Machine Design

Keeping firearms On Target

Gun makers have refined their manufacturing techniques to the point they can almost guarantee that an affordable firearm is both accurate and reliable.

Ron Coburn
Chairman and CEO
Savage Arms
Westfield, Mass.


It takes practice and an accurate firearm to be able to hit a target. Key to a firearm's accuracy is the manufacturing techniques and skilled assembly that went into creating it.

A technician at Savage Arms tightens down the locknut, securing the rifle barrel to the receiver, which prevents the two parts from coming apart when the rifle is fired. He is using an extension bar with a built-in torque wrench that lets go at 70 ft-lb. This maintains consistent tension between the parts.

A myriad of components go into a bolt-action rifle and each plays a role in reliability and accuracy. (Please note that this diagram omits the trigger assembly.)

Firearm accuracy is often described by the average group diameter. It defines the smallest circle that can be drawn around a group of five or three shots fired at a target from a rifle or pistol. The left one is a typical group from an average factory rifle. It is about a 2-in. grouping. The spread of shots is probably due to shooter error or shifting winds. The right grouping is exceptionally tight, about 0.5 in. This would be considered exceptional accuracy for a factory rifle, especially if the rifle had not been tuned by a gunsmith.

Adjustable muzzle brakes let pressure inside the barrel bleed away as the bullet leaves the barrel, thus reducing recoil felt by the shooter, and, theoretically, improving accuracy. Savage Arms' muzzle brake bleeds 360° and reduces recoil by up to 30% with only a 5% loss of bullet energy.

AccuTrigger, an adjustable trigger device from Savage Arms, lets shooters set trigger pull anywhere from 1.5 to 6 lb without sacrificing safety or accuracy.

A full-sized bull barrel (left) sits beside a sporter barrel with the same caliber. The bull barrel is preferred by bench shooters and those striving for extreme accuracy. The sporter barrel is lighter and makes for a more easily carried rifle. But the thinner the barrel, the more that vibrations and harmonics from firing, and they hurt accuracy.

Accuracy is to a rifle what brakes are to a car — absolutely essential. Few devices are as useless and potentially dangerous as inaccurate, unreliable firearms. Fortunately, manufacturing today is so consistent that commercial rifles, shotguns, and handguns are generally accurate and reliable. The trick, though, is to manufacture them as efficiently as possible in a crowded and competitive marketplace. That means using the most automation and fewest man-hours, while building in as much accuracy as possible without compromising safety or reliability.

Making a rifle or any other firearm begins with choosing the right steel, machining operations, and assembly methods. It also requires extremely tight tolerances and components with dimensions that allow for mass production without rework or handfitting.

The average rifle has over 100 components, the most important being the barrel. Typical center-fire rifle barrels are made of carbon or stainless steel. Carbon steel is usually cold-rolled 4140 resulferized steel or stainless-steel 414 (free-machining grade). Both are supplied in the low 200BHN scale (Brinell hardness number), which usually means a special melt (and a special order) from the foundry. 200 BHN is considered medium-soft and suitable for machining. It is ductile and generates long stringers when turned, which helps keep tool-cutter faces clear and prevents galling on surfaces when parts are turned. In comparison, steel used for car bodies is usually 1214 grade, which is softer, letting car makers more easily shape it and maintain a uniform thickness.

To get 414 stainless to the 200 BHN level, it is twice annealed, which adds a 20% premium and bumps the price to $1.67/lb. Meanwhile, plain 4140 is $0.80/lb.

A typical .270 caliber rifle, often used for deer hunting, usually has a 24-in.-long barrel. The barrel starts off as a blank cut from a parent bar of steel that is delivered in 20-ft lengths with a 1.25-in. diameter. The blank is gun drilled, creating the bore that will control the flight of the bullet. The bore is drilled on long horizontal machines that hold the drill steady while rotating the barrel at 4,088 rpm.

Drilling faster than this speed dulls the drill face quicker, creates undersize holes, and traps shavings behind the drill. And although slower drill speeds increase drill life, they also increase manufacturing and cycle costs.

