How are barrels rifled? - Part 1
by B.B. Pelletier
Several readers have asked for this posting, and one reader asked about lapping a barrel, which is supposed to be part of the rifling process. It has all but been abandoned by modern barrelmakers, at least those who make large volumes of barrels. Actually, the first person to request this post asked me to explain how BARRELS are made, but because that answer is included in this discussion, I included it within the talk about rifling.
Rifling was discovered very early in the history of gunmaking. In the beginning, the grooves ran straight down the bore, but soon they were made in a spiral pattern, and immediately gunmakers discovered that a spinning ball was more accurate. There are records of shooting matches in the mid-1500s, where rifled barrels were NOT permitted, because of the advantage they offered. So, the effect of a rifled barrel was known a long time ago.
Three types of rifling
There are three principal ways rifled barrels are made today, and two of them start with a long tube of metal. They are the cut-rifled barrel and the button-swaged barrel. The other - hammer forging - is quite different, so I'll cover it by itself.
The following process refers to both cut-rifled barrels and button-swaged barrels. To get the long tube of metal needed for the barrel, the maker starts with a tube or a solid rod. Some small makers of airgun barrels start with a seamless hydraulic tube that they rifle. If they start with a solid rod, the hole through the center must first be drilled. The task of drilling a deep (long) hole through a solid rod of metal is one of the toughest machining tasks known. In World War II, the M1 Carbine was redesigned to eliminate one deep hole in the side of the receiver, because too many receivers had to be scrapped when the drill broke out of the side of the hole.
Many barrelmakers drill this hole on a lathe, but the precision barrelmaker uses a vertical axis machine to eliminate the effect of gravity on the long drill bit. The drill bit itself has a special cutting surface to reduce the tendency for the bit to wander. Even so, no hole is ever drilled entirely true. The barrel maker has to use other means to true up the hole if he wants a quality barrel.
The hole is reamed and (possibly) lapped
The next step is to ream the hole. A trueness of about 0.001" along the axis is possible with very careful work. If the process is speeded up or the reamer is dull, it will be 0.0015" or even 0.002" of variation along the entire axis. If the maker is a good one, the next step is to lap the bore.
Lapping does not increase the dimension of the bore. It's purpose is to remove the tooling marks left by the reamer, just as the reamer also removed the larger marks left by the drill bit. The finest lapping is done with a lead slug that is cast right in the bore of the gun, so the fit is perfect. The cooled slug is broken free and pushed halfway out the bore, where fine abrasive power and oil are brushed on. This is called charging the lap. I have read in many places that lapping doesn't use abrasives at all, but rather it uses polishing compounds. Well, Virginia, polishing compounds ARE abrasives! They're just very fine. If they weren't abrasive, they wouldn't work.
The charged lap is run up and down the bore, recharging as required to keep polishing the bore. Because the lap is lead, the lapping powder sinks into it before it scratches the steel bore, so this is a laborious process. It's not unlike using J-B Non-Embedding Bore Compound to clean a leaded barrel!
After lapping, the barrel is cleaned. If it's going to have cut rifling, it is now installed in a rifling jig or machine, which looks something like a lathe. A headstock holds one end of the barrel, which is held at the other end so that it can be turned easily. A rifling cutter is a very small tool that fits on the end of a rod long enough to pass completely through the barrel. The cutting rod is mounted on a fixture that causes the rod to spiral as it passes through the bore. Two hundred years ago, this fixture was a wooden positive of the rifling pattern desired. It had the same twist rate that was desired in the rifle. Today, a precision rotating fixture is used. Alternately, the barrel may be rotated and the cutting fixture held still, and the same result will happen.
When a cut is complete, the headstock is indexed for the next groove and another cut is made. When all the grooves have had one pass of the cutter, it is adjusted to cut deeper and another set of cuts is made. Each pass of the cutter will remove about one ten-thousandth of an inch, if the barrelmaker is a good one, so each thousandth of an inch takes ten full passes. If the rifling is 0.005" deep, each groove took 50 passes of the cutter. If the barrel has six grooves, it took 300 passes of the cutter to completely rifle that bore. To speed things up, the cutter can be set to cut deeper, but that means more chances for burrs, gouges and associated tool marks.
Cut rifling has largely gone out of fashion today, though it's still practiced. It does not result in a barrel that's any better, but it does allow complete freedom over dimensions that button rifling does not allow. There is one more step after cutting the rifling, and it's a final lapping. I will cover it after I describe button rifling in the next part.