Barrels The barrel is certainly one of the most important parts of a rifle if it is to shoot accurately. To the unitiated, a barrel is simply the tube sticking out the front of a rifle but there many factors that influence a barrels design and production. Barrel weight is perhaps the most obvious factor in choosing a barrel. The acceleration of a bullet down the barrel propelled by the propellant generates a tremendous amount of heat in the steel. If you are deer stalking or hunting, you may only take a couple of shots a day. In this situation, the barrel doesn't get a chance to overheat so can be made to a slender, light design - ideal for a rifle designed to be carried around. Conversely, a target or varmint shooting rifle may fire multiple shots with little time to cool down and so must be made to a much heavier profile to avoid overheating and even distorting which can cause inaccuracy. Barrel length is also an important factor when choosing a rifle. To a certain extent, there is an optimum length for a barrel. If a barrel is too short, velocity may be lost if there isn't enough time for all the propellant to burn. This can cause excessive muzzle blast as the hot unburnt propellant exits the barrel. An excessively long barrel can lead to lower velocities as the powder will be burned up before the bullet exits and the bullet won't reach optimum velocity. It is clear that there is an balance between barrel length and the burning rate of the chosen propellant. Making a barrel The blank All barrels start out as a bar of either chrome-molybdenum (generally 4140 type) or stainless steel (generally 416 type), usually around 1.25" in diameter. Both materials have their own advantages. Stainless is generally more rust resistant and tends to resist copper fouling better whilst cro-mo steel is more traditional and can be shot at far lower temperatures (stainless barrels becomes dangerously brittle below about -25oC). At this stage, the barrel is just a steel rod and UK ownership is not restricted at all. Drilling and rifling The next stage in the manufacturing process is to drill a hole down the centre of the blank. This is done via a highly specialised process called deep hole drilling. The cutting bit has a single edge which, when correctly ground, is self-centering. The barrel blank is centred in a lathe and spun at high speed. It is then moved onto the cutting tool which remains stationary. Oil is forced down a channel in the cutting tool to lubricate the cutting process and force out the swarf created. At this stage, the hole is drilled undersize to allow for the rifling process and reaming that follow. It is at this stage that most quality barrels are reamed. This is another cutting operation that leaves a very smooth finish in the bore. Again, it uses oil pumped into the reaming bit to flush out the chips created. There are 3 main methods of rifling a barrel. The simplest method is called button rifling. This involved the button, which a rugby ball shaped tool with helical grooves machined into it, being pulled or pushed through the barrel blank. As it passes through the blank, the rifling grooves are printed into the bore, elongating the new barrel and hardening the bore as it goes. At this stage, some stress has been caused in the steel and some manufacturers choose to heat treat at this point which removes these stresses from the steel. If allowed to remain, the barrel my distort slightly when it heats up in use. The second method is called cut rifling. This use a very sharp tool to individually cut each groove into the barrel. Each pass only removes 0.0001" of steel so it can take many hundreds of passes to create the rifling. This is a longer process than button rifling but it induces no stress in the steel so heat treating is generally not needed. It also allows for perfect concentricity in the bore which aids accuracy. There are no hard and fast rules but serious target and benchrest shooters often prefer to use cut rifled barrels. The final common method is called hammered rifling. The blank is drilled and the bore is honed to a very high standard. A mandrel, that forms an exact reverse copy of the desired rifling, is placed inside the drilled blank. It is then rotated between two enormous hammers that literally beat the steel onto the mandrel forming the internal rifling. It is such an extreme process that the blanks usually grow by about a third in length in the 3 minutes it takes to complete the process! Because the steel has been so worked, hammer rifling leaves a very smooth, work-hardened lining in the bore. The downside is that the process induces so much stress in the steel that the barrels must be heat treated. Hammer forging tends to be used for mass produced barrels as it is a quick process but the equipment needed is extremely costly. It is still completely legal to own a barrel in such a state in the UK. Profiling the barrel This stage in the process is determined by whoever designed the rifle the barrel is destined for. It involves choosing the 'shape' of the barrel. The barrel is centred in a CNC cutting machine and is turned to the profile desired. Benchrest shooters may want a simple parllel walled barrel, stalkers may want a traditional barrel where much of the thickness is left around the chamber area and the rest of the barrel quickly tapers down to the muzzle whilst a varmint barrel may have straight taper from the chamber end to the muzzle end. At this stage, fluting may be machined into the barrel to aid weight reduction, stiffness and cooling ability. It is still completely legal to own a barrel in such a state in the UK. Chambering the barrel This stage of the manufacture of a barrel involves the use of a calibre specific reamer, an extremely sharp machine tool that cuts the chamber (the cavity in the end of the barrel designed hold the cartridge). The barrel is centred in a lathe and the reamer is run into the end of a barrel, again with plenty of lubrication to wash away the swarf created. The depth of cut is checked constantly as it has to exactly correct if the barrel is to be safe when installed in the rifle. Mass produced rifles will have what is known as a 'factory' chamber based on standardised specifications but custom reamer makers cater to the custom rifle and wildcatting markets. At this stage in the UK, the barrel becomes a prohibited item and can only be held on a firearm certificate. Threading the barrel This is the final stage in the life of a new barrel. Depending on the rifle the barrel is to fitted to, the chamber end of the barrel is turned down to the desired diameter and threaded so that it can be screwed into the receiver of the rifle.