Typically, the machine drills 2 in. of bore every minute, which equates to a 12-min cycle for a 24-in. barrel. The .270 caliber bore is drilled at a nominal diameter of 0.26 in., plus 0.002 and minus 0.001 in., making the bore between 0.259 and 0.262 in. in diameter.

After drilling, the barrel is reamed at 1,580 rpm and 14.5 in./min to remove slight scores and cutter marks and increase the diameter to 0.270 in. ±.001 to accommodate the bullet. As in the case with gun drilling, reaming faster shortens tool life, while slower speeds stretches out production.

Shallow grooves spiraling up the barrel are added next. Raised portions between grooves are called lands. They grip the bullet as it flies out the barrel, giving the bullet some spin that stabilizes its fight and prevents it from tumbling. For our .270 rifle, lands measure 0.111-in. wide by 0.0035 in. ±.0001-in. deep.

Arms makers "iron" lands into bores using a carbide button, a process called rifling. Pressure on the button, which is either pushed or pulled as it rotates through the stationary barrel, also hardens and polishes the inside of the barrel. Radial forces generated by the button are usually 120,000 lbs or more, letting the button overcome the metal's plastic resistance.

Ballistic experts have determined that a .270 rifle bullet weighing 150 grains requires a 1-in-10 twist (one full rotation per 10 in. of barrel) to stabilize the bullet's flight path. Heavier or lighter bullets require faster or slower twists, respectively.

The gunmaker then chamber drills a pocket into one end of the barrel to accept the bullet casing. It is drilled in two levels, one for the shell casing body and the other to accept the shell nose and hold the bullet tightly.

To make a so-called sporter barrel, the outside of the barrel is tapered from the chamber to the muzzle. This reduces weight and improves the barrel's profile. More than 30% of the material is removed in typical sporter barrels. Though sporter barrels are lighter, they tend to "whip" or vibrate when shot, which degrades accuracy.

Another type of barrel, called a bull barrel, is intentionally not machined down, which reduces harmonics and improves accuracy. Bench shooters claim bull barrels are considerably more accurate. They say bull barrels improve groups at 100 yards by 50%, which means that bullets fired from a mounted and clamped gun will make a pattern 50% smaller than those fired from a nonbull or sporter barrel.

The receiver, the tube that holds the bolt and feeds bullets into the barrel, is then cut to length, turned, broached, and machined similarly to the barrel and using the same steel and tolerances as the barrel.

The barrel and receiver are connected, with some firearm companies pressing the two together and cross-pinning them into position. At Savage Arms, we thread both the receiver and barrel at the chamber-end and screw them together.

The assembly is then secured with a lock nut, and torqued to about 70 ft-lb to control headspacing. Headspace is the free space between the shell face and bolt face, and is measured in thousandths of an inch.

If the headspace is too large (0.006 or greater), a portion of the shell casing is left exposed and not constrained by the barrel or bolt face. Then, when the rifle is fired, the shell is prone to separating or exploding as the powder pressure expands the brass casing past its elasticity limit. A case rupture can release hot gasses back towards the shooter and damage components, including the barrel. Zero headspace is ideal because it provides the best accuracy and safety by completely supporting the shell casing and preventing rearward shunt and twisting.

Next, the trigger group and safety mechanism are assembled. Once the rifle action is complete, it can be proof tested or proofed.

In proof tests, a shooter fires a regular bullet loaded with enough additional powder to generate 130% higher pressures than commercial ammunition. This stresses components to ensure they will withstand the radial pressures created when a shell casing is prevented from moving backwards and the bullet is propelled forward.

If there's no evidence of fatigue or component failure, such as cracks in the stock, a bent recoil lug, (which is supposed to prevent the rifle from shifting back in the stock during recoil) or other twisted or broken components, the rifle is reassembled into a stock, which can be solid or laminated wood, or plastic.

Synthetic stock materials better resist weather, do not warp, and are easier to maintain. They also tolerate bruising and surface damage better than wood.

