_Boring Gun Barrels._—The tool used for this purpose consists of a rod a little longer than the barrel to be bored, with a cutter head at one end. This cutter is about one-half or three-quarters of an inch long, and of a diameter a little larger than the bore that is to be enlarged. It can be made to be pulled through the barrel or to be pushed through when cutting. In the armories where many guns of the same calibre are made, a portion of the shank, next to the cutter, is made of the bore of the barrel, and the cutter made of the size the bore is to be enlarged. The portion that fills the bore acts as a guide, so that the cutter is forced to follow after it, and the enlarged bore is in the same line as the boring previously made. The cutting edges of the tool are, of course, next to the enlarged portion. As the tool is pulled through the barrel, the cuttings are left behind it as it advances; oil is to be supplied while cutting, and care must be exercised not to let it get clogged with cuttings, as a tearing of the surface of the barrel would be the result. When the tool is made to be pushed through while cutting, the cutting edges of the tool is on the end, and it operates like a reamer. This cutting end should be bevelled off so as to follow the bore to be enlarged. _How to make Cutters for Boring._—One way to make these cutters is like a many-fluted reamer, with five, seven, or more cutters. The odd numbers will operate better than even ones. If made with too many cutting edges, there will not be clearance enough for the chips, and clogging will be the result. Then again, the cutters must not be too long, or there will be too much friction, and the barrel will become very hot in working. Also, if the friction is too great, the barrel may be bent and sprung in consequence. One form of cutter is made like the cherry to make an elongated bullet, or like the cherry of a Minie bullet. They may also be made in form of an egg. A common twist drill welded to a steel rod has been used for small bores. A small fluted reamer welded to a rod will do where the enlarging is quite small. When the tool is to be pulled through the cutter may be made like a short twist drill not over an inch long, with the cutting edges next the rod, and not on the end, as these drills are generally made.