One of the things that many builders find daunting about the idea of manufacturing firearms completely from scratch, is manufacturing a rifled barrel. There are two tough nuts to crack with barrel making. The first is deep hole drilling, which is still called in industry “gun drilling,” because the technology of deep hole drilling, which today has many industrial applications, was initially developed for making gun barrels. Gun drilling will be dealt with separately; today, we’re going to talk about rifling a drilled (and, usually, reamed to precise size) barrel blank.
There are at least four processes used in industry to make barrels:
- Cut rifling: A cutting tool is drawn through the bore, using either a spiral guide or a system of gears. This method is used on most high-precision target barrels. It can be used for constant-twist or gain rifling, with an appropriate means of rotating the drawbar.
- Broached rifling: A carefully machined broach, a tool made of very hard material, is drawn through the bore in a single pass, stripping off material from the grooves of the rifling with teeth of gradually increasing depth. It cuts all lands and grooves at once, and produces a constant twist. The broach must be pulled, not pushed, and requires both a heavy, powerful, and specialized machine, and a specialized broach for each caliber, twist, etc. A barrel-rifling broach costs thousands of dollars (for comparison, a steel precision broach for doing the M16 mag well in 7075 forged aluminum is $10k for the tool alone).
- Button rifling: In this process, a carbide or other hard-material “button” is forced through the barrel.It cuts all groves at once (and smooths all lands at the same time), and can only produce constant-twist rifling.
- Cold hammer forging: the barrel is forged by a hammering machine, forcing an oversize bore to close in around a mandrel that is a mirror image of the intended rifling. Developed in Germany, it was first use on H&K weapons, then on TRW’s M14 contract. FN produces barrels this way, also. This forms the lands, grooves and usually the chamber also, all at once around a very expensive precision mandrel.
Of these, the only ones practically adaptable to the small-shop or home shop are cut and button rifling; you can’t hammer-forge or broach barrels on the cheap. It’s what MBAs call “capital-intensive.”
Home Rifle Cutting Lathe Attachment
This elegant lathe attachment, developed by an Australian gunsmith named Tony Small, uses a worm and spur gear to drive the rotating tool for cut rifling, while the barrel is held stationary through the chuck of the lathe. The handwheel drives a (nylon? Delrin?) idler wheel (the big wheel) which in turn drives Interchangeable gears, which are actually the speed-change gears from the lathe, to control the speed of rotation and thereby the rifling twist rate.
The drawbar attaches to the worm wheel and to the rifling cutter with precision tapered cross pins. Because his drawbar is 1/4″ diameter, he’s limited to calibers larger than that, ruling out all the .219-.224 calibers, but it would scarcely be rocket surgery to fabricate a drawbar for small calibers. All components except the speed-change gears were fabricated by Tony, including the worm and wheel. More information about this attachment is available on Tony’s website. That includes images of front and back of lathe chuck and worm wheel, an image of cutting the worm wheel (with a fly cutter in a milling attachment), and a custom-made short chuck he made to de facto extend his lathe’s travel compared to the factory chucks. There’s also a great deal more information in the comments to his YouTube video.
His lathe appears to be a Grizzly unit or something very similar. It is not a lathe that is out of hobbyist reach, even though he’s a professional.
The signal advantage of Tony’s machine is versatility. He can cut a wide variety of rifling pitches and calibers with a single machine. By making his own speed-change gears and perhaps another indexing plate (his present one accommodates up to 8-groove rifling) he could extend that, and make literally any barrel he can have a drilled blank for. This is well-suited for his custom-rifle business in Australia, where he handcrafts rifles for demanding shooters and hunters. (Do take the time to look around his site, and you’ll wish his information about his past work was more boastful, with more pictures of his classic, beautifully fitted and finished firearms. This rifling-machine post will still be here when you come back).
The disadvantages are that he has to make his own tools, and that it is unsuited for series production. It takes far longer to cut-rifle than to button- or broach-rifle a barrel. That is partly why cut rifling these days is the preferred technology of high-end barrel makers.
Hand Cutting Wooden Pattern Machine
This is a very primitive, 18th-Century machine (on 16th-Century principles), in a living-history exhibit. It was a live presentation, so it’s somewhat damaged by all the spectator conversation caught on the mic, but this shows the principles that were used on John Browning’s original sine-bar rifling machine (on display in his shop museum in Utah, it may have been his father Jonathan’s). Browning’s machine was all made of wood, not just the pattern. Of course, the greater rigidity of steel is superior.
This is a more “cottage industry” modern version, a home-fabricated all-metal version for short, i.e. pistol, barrels:
And here’s the crudest version, a failed attempt. This primitive machine is also made mostly of wood (warning: very slow video, very annoying music). It’s an experiment that falls well short of success, but points the way for better approaches:
Sine-Bar Cut Rifling Machines
Here is an American Precision Museum video of a 3D reconstruction of their 1853 Robbins & Lawrence Sine-Bar rifling machine. In this case, instead of a wooden pattern, an iron sine bar driving a rack and pinion imparts the twist:
The exact same principles were used in this sine-bar rifling machine, purportedly “Old Man Savages’s,” recently restored)… (see on YouTube to see the comments)
…and in the famous Pratt & Whitney rifling machines. This one is a circa 1900 one, probably a Nº. 3, but they were produced up to 1945. They are still around, but according to the video maker, most of the ones available these days are well beaten-up, and too expensive for the small shop anyway. There are two videos, and the comments in the original YouTubes here and here are worth reading, too. Part 1:
Home Button Rifling
Here is home-shop or small-shop rifling of a short barrel using a button and a 10-ton manual press to push it through. The manual press means the theoretical speed advantage of button rifling isn’t really achieved. It seems it would be straightforward to substitute a hydraulic press.
For comparison, here is an industrial button rifling machine:
Buttons are widely available, direct from the manufacturers. Danjon Mfg. Corp (warning: crappy autoplay music), for instance, prides itself on selling to the one-man shop as well as to massive Remington. (Remington has used Danjon buttons for something like 50 years).
The advantages of cold-forming a barrel’s bore with button rifling include superior surface finish, surface, and ability to hold very precise dimensions. To get the best of these advantages, the bore must be drilled and then reamed to a very precise size to begin with. Disadvantages include an inability to cut really deep grooves; this process is limited to a depth of about 4 thousandths of an inch (.004″), max.