The Rifle Barrel
By Chuck Hawks
A rifle barrel is usually a long (16 to 30 inches), tapering, tube made of steel with fairly thick walls. It starts as a steel bar, which is then drilled and reamed to the "bore diameter" of the cartridge for which it is to be chambered. For example, the bore diameter is .300 inch for a typical .30 caliber cartridge like the .30-30, .308 Winchester, .30-06 or .300 Magnum. The rifling grooves are then cut into the bore. The distance across the grooves is called the "groove diameter." This varies depending on the caliber, but measures .308 inch for typical .30 caliber cartridges.
The outside of the barrel is tapered by turning it down to whatever contour is desired. This lightens the barrel by removing excess metal. Tapered barrels are greater in diameter at the chamber than at the muzzle, because when a cartridge is fired, the pressure is highest at the chamber end. Some barrels, called "bull barrels" are not tapered at all. These very heavy barrels, designed for extreme accuracy, are usually seen on target rifles.
Materials and barrel life
Rifle barrels are usually made from steel alloys called ordnance steel, nickel steel, chrome-molybdenum steel, or stainless steel, depending upon the requirements of the cartridge for which they are chambered. The higher the pressure and velocity of a cartridge (pressure and velocity usually go up together), the faster it will wear out a barrel.
To give a satisfactory service life, barrels for high velocity cartridges must me made from tougher and harder steel than barrels for lower pressure cartridges. A .270 Weatherby Magnum requires a tougher barrel than a .270 Winchester, which requires a tougher barrel than a .30-30, which requires a tougher barrel than a .22 LR. So manufacturers generally use more erosion resistant steel in barrels for high intensity cartridges, and barrels for magnum cartridges are often made of very hard stainless steel. Harder steels are more difficult to work and raise manufacturing costs (and thus retail prices) which, of course, is not desirable. The consumer has to rely on the manufacturer's judgment in this area. Fortunately, the major name brands like Browning, Marlin, Remington, Ruger, Savage, and Winchester all seem to know what they are doing in regard to choosing appropriate barrel material for their rifles and barrel life is longer today than ever before.
The idea that there is some special "break-in" or cleaning routine required for new rifle barrels has gotten a lot of attention recently. My advice is to follow the suggestions of the barrel manufacturer. If the maker of your rifle barrel prescribes a specific break-in routine, it is probably best to follow their instructions.
Otherwise, my approach has always been to sight-in a new rifle normally, without any special break-in. I try to avoid overheating a rifle barrel regardless of its age. At the range, I generally shoot three shot groups with centerfire rifles and let the barrel cool between groups. With very high velocity cartridges, such as the .257 Weatherby Magnum, I let the barrel cool down after every shot. Heat increases erosion and wear in any rifle barrel. I generally limit range sessions to one or two boxes of ammunition (20-40 rounds) and I clean the rifle normally after I return home. I have never found any special break-in routine to be necessary.
The purpose of rifling is to stabilize the bullet to increase its accuracy. This is called spin stabilizing, and works because of gyroscopic forces acting on the spinning bullet during flight. American rifle barrels traditionally spin bullets to the right, but British barrels spin them to the left.
There are various ways to rifle a barrel. The old way was to cut the rifling one groove at a time on a rifling machine. A more modern method is to pull a gang of broaches through the barrel, which cuts the all the grooves into the bore simultaneously. Another is to insert a very hard mandrel, which bears the reverse of the intended rifling pattern, into an oversize bore; then the outside of the barrel is "hammer forged" (or beaten) to impress the rifling into the bore. A fourth method is to pull a very hot rifling "button" through the bore, turning it as it progresses, which irons (melts) the rifling into the barrel. All of these methods are entirely satisfactory if done properly.
Rifles have been rifled with from two to eight fairly deep grooves. Another approach, called "Micro-Groove" rifling by the Marlin Company, uses many shallow grooves. Barrels for standard centerfire cartridges of .30 caliber or smaller are usually rifled with 4 grooves. Barrels for most cartridges above .30 caliber are rifled with 6 grooves, as are .22 rimfire barrels.
