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. CHAPTER XIX. ON GUN RIBS.