Of all the components involved in constructing a rifle, it is the barrel that intrigues the shooter. The barrel is the determining factor for a hit or miss, and a rifle that is shot out or inaccurate will usually be in the gunsmith's shop for a "re-barrel" job.
Little is understood about the actual barrelmaking process prior to the gunsmith's lathe. "How do you make the grooves?" is one of the most common questions answered by a barrelmaker. Contrary to popular belief, there is no real mystery to barrelmaking. But, in the same token, barrelmaking requires skilled technicians who fully understand the processes and machines, and has a lot of patience.
In this article I will explain the basic processes involved in turning a chunk of raw steel into a rifled barrel, concentrating on the cut-rifling process, as that is my field of expertise.
The quality of steel used is the first and foremost factor a good barrelmaker considers. Most high-production manufacturers use a Chrome Molybdenum (Cro-Moly) steel, and most target or similar-type makers use stainless steel.
Cro-Moly steel is usually designated as 4140, 4145, or 4150 type steel. Cro-Moly is relatively cheap and readily available, is easily machined, can be hardened by heat treatment, and is easily blacked. Most factory hunting rifles, as well as military rifles, are equipped with Cro-Moly barrels.
Stainless steel barrels are not true autensitic stainless; the better term would be "rust-resistant" steel. Stainless barrels are a 416 type, which is a martensitic class, and can be hardened by heat treatment. 416 stainless has a high Chrome content, and sulfur is added to obtain good machining qualities. It is a more expensive steel, and does not black well due to the chrome content, but the Teflon process has filled that void.
I am often asked how hard barrel steel should be. Will harder steel last longer than softer steel? Well, yes and no. There are two determining factors when selecting steel for barrels: tensile strength and impact strength.
Tensile strength is defined as the measured force required to break a one-inch cross-sectional area of steel by pulling at both ends. Basically it measures how much force it takes to pull a rod of steel apart. Barrel steels should be rated a factor of two over chamber pressures (for a good safety margin), which is usually a tensile strength over 100,000 lb/in^2.
Impact strength is the steel's ability to take a sharp blow without breaking. The tensile strength increases as the steel is hardened, but the steel also becomes more brittle (easier to fracture upon impact - or maybe from the explosion you create in the chamber when you pull the trigger!). There must be some elasticity in the steel, and it has been determined that a 26-32 Rc (Rockwell C scale) hardness is the appropriate, safe trade-off.
Production processes at the steel mill often leave residual stress in the steel. This stress must be relieved prior to machining, for if it is not, the barrel or bore will bend while you are removing material. This relieving process is achieved, either at the mill or in house, by cooking the steel in a high temperature oven and then allowing it to cool at a specific rate. Normally, barrel steel is double stress-relieved to ensure straight, stress-free barrels.
Cryogenics is an additional stress relief technique used by some barrelmakers. A cryoed barrel is frozen at -300 degrees Fahrenheit or more, and then brought to room temperature at a specific rate. Users of cryogenics claim that cryo-treating the steel after the heat treatment process creates a more homogenous microstructure in the material, ensuring a more stress-free barrel.
There are three steps in creating a cut rifled barrel: Drilling, Reaming, then Rifling (in that sequence).
A straight barrel begins with a straight hole. Drilling a straight hole in a rifle barrel is accomplished with special drilling machines, generally known as Gundrills. The drill (drill bit) used is a special application type specifically used for drilling deep holes, known as deep hole drills. Not your everyday twist drill.
The deep hole drill is basically a long, hollow tube with a V-groove formed on the outside. A hollow tungsten carbide tip is brazed to the end of the tube and then ground to specs, including a V-groove to match the tube. The face of the carbide tip is asymmetrical so that it will only cut on one side, and is also ground so that the forces acting on the tip keep the drill centered in the workpiece. The deep hole drill used is .008-.012" under the finished bore diameter, which leaves room for the reaming process.
After a bar of steel has been cut to length and both ends faced square, the "blank" is inserted into the Gundrill. The Gundrill rotates the barrel at 2000-4000 RPM while a stationary deep hole drill is fed into the material through a tight-fitting bushing. A steady rest rides along the drill to keep the tube rigid while drilling. Coolant is pumped through the hollow drill at 1000 psi via the tailstock to clear chips and cool the drill face. The oil and chips are forced back through the V-groove and into the chip tray, where the oil is strained and returned to the reservoir.
