The barrel is probably the most important factor in determining a rifle's inherent or potential accuracy. A basic knowledge of what makes a good barrel will help you understand what it takes to build a great shooting rifle. Understanding barrel characteristics will help you realize what your rifle can do and why, as well as diagnose any accuracy issues that arise. Additionally, you will understand why such things as “a free-floated barrel,” “don't rest your barrel on anything,” and a “fast enough twist” influence a shooter's ability to put bullets on target.
A nonshooter will look at a rifle barrel and see a round metal bar with a hole drilled down the center. They will probably realize where this is the part of the rifle where the bullet leaves the rifle. However, the science involved in building an accurate sniper rifle barrel is much more than simply drilling a hole down a metal bar.
The barrel's length, the type of steel or material used, the rifling, the twist rate, and weight all play very important roles in the accuracy of the barrel.
—Conan the Barbarian
Today most rifle barrels are constructed of some kind of steel alloy. Alloy steels contain elements that are added to modify the behavior of the steel during the heat-treatment portion of the manufacturing. Examples of these added elements include nickel, which increases hardness and tensile strength. Vanadium adds the ability to resist repeated stress. Tungsten adds air-hardening qualities; molybdenum improves high temperature, service wear toughness, and hardness; and chromium improves the stiffness and hardness. The most common steel in rifle barrel building is known as chromoly. This type is very high in strength if manufactured correctly—tensile strengths will vary from 98,000 pounds per square inch (psi) to 180,000 psi! It also contains chromium, manganese, molybdenum, phosphorus, sulfur, and silicon, but no nickel.
The other type of steel used in rifle barrels is stainless steel. Stainless costs more than chromoly. Stainless was originally used for knives and was patented in 1916, but the name was not trademarked, so now the “stainless” designation is used for any steel that resists rust, corrosion, and acids. There are over 50 different stainless steel compositions, and all have their unique properties. In order to get the stainless steel to have the required tensile strength for rifle barrels, the steel will under some conditions rust. Stainless is good at reducing barrel wear and fouling. One should note that the tensile strength of stainless is lower than properly processed 4140 chromoly steel.
In the manufacturing process the steel goes through a variety of heating and cooling stages to condition the steel to optimize its performance. A good steel barrel should be stress relieved during the manufacturing process. Stress relieving is a process of heating the barrel then cooling it in a controlled manner. This process allows the steel molecules to relax and align themselves, which helps reduce any imperfections in the trueness of the barrel.
A “gang broach” for rifling barrels. This method of cutting the rifling into barrels is effective, but making the broach and keeping it sharp are difficult machining operations.
Beautiful rifled shotgun barrel. Most shotguns are smooth bore.
Once the steel blank is properly processed the barrel must be drilled or reamed. Usually the reaming process takes several passes to bring the barrel to the correct diameter. The outside of the barrel has to be machined to the correct size and should be concentric to the bore of the barrel. The rifling process can be done in several different ways: hammer forging, button cut rifling, and single cut rifling.
Rock River Arms Tactical CAR A4 with a 16” Chrome Moly Barrel
In hammer forging, the process involves the reamed barrel being heated. A hard mandrel is inserted. The barrel is hammered into shape around the mandrel, which leaves the rifle grooves once it is removed. This is usually thought of as the least accurate method of rifling, but it is the cheapest, and several manufacturers employ this method in all of their rifles with good accuracy.
In the process of button cut rifling, a barrel blank is center bored and a hard metal button with the rifling ridges is pulled through the barrel, producing the rifle lands and grooves. Button cut rifling is more expensive than hammer forging and in most rifle building circles is thought to be more accurate.
Single cut, also known as hook rifling, involves the same center boring process, but the rifling is slowly cut with a single groove tool. Obviously, this process is very time consuming since it takes multiple passes for each groove. This process causes minimal stress to the barrel and if done properly is thought by many to result in a very accurate barrel. It is also the most expensive rifling process.
Diagram breaking down rifling grooves and lands. The caliber of the barrel is measured from the internal diameter starting with the top of the lands.
The length of a barrel is also an important consideration in building a sniper rifle. Some factors regarding barrel length are obvious. A long barrel is more cumbersome to handle in confined quarters, heavier to carry on long missions, and may make shooting in anything but a supported or prone position impractical. On the upside, longer barrels are generally more accurate than shorter barrels, especially at longer ranges. The improved accuracy with a longer barrel is secondary to the added weight—less recoil, less movement when the rifle is fired, and higher bullet velocities that result in a flatter trajectory. The higher velocities produced by longer barrels are generally true, which also implies that they are not always true.
When the powder in the cartridge is ignited, the resulting hot gases expand 800–1,300 times. Since the gas is contained in the brass case, there is only one way for the gas to escape—by pushing the bullet down the barrel. The barrel should have nothing in it except the bullet and the push of expanding gas. The burning of the cartridge powder should occur in the cartridge case, not in the barrel. Interestingly, despite the fact that shorter barrels produce less bullet velocity, the drop in speed and accuracy is small enough that it is of little concern to most hunters. Hunters that seldom shoot over 100 to 150 yards will not notice any discernable drop in accuracy or velocity in a 20- or 22-inch barrel compared to a 24–26-inch barrel. The maximum velocity for any barrel length will vary with the powder, bullet, cartridge, and other factors. After leaving the muzzle of the barrel, the bullet immediately begins to lose speed, hence the highest speed of the bullet is directly at the muzzle.
