A nice article on rifle barrel cutting

[Hunter]

I found this very interesting and read it to the last word to add to my rifle knowledge. Hope you find it ineresting/informative too

 

 

 

 

 

EVER WONDER HOW THAT PRECISEN SPIRAL PATTERN WAS FORMED INSIDE YOUR BARREL? HERE IS A BRIEF LOOK AT RIFLING MET HODS, BOTH SPACE-AGE AND CENTURIES OLD.

 

If you visit the gunsmith's shop in Colonial Williamsburg, you will see a massive, wooden rifling machine that was used to manufacture rifle barrels during the Revolutionary War. If you visit H-S Precision in Rapid City, S.D., you will see the same technology being used today. In Williamsburg, a man provided the power. At H-S Precision, it's done with an electric motor and controls.

 

Three key events directed the evolution of our sport. First was the invention of gunpowder in the 14th century. Second was the invention of the rifled barrel in the 18th century. Third was the inception of the metallic cartridge case in the mid-1800s. And now, the 21st-century rifleman is blessed with the finest rifles ever produced: stable, durable, synthetic stocks and optical sights of the finest quality. Modern ammunition is supremely accurate and reliable, and its Noncorrosive properties free us from constant cleaning and maintenance.

 

With this abundance, we easily forget that there is one feature still essential to making our rifles perform - rifling. While there are modern methods of rifling a barrel, the same basic procedure developed by 18th-century gunsmiths is still in use today. Understanding how barrels are rifled may not make you a better shooter, but it is sure to increase your appreciation for your rifle's performance and of the history of our craft.

 

Five Rifling Methods

 

The basic mechanics of barrel production have remained essentially the same. A cutter is drawn through the barrel at a specified rate of twist and slowly cuts one groove at a time. Called cut rifling, it was the first method ever used to rifle a barrel. A small hook-shaped cutter is pulled through the barrel to remove a tiny amount of metal. When a pass is complete, the barrel is rotated or indexed to the position of the next groove.

 

Once each groove has been given the first cut, the tool is adjusted to cut slightly deeper. The whole process is repeated until the desired groove-depth is reached. In the Williamsburg shop, a small shim of paper was placed under the cutter to adjust it. Modern tools use a screw adjustment that can increase the depth of the cut by as little as 0.0002 inch.

 

Cut rifling was the standard method of manufacturing rifle barrels up through World War II. You may also have heard of the famous "two-groove" Springfield rifle barrels that were a wartime expedient. The process reduced the number of rifling grooves from four to two, and production was doubled with no practical loss of accuracy or function.

 

Shooters reported that the two-groove barrels actually shot quite well. And since they were dirt-cheap after the war, they were widely used by cost-conscious shooters. Cut rifling is the most time-consuming method of rifling. Even today, 45 minutes or more of machine time is required to complete a single barrel.

 

Shorter handgun-length barrels are rifled through a cutting process known as broaching. A broach is a complex cutting tool with a series of cutters that increase in diameter by small increments. This segmented cutting tool begins with a diameter that will just enter the reamed bore. As it travels through the bore, each fractionally larger diameter segment cuts the desired rifling pattern slightly deeper.

 

It is not unusual for a rifling broach to have 20 or more sections. It can do an exceptional job of cutting rifling and has the advantage of cutting each of the multiple grooves at once, in a single pass. The broach is pulled through the workpiece at a slow and constant feed, and it turns as it goes to give the proper twist rate.

 

By far, the most common method of rifling barrels today is the process known as button rifling. This is a swaging process that can be both fast and economical. It is also capable of producing extremely accurate barrels. Unlike cut or broached rifling, the extremely hard carbide button impresses its pattern into the metal rather than cutting it from the bore. The button is highly polished and carries what amounts to a negative of the desired rifling pattern. After the button passes through the bore, the steel will actually spring back to a certain extent. It takes a lot of experience to know the specific properties of the steel being worked in order to end up with the desired internal dimensions.

 

Another method of barrel rifling is rotary-hammer forging. Like button rifling, this is a cold swaging process. A full-length mandrel that has the negative image of the desired rifling is inserted into the drilled and reamed blank. Three or four powerful hammers then beat upon the outside of the barrel blank and force the metal down around the mandrel. This is probably the fastest method of rifling, but the machine required is incredibly expensive.

 

This entire operation takes about 90 seconds at a rate of over 7,000 blows per minute. It is fascinating to watch this machine work, for it can also define the outside contour and dimensions of the barrel. Since metal is swaged around a mandrel, the blank actually gets longer during the process.

 

 

The newest rifling method is electrochemical. In this process, a carbon electrode engraved with a rifling pattern passes through a prepared tube. An electrolytic fluid completes the circuit, and the metal is removed and carried away in the fluid. This is a fast process, but the equipment is expensive. It is best suited for relatively short barrels.