The drill is fed into the material at a rate of about 1"/minute, so a 28" blank will take approximately 30 minutes to drill.
A good bore reamer maker is a barrelmaker's best friend. The reamer is ground to finished bore diameter, and will bring the drilled hole to size as well as leave a good surface finish. The reaming process is what determines the finish of the tops of the lands.
The reamer is mounted on a long hollow tube to allow coolant to be pumped to the tool. The reamer is pulled through the barrel on a special reaming machine at around 200 RPM and a feed rate of 1"/minute.
After the reamer is pulled through, it is inspected for flaws in surface finish and air-gauged for dimensional uniformity. It is now ready to be rifled.
Cut rifling has been around for 500+ years. It was invented in Nuremberg circa 1492 and is still the optimum method of creating precise spiral grooves. As the barrel steel has improved, so has the cut rifling technique.
Rifling is produced using a cutter, sometimes called a "hook cutter," which scrapes metal out of the bore. The cutter rides in a hardened hollow steel cylinder, or "rifling head," which is ground just under the reamed bore diameter, and contains a lift mechanism and feed screw. The rifling head is mounted on long, straight hollow steel tubing so that coolant can be pumped to the cutter. The tubing is fitted with an adapter so that it can be attached to the machine.
The cutter is ground to fit in a slot milled in the rifling head, and is made specific to a caliber and twist rate. Cuttermaking requires great skill and a lot of patience. A well made rifling cutter produces some of the most uniform groove circles found today, and a well maintained cutter will leave a superior finish that requires minimal lapping.
The rifling is cut on rifling machines, which pull the cutter through the bore. Then the rod is pushed back through the bore until the barrel is indexed for the next groove to be cut. After a cycle has been completed (the first pass has been made on all grooves) the machine activates the lift mechanism, which increases the cutting depth. Additional cycles are completed until the grooves are cut to size. The average cut is a ten thousandth of an inch per groove per cycle, and it can take over an hour to fully cut a barrel.
The machines used for cut rifling are specialized for barrels only. There are two basic types of riflers: Sine bar and Hydraulic "B" riflers.
Sine bar riflers, usually Pratt & Whitney or Diamond Machine riflers, are single-spindle belt-driven machines that utilize a sine bar to change the rate of twist. The rifling slide is attached to the sine bar and the spindle rotates at the determined twist rate as the slide moves back and forth on the machine. Most of these machines are of WWI vintage, and are still very accurate.
A series of "new" riflers was introduced by Pratt & Whitney during WWII. Dubbed "B" riflers, they are hydraulic powered machines that have two spindles, making them capable of rifling two barrels at the same time. A leader bar, or lead screw, replaced the sine bar to determine the rate of twist. The rifling slide is attached to the machine, with a nut that follows the lead screw. The nut is held against the lead screw with the aid of a large clock spring to limit backlash and provide a uniform spiral.
The last production rifler was the Pratt & Whitney "B" rifler, as higher production rifling techniques emerged after WWII. The "B" rifler is the zenith of cut rifling machine technology to date.
The final stage for the barrel in raw form is lapping. Lapping the barrel ensures a dimensionally uniform bore, end to end, and provides a uniform, clean interior surface finish.
Good barrels are hand-lapped, and lead is the lap of choice among barrelmakers.
A lap is made by pushing a lapping rod, a cleaning rod without jag, up the bore 3-4 inches from the end. The barrel is then swung vertical and molten lead is poured into the bore. The lead freezes to the end of the rod and makes a cast of the rifling. This is the lap. Next, the lap is pushed out the bore, de-burred, and smeared with a lapping compound, which is grit suspended in a greasy, lubricating medium. The lap is pushed and pulled through the barrel until the barrelmaker feels an even resistance, which can take a few hundred strokes. The result is a uniform, polished finish that follows the direction of the cut groove spiral, eliminating much of the break-in process.
In this article, I can only account for the manufacturing attributes of the cut-rifled barrel. Although new developments in bore-grooving have erupted in the last 50 years, the age-old art of cut rifling has remained the staid force in accurate barrels. I hope I have given you at least a glimpse of what is endured in the process.