This point-blank, therefore, depends on the class of gun, the charge, and the height above the water.
292. A preferable definition of this distance is "range at level."
293. The aim is always supposed to be directed at the water-line. But, with the sight-bar at level, if a gun is aimed by it at the water-line of a vessel at point-blank range, the shot would strike short of the point aimed at by about one-quarter of the distance; or, if aimed, under similar conditions, at the upper part of the hull, the shot would fall a distance below the point aimed at equal to the height of the gun.
294. In firing at small objects, particularly boats, within point-blank range, it is therefore important to attend to this source of error. It is desirable that all sights should be marked, from 100 yards to the greatest range, and thus avoid all consideration of point-blank.
In fitting new guns, or those upon which the sights require replacing, the sight-bar will be fitted as described in Article 289, and graduated from 100 yards downwards.
For shell-guns the ranges are marked for shells on the sight-bars, in the same manner as those for the shot of shot-guns.
295. These sights being each adjusted to a particular gun, and marked with its class and number, do not, in strictness, admit of being transferred to other guns, even of the same class.
296. When used, the stem of the breech-sight must be raised or lowered, to correspond with the ascertained or estimated distance, in yards, of the object aimed at, and firmly secured there by the thumb-screw. Then, if the ship be steady, elevate or depress the gun until the line of sight from the bottom of the notch of the breech-sight, the top of the reinforce-sight, and the point to be struck, will coincide; but if the ship have a rolling motion the gun must be so laid, after the sight is set for the distance, that this coincidence may be obtained, if possible, at the most favorable part of every roll which the ship makes.
297. The inclination of the line of metal to the axis of the bore varies in guns of the same class, as well as in those of different classes. Aiming, therefore, by the line of metal cannot be relied on for definite ranges; besides that, within those ranges, it is apt to mislead by giving too much elevation to the piece. Therefore, when the established sights are not furnished, or have become unserviceable, wooden dispart-sights lashed on the reinforce should be immediately substituted. A narrow groove in the upper surface of the wooden sight, made to coincide with the plane of the line of sight marked on the gun, will assist the Gun Captain in getting the true direction quickly.
Half the difference between the diameters of the gun at the base-ring and swell of the muzzle, or at any intermediate point on the line of metal, will give the proper height of the dispart-sight at the point where the least diameter was taken, to which must be added the height of the lock-piece above the base-ring, in order to get a line of sight over it, parallel to the axis of the bore.