The ASTRAGAL AND FILLETS in field guns, and the _chase ring_ in other pieces, are the mouldings at the front end of the chase. [Sidenote: 14]] The NECK is the smallest part of the piece in front of the astragal or the chase ring. The SWELL OF THE MUZZLE is the largest part of gun in front of the neck. It is terminated by the _muzzle mouldings_, which in field and siege guns, consist of the _lip_ and _fillet_. In sea-coast guns and heavy howitzers and columbiads, there is no fillet. In field and siege howitzers, and in mortars, a _muzzle-band_ takes the place of the _swell of the muzzle_. The FACE of the piece is the terminating plane perpendicular to the axis of the bore. The TRUNNIONS are cylinders, the axes of which are in a line perpendicular to the axis of the bore, and in the same plane with that axis. The RIMBASES are short cylinders uniting the trunnions with the body of the gun. The ends of the rimbases, or the _shoulders_ of the trunnions, are planes perpendicular to the axis of the trunnions. The BORE of the piece includes all the part bored out, viz.: the cylinder, the chamber (if there is one), and the conical or spherical surface connecting them. The CHAMBER in howitzers, columbiads, and mortars, is the smallest part of the bore, and contains the charge of powder. In the howitzers and columbiads,[1] the chamber is cylindrical; and is united with the large cylinder of the bore by a conical surface; the angles of intersection of this conical surface with the cylinders of the bore and chamber, are rounded (in profile) by arcs of circles. In the 8-inch siege howitzer, the chamber is united with the cylinder of the bore by a Spherical surface, in order that the shell may when necessary, be inserted without a sabot. [1] The new columbiad is made without a chamber. [Sidenote: 15]] The BOTTOM OF THE BORE (to facilitate sponging) is a plane perpendicular to the axis, united with the sides (in profile) by an arc of a circle the radius of which is one-fourth of the diameter of the bore at the bottom. In the columbiads, the heavy sea-coast mortars, stone mortar, and eprouvette, the bottom of the bore is hemispherical. The MUZZLE, or mouth of the bore, is chamfered to a depth of 0.15 inch to 0.5 inch (varying with the size of the bore), in order to prevent abrasion, and to facilitate loading. The TRUE WINDAGE is the difference between the true diameters of the bore and of the ball. 27. What is the vent? The aperture through which fire is communicated to the charge. 28. What is to be observed in reference to the diameter of the vent? It should be as small as the use of the priming wire and tube will allow. 29. Why?