Rifling twist rates
The rate of twist, expressed as one turn in so many inches (i.e. 1 in 10"), is designed to stabilize the range of bullets normally used in a particular caliber. It takes less twist to stabilize a given bullet at high velocity than at low velocity. At the same velocity in the same caliber, longer (pointed) bullets require faster twist rates than shorter (round nose) bullets of the same weight and heavier bullets require faster twist rates than lighter bullets of the same shape. It is undesirable to spin a bullet a great deal faster than necessary, as this can degrade accuracy. A fast twist increases pressure and also the strain on the bullet jacket.
Fortunately, the rate of twist chosen by the rifle maker is usually appropriate for the intended cartridge. Anyone ordering a new barrel for a rifle will generally do well to specify the standard twist as supplied by the major rifle manufacturers for that caliber.
Once in a great while, though, a manufacturer makes a mistake. One such case involved the .244 Remington. When first introduced, barrels for this caliber were made with a 1-in-12 twist, because Remington anticipated that their new cartridge would be used primarily for varmint shooting. The 1 in 12 inch twist is ideal for best accuracy with varmint weight bullets (70-85 grains) in a high velocity .24 (6mm) caliber rifle. The heaviest spitzer bullet that a .244 with a 1 in 12 inch twist barrel could stabilize was 90 grains. The customers, however, also wanted to use their new .24 caliber rifles for hunting medium size big game, with 100 grain bullets. Needless to say, customers ignored the new .244 Rem. Remington soon saw the error of their ways, and changed the rifling of their .244 barrels to 1 turn in 9 inches, but the damage was done. Sales remained so slow that eventually Remington had to discontinue the .244. The following year they reintroduced the exact same cartridge as the 6mm Rem. and produced all 6mm rifle barrels with 1 in 9 inch twist barrels, which can stabilize all .24/6mm bullets.
The usual twist rates for some of the more popular rifle calibers are given below. (For a more comprehensive list, see "Common Rifle Barrel Twist Rates" on the Rifle Information Page.)
.22 Short = 1 in 24"
Not all rifle barrels of the same caliber have the same twist rate. A fellow ordering a custom rifle may have his own ideas about twist, as may the builder. For example, some .270 Win. barrels are rifled with a 1 in 12 inches twist, some .30-06 barrels are also rifled 1 turn in 12 inches, and some .300 Magnum barrels are rifled 1 turn in 14 inches. Usually these variations make no great difference. These slower twists may give slightly lower pressure, as well as very slightly better accuracy with the lighter bullets in each caliber. They will still stabilize the heavy bullets over practical hunting ranges. However, they might not be such a good choice, or quite as accurate, for shooting heavy bullets at extreme range (like 600-1000 yards).
For those who like to do it themselves, here is a rifle twist formula:
Barrel weight and diameter
The barrel is usually the heaviest part of a modern hunting rifle. Thus the outside diameter (and hence the weight) of a rifle barrel is an important factor contributing to the overall weight of the finished rifle. Rifle barrels are often described as heavy, medium, or light. This refers to the thickness of the steel surrounding the bore.
Removing excess metal from a rifle barrel is an effective way to lighten a rifle. (So, to a lesser extent, is shortening the length of the barrel.) A lighter rifle is easier to carry long distances and faster to get into action. Mountain rifles, for example, traditionally have short, light barrels. However, there is a price to be paid in performance.
Heavy barrels take longer to heat-up, thus maintaining good accuracy for more shots. They are also usually more consistent in the way they vibrate as a bullet passes down their length, which is very important for good accuracy. They resist outside bending forces, like changes in forearm pressure or pressure from a sling pulling the forearm against one side of the barrel, better than light barrels. They are less sensitive to how they are bedded in the stock. Their weight (within reason) makes it easier to hold the rifle steady. For all of these reasons, heavy barrels are generally more accurate than lighter barrels.
