This is a great thread, Rich.
It would be helpful to know how essential the sap/pitch thread sealer is on these old barrels. Without it, would these barrels have sealed adequately?
The fact that thread sizes were not standardized seems likely to have affected practice back then. It may be that people were more likely to use a shoulder technique if they were using a standard set of taps. If they were making taps to whatever random size they needed, then perhaps they were less like to use a shoulder.
The contrast between smooth bore guns and rifles may also be a big factor in the contrast. (Pukka Bundook's comment.)
And within those factors, there's also the historical contrast between "what was done" and "what was done well". Some old breachs failed disastrously, with death and injury the result. The old records make that clear. (The fact that an old firearm has survived thus far doesn't in itself indicate anything about the quality of the craftsmanship or techniques used in its original production. Some old guns survived simply because the mainspring or cock broke and so they didn't get used much after that. So in those instances, the survival was a factor of the POOR quality of the materials or craftsmanship or technique. So we have to use our heads when studying old techniques.)
Here are two historical documents that show that this discussion about technique is not a new one. Both of these mention bottoming taps. Note that the second specifically addresses other techniques and recommended against some methods. I'm not meaning to arguing in favor of a contemporary technique. But I do think it is helpful to note that some old techniques may have been frowned upon even back in the day.
From Sprengle. “The Gunmaker and Gunstocker”, published in 1771. Translated in JHAT, June 1988, III, p 25-28) (Figures refer to a woodcut image published on page 26 of the JHAT volume. All notes here are from that translation.)
(p 26) As is well known, a breechplug [Schwanzschraube, or literally tail screw] is screwed into the breech [here unterste Mündung eines Laufs or “lower mouth of the barrel”], and therefore threads must be cut into this opening. The pointed thread tap [in] Fig.XIII “a” [starts the thread in] the hole first, and in order for this stout tap to be easily turned, a tap wrench [Windeisen, or winding iron] “cd” is attached to its square shank “c”
(p28) while tapping. The same is also true for the bottoming tap [kalibermässignen Bohrer or gauged bit] Fig. XIII “e,” with which the female thread is finished out for the breechplug. The last tap therefore must be of a [constant diameter for its full length], like the breech plug. The [threads of the] breech-plug are cut with the screw plate [the word Schraubenmutter is also used here; it also signifies a nut or female thread] Fig. XIII “f”, which the bottoming tap “e” fits exactly. Therefore, the bottoming tap “E” with its accompanying screw plate “f” form a powerful cutting mechanism.
From Stelle and Harrison. The Gunsmiths Manual, first published in 1883. reading here from 2013 edition. p 166- 169.
Stelle and Harrison first note that they have occasionally seen breeches tapped with a “blacksmith’s tapered tap” and plugged with an accompanying tapered plug. But they criticize that technique (as well as breeches made with a crooked thread), and pick up with the following:
Let the workman discard all such ways of breeching guns. Let him procure a set of taps of the sizes and threads as noticed in the beginning of this article, and “stick to these sizes.” … Breeching taps should be made in pairs, one tapered a little and its mate made straight and with a full thread, so as to cut full at the bottom where the thread terminates.
Stelle and Harrison also describe how to make a tap with a protruding “stem or projection” that will help the tap align with the bore. (They provide woodcut image, figure 36, and the image shows the tap made with bottoming threads. A similar tool, tool #1, is shown on page 64, plate 46, of Dillon’s “The Kentucky Rifle”, 1967 edition.)
Again, just to emphasize it, I'm not meaning to argue for anything regarding contemporary methods. I hope this thread doesn't get into that debate. Keeping it to Rich's original topic would make me happy. Also, sorry for the long post. Not meaning to hog the mic.
