How to rifle a cannon… the British version

Frequent commenter and blogger from the continent, Jan D, has asked, not once but twice, for posts regarding the construction of cannons. Such is a broad subject, but one I’ve touched upon in several posts over the years. Still I’ll have to work a few more in. Specifically Jan has asked about the method for rifling cannons. So let me start there.

The concept of rifling artillery is in concept like rifling a musket or other small arm. In order to build the grooves, the gunmaker used a cutting or rifling head to gouge out the bore in the required pattern. Simple, right?

Consider, however, at the time of the Civil War several variations of rifling existed. The size and contour of the grooves differed from design to design. While most Federal makers favored flat grooves, Confederate often used “hook-slant” grooves or Brooke’s triangular style. Many Blakely rifles from England featured a “saw tooth” profile. And don’t forget Norman Wiard’s “U-groove” rifling or Whitworth’s hexagonal patterns. Indeed those of us who study artillery could use an authoritative study of just the rifle groove profiles!

And the variations were not limited to the profile geometry. Inventors on both sides of the Atlantic proposed different pitches or twist ratio for the guns. Robert P. Parrott’s guns featured an increasing pitch, as a means to reduce stresses of firing. And of course there were variations as to the number of grooves and their spacing.

These variations stemmed from differences between the gun designers – differences both understanding and reaching the desired projectile behavior. Although lacking computer modeling software and high speed cameras of today’s weapon designers, those of the Civil War period still placed exacting standards on the final product. Gunmakers couldn’t just “eye-ball” the rifling process, but rather had meet those standards to avoid any undesired effects in performance. All done of course with the decidedly “manual” equipment of the day.

Descriptions and diagrams of the machinery appear in contemporary books (usually with the word “treatise” in the title). The best example that I have ready access to is a guide to ordnance production for British needs. A Treatise on the Construction and Manufacture of Ordnance in the British Service Prepared in the Royal Gun Factory, has a dry and unromantic title, but its great reading for those interested in the fine particulars of the craft. I’ll reference the 1879 edition here. While techniques varied between gunmakers, the general principles were the same. But, as the Treatise warns, “This operation … though simple in its action, is not very easy to describe without using technical terms.” I’ll try to avoid technical overload here for brevity.

After casting or other construction technique, the gunmaker had to bore out the gun block (unless of course he was working on a hollow-cast Rodman). With the bore cleared, the next step was the introduction of rifling. The cannon was fixed in place on a stand so the cutting head could be run into the bore as needed. The rifling head ran down the bore at the end of a long, hollow shaft. Inside the shaft was a bar used to adjust the pattern, angle, and depth of the cut.

The way this worked, the gunmaker could work the bar against the saddle on which the cutter sat. This allowed the cutter to extend or retract as required. Another bar, outside the shaft and known as the copying bar, was used to regulate the movement of the entire apparatus and thus determine the pitch of the rifling. The copying bar was curved in relation to the desired rifling twist. As the rifling head was withdrawn through the bore, the shaft, and therefore the head itself, turned in accordance to the curvature of the copying bar.

To ensure the rifling started at the appropriate depth for use, the gunmaker measured chamber space taken up by the smallest charge for the gun. From there he figured the location the projectile would seat when loaded. The rifling would start an inch deeper, just as a precaution.

The Royal Gun Factory preferred to cut one groove at a time. So the entire setup attached to a pawl. When each groove was complete, the gunmaker moved to the next notch on the pawl. The preference was to work a rough, shallow cut for the first iteration. With the basic pattern set, the gunmaker would repeat the movement through the full range of the notches, each time working the rough cut into finer detail. The cutter itself was steel, sufficient to cut bronze and cast iron of the time. As the rifling head withdrew from the gun each time, it pushed the metal shavings and chips forward out the bore. (Can’t you just imagine the screeching noise as you read this?…. eeeeeeekkkkkkkk.)

The British manual provided a nice formula for determining settings of the copying bar to reach the desired pitch:

Got it? Practical application of geometry! Anytime you see more than one π I say it is time to pull out some reference table. Far better to depend on someone else’s math than make some horrible miscalculation on your own.

So there you have it in perhaps more words than one might imagine could be wasted explaining the rifling process… with just a smattering of the technical terms. From the 1860s onwards, the patent files are full of improvements to the basic system. But that’s the way it was done in the days before laser guided, diamond bit, computer aided cutters.