The medium weight barrel is an attempt to find some reasonable compromise between portability and performance. Standard weight hunting rifles usually have medium contour barrels, which have proven to be suitable for most purposes.
The arms makers try to select a barrel contour that complements the purpose for which the rifle is intended. To use the popular Remington Model 700 line of bolt action hunting rifles as an example, the general purpose 700 CDL is supplied with a medium weight barrel. The 700 Mountain Rifle has a lightweight barrel, while the 700 Sendero long range rifle has a heavy barrel.
Barrel length, accuracy and ballistics
It is worth mentioning that a longer barrel is not inherently more accurate than a short barrel. Intrinsic accuracy is a matter of quality, not length. However, a longer barrel is generally better in terms of practical accuracy, because a longer and therefore heavier barrel (within reason) is easier to hold relatively steady from unsupported positions; thus it is easier to shoot a long barreled rifle accurately.
The length of the rifle barrel has a direct influence on the velocity obtained from the cartridge for which it is chambered. Ballistically, for centerfire calibers, longer is usually better. However, for carrying, handling and maneuvering in close quarters (like thick brush or getting in and out of vehicles), shorter is usually better. Some sort of compromise must therefore be reached.
Very long 27-30 inch barrels are seldom seen these days on repeating hunting rifles, although they are still occasionally found on single shot hunting rifles and target rifles. The longest barrels usually seen on hunting rifles today are 26 inches in length.
26 inch barrels are usually found on rifles chambered for high velocity magnum cartridges. A long barrel is required to burn the large amounts of slow burning powder used in this type of cartridge. Unfortunately, most repeating rifles with 26 inch barrels balance too far forward; they are muzzle heavy and slow to swing. The long barrel seems to hang up on every limb and outcropping of rock in the area and a hunting rifle so equipped can be very awkward carry in steep terrain.
For this reason, many magnum rifles now come with 24 inch barrels, which sacrifice some of the magnum's velocity. 24 inches is about the minimum barrel length practical for most magnum cartridges. Cut a magnum's barrel down to 22 inches and the muzzle blast and flash become intimidating. Also, magnum cartridges, such as the popular .270, 7mm and .300 numbers, lose so much velocity in a 22 inch barrel that they show little ballistic advantage over standard calibers like the .270 Win., .280 Rem. and .30-06.
The typical barrel length for a repeating hunting rifle chambered for high intensity cartridges, such as the .243, .260, .270, 7mm-08, .308, or .30-06, is 22-24 inches. These are useful all-around barrel lengths for such cartridges. The highest velocity standard cartridges (.243, .25-06, .270 Win.), which achieve muzzle velocities around 3000 fps, are at their best in a 24 inch barrel and 24 inches is the SAAMI standard for almost all American centerfire rifle calibers. However, for cartridges such as the 6.5x55, 7x57mm, .308 Win., .30-06, .338 Federal, .35 Whelen and .350 Rem. Mag., which typically operate at 2500-2800 fps, the velocity loss in a 22 inch barrel is not extreme and a rifle with a barrel of this length usually balances and swings well.
Cartridges with smaller cases that operate at lower velocity, such as all .22 Rimfire cartridges, the .30-30, .32 Special, .35 Rem., .38-55 and similar numbers do reasonably well in 20 inch barrels. The very popular carbine versions of classic lever action rifles like the Winchester 94 and Marlin 336 usually come with 20 inch barrels. These short rifles ride well in a saddle scabbard, are easy to carry in rugged terrain and handle very fast in close quarters. Because they are lightweight rifles with under barrel tubular magazines, they balance well with a 20 inch barrel. Muzzle blast from these cartridges in a 20 inch barrel is less severe than from the larger, high intensity cartridges in a 22 inch barrel. Velocity loss is still considerable, however, usually 150-200 fps.
