The Basics Of Gundrilling Rifle Barrels

A little history on gundrilling and the use of modern machines and tools.

by Charles J. Moore

Prior to the Industrial Revolution, the hole through gun barrels was formed by forging red-hot iron skelp around a mandrel, then forge-welding the edges together. As you can imagine, this was a rather time-consuming and labor-intensive process. It was also the type of task which required the skill and experience of a master blacksmith to ensure that the job was done properly. It also limited gun barrels to the low-pressure black powder loads that would be safe in a wrought iron barrel. The Industrial Revolution brought steam, and later electric, motors onto the scene and these inventions allowed the development of ever more sophisticated machining tools. At about the same time, there were also several innovations in the field of small arms development, most notably the self-contained cartridge and nitrocellulose (smokeless) powder. The pressures that could be produced with the combination of smokeless powder and brass cartridge cases was awesome and demanded that gun barrels be made of stouter stuff than wrought iron. Fortunately, the machine tools of the period were fully capable of cutting the strongest steels known.

Although the machinery of the period could be made to serve the purpose of creating gun barrels quite readily, the drills used to cut the holes through the barrel blanks were still in their primitive forms. The twist drills used for most types of drilling were the first adapted to the task. These drills were manufactured with a small hole through the shank through which oil could be pumped. The oil flushed the chips backwards out of the hole effectively, but the drills did not perform all that well. Twist drills have a tendency to wander off-center when employed to drill deep holes due to their geometry. Twist drill geometry incorporates two opposed cutting edges of equal length which are set at equal angles in relation to the shank of the tool. This means that there is no net lateral force produced by a twist drill, at least not if the cutting edges truly are equal and opposite. The problem is that they can seldom be ground to be perfect mirror images of each other, and so the drills tend to wander a bit. And the fact that their central webs prevent them from being truly free-cutting made it obvious that a more effective tool must be developed for the particular job of drilling gun barrels.

The first improvement in gundrill design came in the form of…

Read more in the October 2015 issue.

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