After final assembly, the firearm is tested with conventional ammunition, checked again for integrity of the safety lock, and then serialized with an indelible number that is maintained permanently on file at the factory. The serial number is usually placed on a face of the receiver that is in open view and not hidden.

When a rifle recoils, it puts upward pressure on the stock and pushes back towards the shooter at the same time. If a stock forend presses on the barrel, especially a sporter barrel, it changes the harmonics and deflects the bullet's flight path. Ideally the barrel should be allowed to free float.

To reduce recoil, some rifles have muzzle brakes attached to the end of the barrel. They let gasses escape sideways, which cuts down recoil, something that benefits smaller-framed shooters and improves accuracy.

Rifles are also designed to be lighter-weight so they can be shouldered easier. This also makes them easier to carry. Rifles can be made lighter by choice of stock material and using lightweight metals such as titanium.

However, as every engineer knows, for every action there is an equal and opposite reaction. Consequently, lighter firearms have more recoil, which can affect accuracy as shooters flinch in anticipation of the recoil slap. Firearm designers must therefore balance safety with function.

Accuracy also depends on how easy it is to pull the trigger, which releases and, in turn, releases the firing pin. The lighter the trigger pull, the easier to shoot the firearm and not jerk it out of alignment with the target. Only about 0.010 to 0.015 in. of metal holds the sear against the trigger release, so it can easily be tripped with minimal finger pressure.

Typically, a factory trigger is set at 5 to 6 lb, meaning it takes that much pull on the trigger to fire the rifle. This relatively high level is more a defense against potential product liability lawsuits that claim accidental discharge than any consideration for accuracy. Still, the lighter the trigger, the easier it is to accidentally "jaroff" the firearm if it is shocked or dropped.

Savage has considered that dichotomy as an opportunity to provide the best of both worlds: Adjustable triggers that let consumers reduce trigger pull and yet not permit accidental firing if the firearm is dropped or hit. Enter the AccuTrigger, a patented technology setting new standards in the firearms industry. It has a secondary lever floating inside the trigger that has to be depressed before the trigger releases the sear, which in turn releases the firing pin.

Once a shooter depresses AccuTrigger's secondary lever, his finger connects with the trig-ger and pulling it will fire the rifle. If the firearm is dropped, the sear may start to rotate, but the secondary lever prevents the sear falling away completely, effectively holding back the firing pin. The action would then need to be reset to shoot the firearm (resetting requires opening the bolt and closing it again, which pulls back the firing pin and recocks the mechanism).

Another peculiarity of firearms is that they are rarely obsolete. Savage is 110 years old and it is certainly possible that a rifle it made in 1895 is still serviceable and can be safely used today. Few other consumer products made that long ago are still in everyday use.

This kind of longevity requires a special emphasis on material, safety, and fitness for use. For example, we have designed and built equipment that cycles rifle actions and stresses components to a level far above what we consider normal use in an attempt to uncover critical fatigue elements that could cause premature failure. Those elements could be material choice, hardness, elasticity, abrasion and erosion or corrosion. Our standard tests stress components 200% above actual applied use. A test to determine how many cycles a component can endure may, for instance, call for cocking and firing the firearm 200,000 times to discover wear, fatigue, or fracturing.

Tests also look at components that are marginally sacrificed so that others will maintain their role and overall integrity over a reasonable life cycle. For instance, the sear is marginally harder than the trigger release, so the sear keeps it shape and sharp edge while the trigger is allowed to erode. (Typically, triggers and sears are made out of a metal with hardness in the 50 Rockwell C range so they can resist wear and erosion.)

There does not appear to be anything extraordinary evolving in the firearms industry, in terms of really new technology or alternate methods of propelling small objects long distances. The military has its rail guns, sonic weapons, and who knows what else in their arsenals. But for the general public, firearms are not only adequate, they are also affordable. (A midpriced shotgun sells for about $300, while a hunting rifle sells for $400).

In reality, firearms today are exceptional values, offering safety, accuracy, reliability, and minimal maintenance. In short, a firearm is a tool that has evolved to where it is often taken for granted and may be passed down within a family for several generations. Try doing that with your computer or iPod.

TAGS: Technologies
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