_Plugs for Nipples._—Plugs can be made from a rod of iron of suitable size that is sound and free from flaws. The size of the part that screws into the barrel is about three-eighths of an inch, and the thread is twenty to the inch. This is the size and thread generally used. There may be exceptions that will require a different size and a different thread, but, if possible, let the sizes given govern the work. The end on which the screw is to be cut can be turned in the lathe, cut down with a hollow mill in the method given for making breech pins, or it can even be filed to shape in absence of any other way to make it. Before cutting the plug to the length where the nipple is to be seated, screw it into the barrel, mark the place for the nipple which can be found by letting down the hammer of the lock, then remove drill and fit the nipple. Cut off all that is not required beyond the nipple and screw into place, the nipple forming a shoulder on which to rest the wrench in screwing it home. The finished plug is shown in Fig. 53. [Illustration: FIGURE 53.] It is best to remove the barrel from the stock in screwing in the plug. With an improper wrench, it often happens that a nipple will be broken off at the square if it be very hard, and then it is difficult to remove. To obviate all danger of this, make a wrench with a handle at each end, and in the middle have a hole that will fit the plug, and then file an opening so that it will just fit over the nipple. It should fit close to the nipple at the square, and be free so as not to have a bearing at the end of the nipple where the cap is put on. Plugs are sometimes left bright, but if blued it improves their appearance and there is less tendency to rust. If desired to be very durable, case-harden them. The better polish that can be made on them will make a better blue or case-hardened surface. Many of the cheap plugs in market are nothing but cast malleable iron. The best material of which to make them is decarbonized steel, or what is known as “soft steel.” It comes in smooth round rods and is homogeneous and easy to work. Cold-rolled iron rod makes very good plugs. The diameter may be half-inch or nine-sixteenths, the latter size being most preferable. With a three-eighth size screw the half-inch rod gives a one-sixteenth shoulder, which must fit tight to the barrel. If not fitted tight, the gas from the powder when the gun is fired will soon cause a leak which is difficult to remedy. In some localities the name plug is discarded and the term “cylinder” adopted. CHAPTER XXV. MAINSPRINGS. _Mainsprings._—Mainsprings are made for right and left side locks, and are known as right and left. Very few gunsmiths make these springs, as they are kept in stock and can be purchased of the dealers at any time. Fig. 54 is called the forward or side-action hook spring; Fig. 55 the forward or side-action swivel, and are used in bar locks.