sophisticated hammer forging machines that perform this can also chamber and profile the barrel in one process. Hammer forging usually creates radial stresses in the barrel that cannot be completely relieved by conventional methods. Precision shooters tend to avoid hammer forged barrels but any residual stresses have little effect on pistols and typical hunting rifles.

Barrel Profiling

The exterior of the barrel is usually turned down to the required profile (unless it was done during hammer forging). Since both button rifling and hammer forging induce stresses in the barrels the inside dimension can grow when the profile is cut down. The barrel is usually heat treated prior to profiling to prevent this.

There are many barrel profiles available; government (pencil), M4, SOCOM, Bull, HBAR (heavy barrel).

Barrel Lapping

Lead lapping is the process is inserting a rod into the barrel, pouring molten lead around the end and then drawing the solid lead plug through the barrel adding lapping compound to polish and hone the barrel. This also removes burrs and tends to even out any high spots in the barrel. The process can be done by hand but some shops have automated the process. The plug is usually pulled back and forth several hundred times. A lapped barrel usually shoots well from the get go while a non-lapped barrel could need several thousand rounds before it shoots to its best potential.

Barrel Twist

AR barrels are available in a variety of twists. The best twist for your barrel will depend on what cartridge (bullet) you are planning on shooting. A 1:9 twist (one complete 360° twist in 9 inches) is good all-around twist that can stabilize most bullets 55 to 79 grains. Generally you want more twist with shorter barrels and higher bullet weight. Also long bullets need more twist to stabilize.

Rules of thumb:   1:14   Older M16 military       40 - 55 grain
               1:12   usually longer barrels      40 - 62 grain
               1:10                       55 - 69 grain
               1:9                        55 - 79 grain
               1:8         16” carbine           75 - 80 grain
               1:7         pistol length         70 - 90 grain

Also     77 grain likes 1:7
       55 grain likes 1:9
       40 grain likes 1:12

The twist won’t guarantee accuracy but matching the twist with the bullet weight and type will get the bullet to stabilize better.

Barrel Length

Rifle barrels by (ATF) definition are from 16” (SBR = less than 16” barrel) to 26” and longer. Having a shorter barrel does not necessarily mean it will be less accurate. As long as the bullet has stabilized there is little difference in accuracy.

A long barrel will propel the bullet faster (all other things being equal), the difference between a 13.5” barrel and a 26” barrel is only 315 FPS for a .308 168 grain Hornady TAP round.

Barrel Crown

Barrels usually have a slight bevel cut in the muzzle end of the barrel. This is supposed to protect the end of the barrel if it struck again something. It can also be an indentation that recesses the end of the rifled hole at the muzzle. Many times the crowning operation is done by hand to prevent a burr from forming as the rifling transitions at the end of the barrel. Any burr or inconsistency at the muzzle exit can destabilize the bullet when it exits the muzzle. A 11 degree (match) crown is common.

Barrel Choke

This is a narrowing of the barrel diameter near the end of the barrel right before the bullet exits the barrel. Generally this considered an advantage for pellet and 22LR rifles. All other rifle calibers should generally not have any choke in their barrels.

Barrel Accuracy

What makes a barrel accurate is the consistency in the bore and groove dimensions and that they are uniform down the length of the barrel, that the twist rate is uniform and that the groove circle is concentric with the bore. The groove diameter must be same or less than the bullet diameter. It does not seem to matter how much less it is, one tenth of a thou' (.0001) or one thou' (.001) so long as it is less. The most accurate barrels are usually cut rifled or broached and lapped. The profile of the rifling (lands and grooves) seems to matter little in accuracy (BHW & HK would disagree). While the depth of the grooves will affect the bullet, the shallower grooves will distort the bullet less but those shallow groves won’t last as long in the barrel.

AR Rifle Barrels

You won’t see many (if any) AR barrels that are cut rifled. Broaching and button rifling is most common with larger manufacturers using hammer forging. Generally stainless steel match grade barrels are going to be the most accurate AR barrels. There is always a tradeoff between accuracy, longevity and ease of maintenance. Most of this matters little if you are shooting less than 300 yards. Most AR folks would be better served with a barrel that is chrome lined. It will last a long time, be easy to clean and be more than accurate enough. For the budget conscious any mag-phosphate in the proper twist will do. If you can afford to shoot out this barrel then you can definitely afford a new barrel.

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