The first thing a rookie forensic firearms examiner will learn is how firearm barrels are created. To understand and perform laboratory bullet comparisons and give fact-based testimony that can affect someone’s life, every examiner must be intimate with the rifled gun barrel.
To make a gun barrel, you need a steel rod with a hole down the middle. For a couple of centuries, that hole has been drilled. There are four ways to turn that drill hole into a rifled barrel; two ways cut grooves into the steel, and two ways swage them under pressure.
The earliest rifling was simple and intended to grip the cloth patch around a round lead ball, not to engrave the lead. This also eased the task of loading. Given the soft steels of the time, it was relatively easy to produce the first type of rifling: single-point rifling.
A hook scraper cuts one groove at a time. Protruding from the side of and near one end of a long rod, it is adjustable for “bite,” shaving off tiny amounts of steel when pulled through the bore. After each pass, a wedge between the scraper and the mounting rod is adjusted to slightly raise the scraper tip so it removes a little more metal from the same groove on the next pass. This repeats for each groove until the desired depth is achieved. Then you rotate the work piece and start another groove.
To create twist, most old rifling machines had a long wooden drum with a gently spiraling groove cut into the outer surface. A pin riding in that groove rotated the barrel while the scraper rod was pulled through.
This produced excellent barrels, but what worked for a gunsmith making a dozen Pennsylvania rifles a year was not scalable to a major arsenal. Mass production required quality rifled barrels in volume.
The answer was the broach. Multiple cutting surfaces to distribute stresses are arrayed on a long steel rod. Starting with a pilot section, a rifling broach has multiple rings of cutters whose axial cross-sections match the rifling layout. If the barrel will have six grooves, each ring will have six cutters. The diameter of each cutting ring is only a few ten-thousandths larger than the one ahead, shaving a tiny amount of steel. There can be over 20 of these rings on a broach, each ring making the grooves incrementally deeper. Broaches are long, often 18 inches or more. One pass through the barrel and the rifling is fully cut and ready for final lapping.
Compared to the tooling costs for single-point rifling, one broach is staggeringly expensive. However, this cost is justified when you can measure factory output in rifles per hour instead of rifles per fortnight.
Advances in metallurgy gave us more durable steel alloys and specialty materials like tungsten carbide; these put faster methods of rifling with lower tooling costs in reach. With super-hard steel or carbide, it was possible to create smaller and much cheaper tooling that moved metal rather than removing it. This is button rifling.
The “button” is a piece of carbide tapered at both ends. In cross-section its profile is the inverse of the finished rifling. I have a worn-out .22-caliber button. The tool is only 1.5 inches long, and the working surface is probably less than 0.150 inch long. Secured to the end of a long rod, these are dragged through the tube from breech to muzzle. Apply a lot of force to a very small area and steel will move.
Another advance, electron discharge machining (EDM), made the fourth rifling type feasible. EDM allows complex shapes to be precisely etched into metal. Instead of directly cutting the barrel with EDM, the process makes a mandrel, a reverse or “negative” version of the inside of a barrel. Ridges on the mandrel will produce grooves in the finished barrel.
The mandrel slips into a barrel blank whose bore is slightly larger than the largest diameter of the mandrel. This package is run through a rotary hammer forge where six or more hammers move down the barrel beating the tube onto the mandrel while spinning to evenly distribute stresses. When done, the slight springback of steel releases the mandrel, which is withdrawn and used for the next barrel. This is hammer-forged rifling. Because you can use any mandrel shape that you can extract when done, polygonal rifling is usually hammer-forged.
Is one method better than another? Manufacturing advantages aside, no. All four methods have created match-winning barrels. Broached rifling is probably better at making deeper, wider grooves for lead bullets, but jacketed bullets seldom have problems with swaged rifling.
To confidently match a fired bullet back to one gun, the rookie forensic examiner must learn where to look for true unique markings transferred to a bullet. Carry-over marks from one barrel to another are infinitesimally rare. I’ve seen one verified example, caused by a badly chipped rifling cutter. Carry-over marks will show in the grooves of the barrel. They are typically larger than the normal striations relied upon for comparisons, not to mention being blatantly atypical. We easily avoid tooling carry-over problems through experience, careful consideration of the entirety of the set of markings, and focusing on areas that contacted the lands of the barrel.
Toolmarks on the lands are created by the bore drilling operation and run perpendicular to the axis of the bore. Because of how drills cut and the “additive” nature of patterns a bullet accumulates in traversing the length of the barrel, the lands produce the markings on a bullet that are truly unique to one and only one firearm. Even in hammer-forged rifling, it is the remaining drilled surface that makes unique toolmarks. In addition, less dramatic actions—like removing the mandrel and crowning the muzzle—can impart unique toolmarks.
The average shooter doesn’t need to worry much about how the barrel is made, but the processes are interesting nonetheless. A cast bullet shooter probably wants deeper grooves, but beyond that, any of these methods will produce a barrel that will out-shoot most shooters.