The Nordenfelt mechanism consists of a breech block (fig. 35) and a wedge to secure it. A hand lever on the shaft is pulled to the rear, and this works the action cam, which pulls down the wedge; the breech block is then caused to rotate and falls back to the rear. This motion of the breech block actuates the extractor and extracts the case. While the wedge is being withdrawn the firing pin is pulled back and cocked for the next round. The mechanism is closed by reversing the hand lever; this rotates the breech block upwards and pushes home the cartridge case, and the wedge is then forced up and secures the breech block.
Fig. 32.—Krupp Breech Action.
These small type Q.F. guns, which were introduced to cope with torpedo boats, are now, however, of little account, since experiment has proved that nothing smaller than a 12-pounder is sufficient so to injure a modern torpedo boat as to stop it. Most of these small guns are therefore in the English and in some other Services being converted into “sub-calibre” guns for exercise purposes. These sub-calibre guns retain their ordinary breech mechanism, but the bodies are fitted with a strong steel plug screwed on the outside in a similar manner to the breech screw of the parent gun. The sub-calibre gun is placed in the parent gun and the screwed plug engages in the threads of the breech opening.
There has been a gradual development of ideas regarding the repelling power required by a vessel against torpedo boat attack. The 12-pounder Q.F. 40-calibre guns were replaced by the more powerful 12-pounder Q.F. 50-calibre gun; this again by the 4-in. high power gun of 50 calibres, and now 6-in. guns are being used.
One other form of sliding mechanism is of importance owing to its adoption for the 75 m/m. French long recoil field gun (see below: Field equipments). This mechanism is on the Nordenfelt eccentric screw system and is very similar to that proposed by Clay about 1860; it has a breech screw (fig. 36) of large diameter mounted in the breech opening, which is eccentric to the bore. For loading, the breech block has a longitudinal opening cut through it, so that when the mechanism is in the open position this opening coincides with the chamber, while a half turn of the breech screw brings its solid part opposite the chamber and closes the gun. The mechanism is very simple and strong, but it is only suitable for small Q.F. guns using cartridge cases; the firing gear is similar to that applied to other types of mechanism, and the fired case is extracted by an extractor actuated by the face of the breech screw as it is opened.
With the swinging type of breech mechanism we are confronted with numberless patterns, many of undoubted merit and claiming certain advantages over others, and all showing the vast amount of ingenuity expended in so designing them that they may be as simple, and, at the same time, as effective and quick acting as possible. It is impossible to deal with all these, and therefore only the more important systems will be described. The special feature of this type is that the breech is closed by an interrupted breech screw; the screw is either supported in a carrier ring or tray hinged near the breech opening, or on a carrier arm which is hinged near the outer circumference of the gun.
The screw may be of the cylindric interrupted, Welin and coned types; these, or their modifications, practically embrace the various forms used. The cylindric form (fig. 37) is the simplest; it consists of a strong screwed plug engaging with a corresponding screw thread cut on the interior of the breech opening of the gun. The screw surface of the breech plug is cut away in sections equally divided and alternating with the threaded portions. The screw surface of the breech opening is similarly cut away, so that the plug can be pushed nearly home into the breech opening without trouble; by then revolving the breech screw through a small angle the screwed portions of the plug and breech opening engage. Thus if three screwed sections alternate with three plain sections the angle of revolution necessary to ensure a full engagement of the screw surfaces will be 60°. The Welin screw (fig. 38) is an ingenious adaptation of the cylindric type; in this the surface is divided into sections each formed of two or three cylindrical screwed steps with a single plain portion; thus if there are three sections, each section of which has one plain division and two screwed divisions, there will be in all six screwed portions and three plain. The breech opening is correspondingly formed so that the screwed threads would fully engage with 40° of movement. There is consequently a greater amount of screwed circumferential surface with the Welin screw than with the ordinary cylindric interrupted type; the latter form has 50% screw surface while the Welin has 60%. For equal screw surface the Welin can therefore be made shorter.
For medium guns the Elswick type of coned screw (fig. 39) has found much favour, and this mechanism has been fitted to guns of all calibres from 3-inch to 6-inch, both for the British and numerous other governments. The coned breech screw is formed with the front part conical and the rear cylindrical, to facilitate its entrance into the gun, and also its exit; this form, moreover, is taken advantage of by cutting the interruptions in the screwed surface alternately on the coned part and on the cylindrical part, so that there is a screwed surface all round the circumference of the breech screw. By this means the stress is taken all round the circumference, both of the breech screw and in the gun, instead of in portions alternately, as with other forms.
The Bofors breech screw is a modification. The surface is formed of a truncated ogive instead of a cylinder and cone, and the threaded portions are not alternate.
Fig. 33.—Krupp Breech Action.
In the older types of mechanism for heavy B.L. guns the breech was opened in from three to four different operations which involved considerable loss of time. Fig. 40 shows the general type for 9·2-in., lo-in. and 12-in. B.L. guns. To open the breech the cam lever C was folded up so that it engaged the pin B in connexion with the ratchet lever E. This was worked and so disengaged the breech screw from the threads cut in the gun; the cam lever was then folded down as to to start the breech screw, and the winch handle Q rotated and so withdrew the screw and swung it clear of the breech opening. During these operations the firing lock was actuated and made safe, but the fired tube had to be extracted by hand. To close the gun these various operations must be reversed, and to open or close the gun would certainly occupy at least half a minute with trained men.
To compare with this a modern 12-in. breech mechanism is shown in fig. 41. In order to open this breech it is only necessary to turn