Old chart showing different rifling patterns for the Enfield rifle
50 cal.
The bore of the barrel should be as smooth as possible. The reaming and rifle cutting process will leave small irregularities that need to be removed by either smoothing the barrel with a lapping process and/or the break-in process. The break-in process varies with barrel manufacturers but usually consists of firing bullets in a certain number while cleaning the barrel between rounds. This process helps smooth out the small imperfections left by the reaming and rifling process.
Despite the strength of the steel, firing high velocity bullets down the barrel causes all barrels to eventually wear out. As the barrel wears, the accuracy will deteriorate. Barrel wear is first noticeable in the area immediately in front of the chamber.This is known as the throat of the barrel. Generally, higher velocity cartridges wear barrels faster than slower cartridges. Depending on the cartridge, the load used to propel the bullet can last from three thousand rounds in an ultrahigh velocity wildcat cartridge to ten thousand rounds in a .308 barrel. Keeping an accurate log of the rounds through a barrel is important because of this fact. As a barrel ages, a shooter will notice the group spread progressively getting wider.
Tactical Operations .308 X-Ray in an Accuracy International Aluminum Chassis
Rifling is the grooves cut into the bore of the rifle barrel. The rifling is cut in a helical or spiral pattern over nearly the entire length of the barrel after the chamber.
(Note: Most barrels have a small section of the barrel after the chamber that is known as the free bore or jump. This portion of the rifle does not have rifling. This jump length is very short and in some custom rifles can be eliminated, but more about this later.)
Muskets of old and shotguns do not have rifling. They are referred to as smooth bore. The addition of rifling to barrels was first found in the late 1400s and early 1500s. The rifling grooves and their helical (spiral) shape cause the bullet to turn on its axis as it travels down the barrel and then spin at a high rate of speed as it exits the barrel on the way to its target. This spinning motion greatly enhances the bullet's stability just as a perfect spiral motion enhances the accuracy and flight of a well-thrown football.
Internal to external bullet transition
The amount of twist is important to the accuracy of any bullet. Small differences in twist rate can have major accuracy influence. A good rifle barrel, with the good quality bullet can perform magical things with the right twist. There are many theories on what twist is best for different-sized bullets. Today, most people acknowledge that there is an ideal twist for each caliber, bullet, and barrel length, but when in doubt, it's better to go for the higher twist rate. For hunting and all around rifles, the twist should be chosen that will work satisfactorily with the biggest and heaviest bullets made for that rifle. For precision work, the twist should be chosen that will best match and meet the needs of the bullet chosen to do the work. Fortunately for all of us there are smart engineers and gunsmiths working this out for us, so we don't have to lose sleep at night wondering if our twist ratio is appropriate for our ammo.
Proper twist rate requires that we consider multiple factors. In general, to achieve the correct twist we must consider the following: a) a bullet that is longer in proportion to its diameter will require a faster twist to stabilize it; b) long-nosed bullets, such as hollow point boat tails, are good to reduce drag and have increased aerodynamics, but they are harder to stabilize; c) the density of a bullet is also a factor; longer and heavier bullets can be stabilized with a twist suitable for a lighter bullet if we can increase the velocity of the bullet; and d) a slow bullet requires a faster twist.
Caption TK
Other “twisted” facts about twist:
1. If the rifling lands extend to the end of the chamber, less gas escapes and pressure is increased. This permits a lower powder load for the same velocity.
2. Long, pointed bullets are harder to stabilize because any error in manufacturing will cause problems with the center of gravity of the bullet and affect its gyroscopic stability in flight.
3. Plain, short bullets work best with a slower twist.
4. If two bullets are fired simultaneously, with the same velocity and spin rate, the heavier long bullet will keep its speed of rotation better than the shorter, lighter bullet.
5. The best rifle twist is governed by the bullet's length, not its weight.
6. Barrel manufacturers have tolerances for the actual twist, i.e., a 1 in 12 twist may be a 1 in 11 or a 1 in 13. Custom barrels can be made more precisely, but at an increased cost.
7. Increasing twist for a faster rotation speed will not give a flatter trajectory.
8. Rechambered rifles commonly do not shoot as expected because the twist is not correct for the new caliber.
9. If the twist was not correct for a bullet, switching to a heavier, longer bullet will not improve accuracy because the twist is too slow.
As noted above, a tremendous amount of science goes into building an accurate sniper rifle. To be a top sniper does not require that one know or understand all the nuances of the sciences of metallurgy, propellant (powder) chemistry, or ballistics. However, the more shooters understand the science, the better they will understand why certain shooting facts make sense and in turn use these facts to become better. There is a tremendous amount of science involved in building a great bolt-action sniper rifle. The same can be said for what happens to the sniper's rifle once the trigger is squeezed.