 

It All Begins With A Hole

 

All five of these processes require drilling a basic hole through the barrel blank before any rifling operation can be performed.

 

At first glance this seems simple, but it is actually very difficult. If you've ever drilled a hole through a relatively thick piece of steel, you know that drill bits tend to wander quickly. The deeper the hole, the greater the tendency for the drill to depart from a straight and true path. The best rifling in the world isn't worth much if the original hole isn't straight. Very special drills are used for this type of work.

 

Deep-hole drilling machines throughout the industry are pretty much alike. But how they are operated can vary. Of course, the diameter of the bore and type of steel are significant. But the rate at which the cutting is done governs both how smooth and how straight the hole will be. If the drill is advanced too fast, the cut will be rough and more likely to wander. Drill too slowly, and production rates will suffer. The drilling is done at high speeds -- around 6,000 rpm -- and the cutter is advanced at an average rate of around two inches per minute.

 

 

The drill bits themselves are very special. They are hollow, and a cooling liquid is forced through a hole right at the point of the drill. These are not common twist drills but rather single-flute cutters with a channel down the side. As the drill is pushed forward, there must be some place for the chips to go, and they are washed to the rear and out of the barrel blank by the constant stream of very high-pressure cutting fluid.

 

After gun drilling, the rough bore must be reamed to establish the desired internal diameter and to improve surface finish. Reaming is done at low speeds of around 250 to 400 rpm. The reamer is pulled through the barrel at a rate of approximately five inches per minute. This rate is possible because reaming removes very little material.

 

In the absence of a lapping, the quality of the reaming operation establishes the interior finish of the bore. If the reamer leaves behind deep tool marks, they will probably be there forever. Even though rifling operations may smooth things out, they normally will not remove tool marks left from the reaming operation.

 

The Finishing Touch

 

Many custom barrels are finished with a lead-lapping process after rifling. This process has not really changed since colonial days, and involves using the barrel as a mold to cast a lead slug onto the end of a long rod. Taking care never to remove the resulting lead lap completely from the bore, it is then coated with a fine abrasive compound and pumped carefully through the bore until the desired level of polish is attained. This can either be done manually or by a machine. There is also an electrochemical polishing process that can be used in high-volume production.

 

Over the years, I have been very fortunate to visit factories, both large and small, where barrels are made. Not long ago, I was able to visit Douglas Barrels in Charleston, W.Va. When, way back in the 1950s, I built my first centerfire rifle on a surplus Mauser 98 action, the unanimous recommendation was to use a Douglas Premium barrel. Today there are a number of shops making custom barrels, but Douglas is still a leader.

 

A visit to the Douglas plant is, in a way, a trip back in time to witness barrels being made as they have been for generations. The equipment is spotless, but it has actually changed very little since World War II. In fact, almost every barrel-making shop I've seen uses rather elderly machines that have been rebuilt and modernized over the years. Nothing wrong with that.

 

Good, Better And Best Barrels

 

For as long as there has existed more than one way to rifle barrels, there has been discussion, and even bitter arguments, over which method is best. Let me say from the outset that I don't think any method of rifling is automatically superior to any other. It is far more complicated than that. Each of these methods has produced match-winning barrels. The real question that remains unresolved since the earliest days of the rifle is why all barrels produced by the same manufacturer and on the same equipment do not perform equally.

 

Frequently the quality of the barrel is blamed when a rifle shoots poorly. While that may be the cause, there are other factors that must first be eliminated. Was the barrel installed into the action properly? Is the chamber cut concentrically and to proper dimensions? Is the muzzle crown true and undamaged, and is the action bedded properly into the stock? If these details are not correct, the best rifling in the world isn't going to help.

 

Even if everything is done correctly, sometimes we encounter a barrel that just won't shoot up to expectations. We can look at it, measure it, and study it 'till the cows come home and still not always understand why one barrel shoots poorly and a supposedly identical barrel shoots one-hole groups.

 

These days, a barrel that genuinely will not group is so uncommon as to be rather remarkable. Regardless of the manufacturing process used, modern barrels are uniformly quite good. That colonial gunsmith who hand cranked his wooden rifling machine to produce a usable barrel would have leapt for joy if he could have made a product as accurate and as precise as our modern barrels.

[Dave-G]

Very informative - I'd imagined they started as tubes Thanks hunter

[Baldrick]

A very good article, it certainly answered a few things for me. Incidentally, I've had a lot of use and information from the new book "The Art of Gunsmithing" (Lewis Potter, http://www.crowoodpress.co.uk/2004/book_de...N=1+86126+815+7). It certainly has a lot of information about barrel honing/lapping/polishing. Whilst it's shotgun-orientated, I was told that the author may be writing a rifle-specific volume.