Combine a high intensity cartridge with a 20 inch barrel and the velocity drops sharply, while the blast becomes annoying. Exceptions might be the .300 Savage 180 grain, .356 Win. 200 grain, .358 Win. 200 grain and .375 Win. 200 grain loads, which are still effective woods cartridges when fired from carbine length barrels. These cartridges retain enough velocity in a 20 inch barrel for medium range shooting and the stubby barrel is less likely than longer tubes to get hung up on branches, overhangs and so forth. However, the muzzle blast is considerable.
To my mind it is hard to justify barrels shorter than 20 inches for any purpose. I have owned rifles with 18-19 inch barrels and in every case I wished that they had come with at least a 20 inch barrel. Very short barrels of standard contour (not bull barrels) tend to make the rifle muzzle light and unsteady to hold and swing. I like a rifle to balance between my hands, not toward the butt. Even .22 rimfire rifles balance better with 20-22 inch barrels, although in this instance the longer barrel has no ballistic advantage, since the .22 LR cartridge burns all of of its powder in about 16 inches. Very short barrels also increase the muzzle blast from high intensity cartridges to very annoying levels and the velocity loss is excessive.
Revolver cartridges adapted to rifle actions can, of course, be fired from barrels at the legal minimum length and still show large velocity gains compared to the same cartridges fired from typical 4" to 8" revolver barrels. Carbines chambered for the .357 Magnum, .44 Magnum and .45 Long Colt revolver cartridges are often supplied with 18" barrels and are fun to shoot.
Velocity loss (or gain)
It is worth noting that the velocity figures published in ammunition brochures and reloading manuals are sometimes taken in barrels different in length from those supplied on many rifles. I have seen various estimates of how much velocity is lost (or gained) when a barrel is not the same length as the test barrel in which a cartridge was chronographed. Here are some of them.
The 2001 Edition of the Shooter's Bible states, in the introduction to the Centerfire Rifle Ballistics section, "Barrel length affects velocity, and at various rates depending on the load. As a rule, figure 50 fps per inch of barrel, plus or minus, if your barrel is longer or shorter than 22 inches." However, they do not say what category of load to which this 50 fps average pertains.
Jack O'Connor wrote in The Rifle Book that, "The barrel shorter than standard has a velocity loss which averages about 25 foot-seconds for every inch cut off the barrel. Likewise, there is a velocity gain with a longer barrel." He went on to illustrate this using a .30-06 rifle shooting 180 grain bullets as an example, so his estimate was obviously for rifles in that general performance class.
Other authorities have tried to take into account the different velocity ranges within which modern cartridges operate. The Remington Catalog 2003 includes a "Centerfire Rifle Velocity Vs. Barrel Length" table that shows the following velocity changes for barrels shorter or longer than the test barrel length:
MV 2000-2500 fps, the approximate change in MV per 1" change in barrel length is 10 fps.
The 45th Edition of the Lyman Reloading Handbook also has a table showing Center Fire Rifle Velocity Vs. Barrel Length. Their figures apply to barrels between 20 and 26 inches in length and agree with the Remington figures. The Lyman table shows the following approximate velocity changes:
For rifles with muzzle velocities in the 1000-2000 fps range, the change in velocity for each 1" change in barrel length is 5 fps.
The 43rd edition of the Lyman reloading Handbook gave some concrete examples of velocity loss for specific calibers and loads. The Lyman technicians chronographed some high velocity cartridges in rifles with barrels ranging in length from 26 inches down to 22 inches with the following results:
The average loss for the .243 Win./100 grain bullet was 29 fps per inch.
For standard high intensity cartridges in the same test, the Lyman technicians chronographed the cartridges in barrel lengths ranging in length from 24 inches down to 20 inches with the following results:
The average loss for the .270 Win./130 grain bullet was 37 fps per inch.
After a bunch of disclaimers, the Lyman people concluded, "The rule of thumb is that high speed, high pressure cartridges shed more speed in short barrels than do the low speed, large bore types." It's funny, but that is what I had suspected all along!
Copyright 2001, 2012 by Chuck Hawks. All rights reserved.