The phenomena described below all happen in the microseconds following the firing pin striking the primer, so it's nearly impossible to really “see” these events.
Once the firing pin strikes the primer, the primer compound explodes and ignites the powder in the cartridge case. The resulting hot gas expands in all directions—not just against the bullet's base and the rear of the rifle chamber. The pressure causes a barrel expansion. The amount of expansion or swelling of the barrel depends on the type of barrel, its thickness, and the type of steel as well as the amount of powder ignited. The expansion is greatest at the breech end of the barrel. This expansion moves down the barrel and decreases as the pressure drops during the bullet's travel down the barrel. If you picture a sausage-shaped balloon being squeezed from one end to the other, the balloon expands just ahead of the pressure. The same happens to a rifle barrel. Experiments have shown that if you fire a bullet into a water tank, the bullet diameter will be larger as you cut the barrel shorter because the barrel expands more at the breach end. It makes sense that heavier, thicker barrels will not expand as much as thinner barrels.
Cross-section of some popular rounds and their compositions. Note the two rounds on the left are full metal jacket, whereas the other two are considered ‘open tip.’ There really isn't a lot of difference between the two M118 rounds, aerodynamically speaking.
Probably not too concerned about rifle twist this split second. SEAL sniper Matt Johnson gets some trigger time in.
In addition to expanding in all directions, the barrel is violently twisted by the bullet being forced to rotate by the barrel's rifling. The force or torque required to start the bullet rotating causes a distortion on the barrel that increases as the bullet moves from the action to the muzzle. This rotation is counter to the rotation of the bullet and precedes the bullet down the barrel. These two phenomena explain the importance of a “free floated barrel” and not allowing the barrel of the rifle to come into contact with anything as the rifle is fired. The barrel must be allowed to move in its natural way. These forces place tremendous stress on the steel, in addition to the heat generated by the friction of the bullet and the hot gasses in the barrel. With enough stress, cracking and eventually barrel failure can occur. Any weak points in the grain texture of the steel are the most common places for these cracks to occur, which highlights the importance of a barrel being “stress relieved” during the manufacturing process.
Different bullet shapes, whether on the tip or shank, are going to affect how a bullet flies and the terminal ballistics of the round.
Additionally, one may think that a rifle barrel is perfectly straight and that the strength of the steel over the length of the barrel would not allow the barrel to droop or sag. Wrong. Any material supported on only one end will sag slightly. As the bullet travels down the bore, the forces exerted on the barrel will try to straighten it. These forces create shock waves that cause vibrations or harmonics. All of these forces cause the barrel to have a whiplike motion. In most instances the barrel will not be perfectly straight as the bullet exits the muzzle. As you sight the rifle in, the optics will correct for this motion—as long as the motion remains the same. This again points out the need for the barrel to not be hindered from moving as it does naturally with every shot. Even improper use of a sling that is attached to the barrel or the stock coming into contact with the barrel can alter the bullet's point of impact. Additionally, this barrel motion will change with different ammunition, different powder loads, and different bullet weights, which again reinforces the saying that high accuracy is a result of high consistency.
Shadowgraph of a supersonic bullet. Note the bow shockwave coming off the front of the bullet, and the air turbulence in the bullet's wake.
In this close up of a different round, you'll notice the boattail design, which will provide better accuracy over distance than the previous bullet's flatter base.
A fair amount of technical intel, but the knowledge taken away is directly applicable in real-world shooting situations. Next time you throw a round off target while resting your barrel against a support, it will all become clear to you. Knowledge is power.
Mural from Iraq. Lovely lady sporting the ever present AK 47, a widely used and reliable weapon.
Co-author in Iraq posing before demo shot (see next photo).
Perhaps a bit more of an explosion than what happens in a rifle barrel. Blowing up some captured surface-to-air missles during the push to Baghdad.
Another way to place undue stress on a barrel… and everyone else around you!
MOA (I'd Like Half an MOA, Please)
An important and commonly used acronym in a sniper's vernacular is MOA (minute of angle). What is MOA? Why is a rifle's potential MOA important?
Again, science, or in this case math, rears its ugly head. A minute of angle is a measurement of angular width. A complete circle is divided into 360 degrees. Each degree is further divided into 60 minutes and each minute has 60 seconds. Therefore one minute is one sixtieth of a degree. If you take that angular measurement from your shooting position it should form a triangle. One line is your direct line of sight (LOS) to the target, the second is a line rising from the muzzle at the one sixtieth of a degree angle, and the third line is the distance from the LOS and the sloping line.
At 100 yards this distance is 1.0472 inches. In most shooting literature, 1 MOA is usually measured as 1 inch at 100 yards. The MOA increases linearly as you go further from the muzzle,i.e., 2 inches at 200 yards, 3 inches at 300 yards, and 10 inches at 1,000 yards.
Co-author taking a break during some stress course work at Marc Halcon's American Shooting Center outside Alpine, California.
