To obtain the true form, it is important that they should be turned. The way of fixing them in the lathe is by having a number of plugs or mandrils, which are perfectly true, and of various sizes, to fit different bores; these are centred and put in the lathe; a carrier is then secured on a part of the plug that projects out of the breech-end of the barrel, and then put into the face-plate of the lathe, which carries it round. The leading screw that travels the slide-rest, is then set in the angle to which the barrel is to be turned (though some lathes have not the power of alteration, but turn all barrels in one angle); the slide is next adjusted to the thickness of the muzzle wanted, and, when all is ready, the lathe is set going; the leading screw is turned at the same moment by the machinery connected, which keeps the tool cutting sufficiently keen to turn a barrel in about twenty-five minutes. This being done, nothing more is required than a fine smooth file to remove the marks of the tool. There can be no doubt of the superiority of this mode of turning barrels, if due care only be taken with the tool. If it get blunted by any scales or impurities, it is apt to tear pieces out of the barrel, similar to the rings that may be noticed in a slovenly bored barrel, owing to dirt getting on the edges of the bit. In turning a barrel by a common lathe, it is fixed in the same manner as before; about an inch of the surface at the breech and the muzzle is turned to the diameter wanted; the rest is then removed, and half an inch more is turned four or five inches from either end; then another half inch, at another distance of four or five inches, and so on, according to the length; making an allowance each time in the depth of the turning, according to the taper of the barrel. The iron between these cuttings is then filed off by floats the lengthways of the barrel, or more frequently ground off; this is a sure mode of getting the barrels perfectly straight on the outside, and without any of those hollows and shades which may be always discovered in an ill-made barrel. It is astonishing how beautifully many barrels are struck by the float. The mode of turning by the lathe is, however, cheaper, and is now confined to military barrels.