How to Put Thimbles on Barrels.—File bright and tin the thimbles where they are to be joined to the rib. Observe if the thimbles fit the rod properly by putting the rod in them and then inserting the rod in place in the gun stock. Mark the place on the rib or barrel where the thimbles are to be fastened, and remove both rod and thimbles from the gun. If to be attached to a rib, file a spot the length of the thimbles where it was marked, and file it of a depth equal to the thickness of the metal of which the thimble is made. Too deep filing may cut through the rib, and too little filing will leave the thimble projecting above the rib, so that the rod will hit or rub as it is being pushed down in place. Also let the joint of the thimble come in the centre of the rib when it is soldered in place. Tin the places filed, by heating the barrel carefully over the forge fire, using the soldering acid as for tinning the thimbles. A common tinner’s soldering copper is best to apply the solder.
When the barrel is cool enough to handle put the thimbles on the rod, and the rod in place as it is intended to be when finished. Confine the thimbles to the barrel with pieces of binding wire, using two pieces to a thimble, one at each end. By putting the rod into the thimbles and confining them thus, there is no danger of their being “askew” after being fastened, and by putting on two wires there is less danger of their moving while being soldered to the barrel.
Make a clear fire in the forge, using charcoal if it can be obtained, heat the barrel very carefully until small pieces of solder will be melted when placed on the inside of the thimble. Have the soldering copper heated, and by using it and applying the acid an even amount of the solder can be applied to the joint outside the thimble where it joins the barrel. When all are soldered let the barrel cool, remove the binding wires and wash with warm water to remove the acid flux, which would rust the work. A stiff brush is best to wash with. Scrape off the superfluous solder, rub the thimbles bright with emery-cloth, or let them remain the black color, as may be desired.
CHAPTER XXI.
ON RIFLING OF GUNS.
Importance of Rifling.—In a rifle the grooving is of the utmost importance; for velocity without accuracy is useless. To determine the best kind of groove has been, accordingly, the object of the most laborious investigations. The ball requires an initial rotary motion sufficient to keep it “spinning” up to its range, and is found to gain in accuracy by increasing this rotary speed; but if the pitch of the grooves be too great, the ball will refuse to follow them; but being driven across them, “strips”—that is, the lead in the grooves is torn off, and the ball goes on without rotation. The English gun-smiths avoided the dilemma by giving the requisite pitch and making the grooves very deep, and even by having wings or lugs cast on the ball to keep it in the grooves—expedients which increase the friction in the barrel and the resistance of the air enormously.
The American gun-makers solved the problem by adopting the “gaining twist,” in which the grooves start from the breech nearly parallel to the axis of the barrel, and gradually increase the spiral, until, at the muzzle, it has the pitch of one revolution in three to four; the pitch being greater as the bore is less. This gives, as a result, safety from stripping, and a rapid revolution at the exit, with comparatively little friction and shallow groove-marks on the ball, accomplishing what is demanded of a rifled barrel, to a degree that no other combination of groove and form of missile ever has. There is no way of rifling so secure as that in which the walls of the grooves are parts of radii of the bore. They should be numerous, that the hold of the lands, or the projection left between the grooves, may divide the friction and resistance as much as possible, and so permit the grooves to be as shallow as may be. [Fig. 41] represents grooves cut in this way, but exaggerated to show more clearly their character. In the Kentucky rifle this law is followed, except that for convenience in rifling, the grooves are made of the same width at the bottom and top, as shown in [Fig. 42], which is, for the grooves of the depth of which they are generally made, practically the same, the depth in the cut being two or three times that generally used.
Figure 41.
U. S. Rifling Machines.—The rifling machines in use by the U. S. Government at the Springfield Armory for cutting their grooved rifles may thus be described: The barrel is placed in a horizontal position in an iron frame, and held there very firmly. The grooves are made by three short steel cutters placed within three mortices, made to receive them, near the end of a steel tube which is moved through the bore of the barrel by slow rotary and progressive motion. The cutters are narrow pieces of steel having upon one side three angular shaped teeth about one-sixteenth of an inch in height, and of the width of the groove, ground to a very sharp edge at the top. It is these which produce the rifling. The three cutters, when inserted in the tube, form upon their inner surface a small opening which decreases toward the inner end. Into this is inserted a tapered steel rod, and is so controlled by a connecting cog-wheel that this rod is pushed, at every revolution, a little further into the tapered opening formed by the inner edges of the three cutters. The effect of this is to increase the pressure of the cutters upon the inner surface of the barrel, and thus gradually, at each stroke of the machine, deepen the cuts as produced by the rifling. The rod makes about twelve revolutions in a minute and it occupies about thirty minutes to rifle a barrel.