When chambering barrels for breech-loaders, the utmost care should be observed to make the chamber exactly concentric with the bore, and have it smooth and well finished. Do not trust to a drill or a flat reamer; neither will a half-round or a common fluted reamer answer the purpose. Take, for instance, a bore for a thirty-eight cartridge. Fig. 38 shows the tool for chambering, and Fig. 39 the tool for recessing for the head of the cartridge. The diameter of the tool for chambering is thirteen sixty-fourths. The tool for recessing for the head is seven-sixteenths. The body of the cutting portion is about seven-eighths or an inch long. An extension is formed beyond the cutting part which must exactly fill the bore of the barrel and serves as a guide to insure the cutting part making the chamber in proper relation to the bore.
Figure 38.
The neck or recess in front of the cutting part is for a three-fold purpose; it forms a receptacle for the chips or cuttings, which otherwise would clog the tool as soon as the teeth were filled, and would probably result in a rough surface by being forced between the wall of the chamber and the tool back of the teeth; it also serves to produce a better shaped tooth, which is done with a fine cut three-square file, and finally by the teeth being made below the surface of the extension a square cut is produced with no feather edge where the chamber terminates.
Figure 39.
The cutting teeth of the chambering tool are made so as to produce a bevel at the end of the chamber, and by this means avoid shaving the ball, as would be the case if it passed over a sharp angle. The recessing tool is made square on the cutting end.
Figure 40.
For larger bores than a forty-four, the tools may be advantageously made a little different, as shown in Fig. 40. The shank of the tool may be made of a cheaper grade of steel and the extension much smaller than the bore of the gun. At a, is shown a steel thimble that is turned to the proper size and has teeth cut on the end. This is tempered and is put on over the extension and held by a wire or pin passing through both thimble and extension. At b is another thimble, made preferably of brass, that fits the bore. It can then be worked in the barrel with less fear of scratching the barrel, as might be the case if it were of steel or iron. The space left between the brass thimble and the teeth forms a recess to receive the cuttings. The diameter of a twelve gauge chambering tool will be about forty-nine sixty-fourths, and the diameter of the chambering tool will be about twenty-seven thirty-seconds. The length of the cutting thimbles can be about one inch. The brass thimbles about three-quarters of an inch. The cutting thimbles should rest against a shoulder, as at c. These thimbles should be fastened, a finish turned on the shanks on which they are to remain, after the shanks are turned. The brass thimbles are held by being driven on the extension, and should be turned up true after being driven in place.
When making these tools, form the cutter for the chamber about one-hundredth of an inch larger than the bore of the cartridge to allow for easy extraction. The brass thimbles ought to be of the size of the interior of the cartridge, which is supposed to be exactly the same as the bore. The tool for recessing the head may be a trifle larger, say about a thirty-second, than the diameter of the cartridge head. If it be made one-sixteenth larger it will not matter. Eight cutting teeth are enough for these tools.