After the hole is drilled it is cupped or a seat formed for the shoulders of the nipple to rest upon. This is done with the tool shown in Fig. 34. The guide is used to get the proper angle of this seat, the same as in drilling. The stem at the end of the cutting portion of the tool, enters and fills the hole drilled, which insures the correct formation of the seat. The tap is held in the guide in the same manner and it follows that the thread has been made at the same angle that the hole was drilled. _Finishing Nipple Seat._—The filing and finishing of the nipple seat is one in which tastes differ or the price of the work may demand. This hint may be given, the seating tool must cut a seat large enough to receive the nipple and in filing keep this seat full size, taking care not to allow the file to take away or reduce any portion of it. A study of this portion of different guns that may come under the observation of the workman is the best guide for forming and shaping these parts. The first effort may be to form a nipple seat from a piece of hard wood shaped like the barrel with its lump, using the drill, the seating tool and even the tap, then finish up with files, as if it were iron, and insert the nipple. Better to correct a fault in a “sham” of wood than spoil a good barrel breech. _The Vent, in Percussion Gun Barrels._—When the old flint-lock was pushed to one side by the introduction of the percussion principle, it was thought by many that there was a difference in shooting and that the flint-lock shot “smoother” than its substitute. In the percussion gun there was thought to be more recoil than in the other form of gun. It was supposed that the hermetically closing of the breech, as the firing took place, was the cause of this, and to remedy the matter a small vent was drilled near the locality of the vent or “touch-hole” of the old-time arm. A few gunsmiths and many users of arms cling to the vent and could “not do without it.” Some claim that this vent is of use, as it enables air to circulate through the barrel, after the ignition of the charge. While both these theories are doubted by others, one thing is quite certain, it increases the certainty and also aids the rapid ignition of the charge, as the air contained among the powder and held there in a more or less state of pressure, being so forced and held by the wadding, retards in some measure the entrance of the fire from the percussion cap. The vent allows this confined air to escape, and that is the only real and apparent good that can be accredited to it. In making these vents make them very small, and in no instance let them exceed a thirty-second of an inch. To prevent their being closed by rust or the debris of burnt powder, drill a larger hole, tap a thread in it and screw in a silver wire and then drill the vent through the wire after it has been finished to conform to the shape of the parts adjacent.