A number of bits, each a little larger than the former, are afterwards successively passed through the barrel in the same way, until the bore has acquired the magnitude intended. By this operation the barrel is very much heated, especially the first time the borer is passed through it, by which means it is apt to warp: to prevent this in some measure, the barrel is covered with a cloth kept constantly wetted, which not only preserves the barrel from an excess of heat, but likewise preserves the temper of the bit from being destroyed. The equality of the bore is of the utmost consequence to the perfection of a barrel, insomuch that the greatest possible accuracy in every other respect will not make amends for any deficiency in this. The method used by gunsmiths to ascertain this is by a cylindrical plug of tempered steel highly polished, about an inch in length, and fitting the bore exactly; this is screwed upon the end of an iron rod, and introduced into the cavity of the barrel, where it is moved backwards and forwards; and the places where it passes with difficulty being marked, the boring bit is repeatedly passed until it moves with equal ease through every part.

In forming the breech, a tap is introduced into the barrel, and worked from left to right, and back again, until it has marked out the first four threads of the screw; another less conical tap is introduced; and when this has carried the impression of the screw as far as it is intended to go, a third one, nearly cylindrical, is made use of, scarcely differing from the plug of the breech intended to fill the screw thus formed in the barrel; the plug itself has its screw formed by means of a screw plate of tempered steel, with several female screws corresponding with the taps employed for forming that in the ban-el. Seven or eight threads make a sufficient length for a plug; they ought to be neat and sharp, so as completely to fill the turns made in the barrel by the tap. The breech plug is then to be case-hardened, or to have its surface converted into steel by covering it with shavings of horn, or the parings of the hoofs of horses, and keeping it for some time red hot, after which it is plunged in cold water.

The above is the usual method of making the common barrels, especially for fowling pieces; but there are some other methods of manufacture by which they are thought to be considerably improved. One kind of these are called twisted barrels, and by the English workmen are formed out of the plates made of stubs, as above described. Four of these, of the size already mentioned, are requisite to make one barrel; one of them heated red hot for five or six inches, is turned like a cork-screw by means of the hammer and anvil, the remaining parts being treated successively in the same manner until the whole is turned into a spiral forming a tube, the diameter of which corresponds with the bore of the intended barrel. Four are generally sufficient to form a barrel of the ordinary length, that is from 32 to 38 inches; and the two which form the breech or strongest part, called the reinforced part, are considerably thicker than those which form the muzzle or fore part of the barrel: one of these tubes is then welded to a part of the old barrel to serve as a handle; after which the turns of the spiral are united by heating the tube two or three inches at a time to a bright white heat, and striking the end of it several times against the anvil in a horizontal direction with considerable force, which is called jumping the barrel; and the heats given for this purpose are called jumping heats.

The next step is to introduce a mandril into the cavity, and to hammer the heated portion lightly, in order to flatten the ridges or burrs raised by the jumping at the place where the spirals are joined. As soon as one piece is jumped throughout its whole length, another is welded to it and treated in the same manner until the four pieces are united, when the part of the old barrel is cut off as being no longer of any use. The welding is repeated three times at least, and is performed exactly in the same manner as directed for plain barrels; and the piece may afterwards be finished according to the directions already given. The advantages of twisted barrels are, after all, somewhat problematical, where there is so much of welding; and that in a spiral form, the welding is more likely to be done in a careless manner, or with some imperfection in some part, than when it is a plain, is an obvious business; nor have we observed that twisted barrels are leas liable to burst than plain ones, where the latter have been well and carefully forged.

The manufacture of rifle barrels, in their first formation, is exactly similar to that of other barrels, except that their external form is generally octagonal, instead of being smooth on the inside, like the common pieces, they are formed with a number of spiral channels resembling those of a screw, except only that the threads or rifles are less deflected, making only one turn, or a little more, in the whole length of the piece. This construction of the barrel is employed for correcting the irregularity in the flight of balls from smooth barrels. The rifle barrels which have been made in England, where they are not very common, are contrived to be charged at the breech, the piece being for this purpose made larger there than in any other part; the powder and bullet are put in through the side of the barrel by an opening, which, when the piece is loaded, is filled up with a screw; by this means, when the piece is fired, the bullet is forced through the rifles, and is projected with greater truth.

The principal imperfections to which gun barrels are liable are the chink, crack, and flaw, the first is a small rent in the direction of the length of the barrel; the second across it; and the third is a kind of scale or small plate adhering to the barrel by a narrow base, from which it spreads out like the head of a nail from its shank, and, when separated, leaves a pit or hollow in the metal. The chink or flaw are of much worse consequence than the crack in fire-arms, the force of the powder being exerted more upon the circumference than the length of the barrel. The flaw is much more frequent than the chink, the latter scarcely ever occurring but in plain barrels formed out of a single plate of iron, and then only when the metal is deficient in quality: when flaws happen on the outside they are of no great consequence; but in the inside they are apt to lodge moisture and foulness, which corrode the iron, and thus the cavity enlarges continually till the piece bursts. This accident, however, may arise from many other causes besides the defect of the barrel itself; the best pieces will burst when the ball is not sufficiently rammed home, so that a space is left behind it and the powder; a very small windage or passage for the inflamed powder between the sides of the barrel and ball will be sufficient to prevent the accident; but if the ball has been forcibly driven down with an iron ramrod, so as to fill up the cavity of the barrel very exactly, the piece will almost certainly burst, if only a very small place is left between it and the powder; and the greater the space is, the more certainly does the event take place.