The first operation, after ascertaining the locality where to drill the hole for the nipple, is to drill this hole. If a double gun the nipples must be inserted as near the centre of the breech as possible to get them and have a direct communication, with no unnecessary angles, in the passage-way of the flame of the cap to the powder in the chamber. From the bottom of the hole drilled for the nipple a small vent, as small as can conveniently be made, must or ought to communicate directly to the powder chamber. The nipple ought to be seated or screwed to the bottom of the hole and the vent be an unbroken continuation of the opening through the nipple. Let there be no chamber or reservoir between the bottom end of the nipple and the bottom of the hole in which it is screwed. [Illustration: FIGURE 33.] But to return to the drilling. From the axial or central line of the barrel, this hole must be drilled at an angle of forty-five degrees so that the nipple when inserted shall stand at that angle. In the cut Fig. 33, _a_ represents the central line of the barrels, _b_ a line at right angles to this, and _c_ the angle of forty-five degrees. If the workman have no lathe and is necessitated to use a bit-stock or breast drill, a piece of iron can be fitted to lay upon the barrels with one end elevated three or four inches with the end turned at an angle in a hole drilled in this end to receive the drill, which should fill the hole. By clamping this upon the barrels, inserting the drill in the hole and having a prick mark where the hole is to be made for the nipple, it forms a reliable guide to insure the correctness of the hole, and also to get any number of holes alike. To insure the hole to be tapped or threaded properly the guide can be kept in position and the hole tapped after being cupped to form the seat. If a lathe be used to drill the hole, a piece of wood or an iron forging can be fitted to the spindle against which the work is held and when made of the proper angle and the barrel confined upon it to drill the hole, no guide is necessary, as the drill being held in the spindle and the spindle running in its bearings operates upon the same principle as the guide clamped upon the barrels. An upright drill can be fitted with a forging or casting to hold the barrels in the same manner. The size of hole to be drilled ought to be that of the smaller sized nipples, as in time frequent re-tappings to insert new nipples will enlarge the hole. Perhaps a broken nipple may necessitate drilling or cutting out and the thread may be injured somewhat, so that it may have to be bored out and a new thread made to receive a larger nipple. [Illustration: FIGURE 34.]