THE BREECH MECHANISM.
The breechblock is of the interrupted-screw type, and is provided with four threaded and four slotted sectors. The front end of the axial recess in the block is closed by a bushing. Four ventholes lead from a cavity in the bushing and permit the escape of gas to the rear in case of a ruptured primer. On the rear face of the breechblock are cut gear teeth, in which the gear teeth of the operating lever bevelgear mesh. The breechblock is concentrically mounted on a hub on the block carrier, in which the firing-lock case is fitted. Its position in the breech of the gun with reference to the axis of the bore is eccentric.
The breechblock is closed or locked by a continuous movement of the operating lever. When the block is swung to the closed position the front face of the block latch comes in contact with the rear face of the breech of the gun, thus forcing the latch out of the notch in the breechblock and back into a recess in the carrier. By continuing the motion of closing the mechanism, the breechblock is then rotated on the hub of the carrier and its threads engage with corresponding ones in the gun. When the breechblock is in the closed position, a lug on the firing-lock case serves to lock the carrier to the breechblock and prevents displacement due to a blowback.
The firing mechanism belongs to that type known as a continuous-pull mechanism; that is, no cocking of the firing-pin is required.
The firing-lock case is eccentrically fitted in the hub of the block carrier, in such a position that the axis of the firing-pin is always in line with the bore of the gun. The vent bushing in the front end of the breech block through which the firing pin passes when in the fired position, is fitted eccentrically with reference to the breechblock. This eccentric arrangement of the breechblock, masks the point of the firing-pin and prevents any possible contact between the pin and the primer in the cartridge case when the block is unlocked. The block will be practically fully locked before any contact between the firing-pin and primer can take place.
Nomenclature of important parts of Breech & Firing Mechanism.
Mod. 1905:—
| Name of Part | Where located |
| Breech Block | On block carrier |
| Vent Bushing | Front end of block |
| Block Carrier | Hinged to jacket; supports block |
| Block Stop | Screwed into front face of carrier |
| Hinge Pin | Hinges carrier to jacket |
| Hinge Pin Catch | In hinge pin |
| Extractor | In breech recess |
| Extractor Lever | Mounted on hinge pin |
| Operating Lever | Pivoted on block carrier |
| Lever Pivot | Pivots lever on block carrier |
| Lever Latch | In operating lever |
| Lever Latch Spring | In operating lever, lower part |
| Lever Latch Pivot | In operating lever, lower part |
| Block Latch | In recess in carrier |
| Block Latch Spring | Around latch bolt |
| Firing Lock Case | In hub of the block carrier |
| Locking Bolt Nut and Pin | On firing lock case, rear face of carrier |
| Firing Pin | In axle hole, center of firing lock case |
| Firing Pin Spring | Around firing pin |
| Firing Spring Sleeve | Around firing pin spring |
| Sear | In slot in firing lock case |
| Trigger Fork | Rear end firing lock case |
| Trigger Shaft | On rear end firing lock case |
| Trigger Shaft Detent | On trigger shaft |
| Firing Pallet | On pallet shank |
| Pallet Shank | On recoil lug of gun |
| Firing Handle | On firing handle shaft |
| Firing Handle Bracket | Attached to right side of cradle |
| Firing Handle Shaft | Assembled in bracket, right side of cradle |
| Trip Latch | Attached to trip latch plunger |
| Trip Latch Plunger | Assembled to firing handle |
| Adjusting Screw | Assembled to firing handle bracket |
| Check Nut | Assembled in adjusting screw |
3 Inch Gun Carriage, Model of 1902.
Plan